ail)p  S.  1.  Bill  ?jtbrarg 


?5nrtb  (Taroliim  ^tatp  (Eollpnp 


coD.r 


>• 


s 


This  book  is  due  on  the  date  indicated 
below  and  is  subject  to  an  overdue  fine 
as  posted  at  the  Circulation  Desk. 


r^.3„ 


>-'•<-' 


J«JiM  1  S  1979 


n  26  19,3 


DEC  2  4  1987 

APR  2 1  mn 


i- 


THE    CELL 


IN    DEVELOPMENT    AND    INHERITANCE 


Columbia  ^Snibcrsito  Biological  Scries. 

« 

EDITED   BY 

HENRY    FAIRFIELD   OSBORN 

AND 

*       EDMUND    B.    WILSON, 

1.  FROM  THE  CREEKS  TO  DARWIN. 

By  Henry  Fairfield  Osborn,  Sc.D.  Princeton. 

2.  AMPHIOXUS  AND  THE  ANCESTRY  OF  THE  VERTEBRATES. 

By  Arthur  Willey.  B.Sc.  Lond.  Univ. 

3.  FISHES.  LIVINC  AND   FOSSIL.     An  Introductory  Study. 

By  Bashford  Dean,  Ph.D.  Columbia. 

4.  THE  CELL   IN    DEVELOPMENT   AND   INHERITANCE. 

By  Edmund  B.  Wilson,  Ph.D.  J.H.U. 

5.  THE   FOUNDATIONS   OF  ZOOLOGY. 

By  William  Keith  Brooks. 


COLUMBIA    UNIVERSITY  BIOLOGICAL   SERIES.     IV 


THE    CELL 


IN 


Development  and  Inheritance 


BY 


EDMUND    B.   WILSON,   Ph.D. 

PROFESSOR    OF    ZOOLOGY,   COLUMBIA    UNIVERSITY 

SECOND   EDITION 
REVISED  AND  ENLARGED 


"  Natura  nusquam  magis  est  tota  quam  in  minimis 


PLINY 


THE    MACMILLAN    COMPANY 

LONDON:   MACMILLAN  &  CO.,  LTD. 
3>3oV^vv\>A\—    1904 

AN  rights  reserved 


Copyright,  1896, 
By  the   MACMILLAN   COMPANY. 

Copyright,  1900, 
By   the   MACMILLAX   COMPANY. 


Set  up  and  electrotyped  October,  1896.      Reprinted  September, 
1897;  September,  1898. 

New  edition,  revised,  set  up  and  electrotyped  January,  1900; 
March,  1902  ;  June,  1904. 


XorSuooti  39rc33 

J.  S.  Gushing  &  Co.  —  Berwick  &  Smith 
Norwood,  Mass.  U.  S.  A. 


Co  mo  fxitnH 
THEODOR    BOVERI 


144370 


PREFACE 

This  volume  is  the  outcome  of  a  course  of  lectures,  delivered  at 
Columbia  University  in  the  winter  of  1892-93,  in  which  I  endeavoured 
to  give  to  an  audience  of  general  university  students  some  account 
of  recent  advances  in  cellular  biology,  and  more  especially  to  trace 
the  steps  by  which  the  problems  of  evolution  have  been  reduced  to 
problems  of  the  cell.  It  was  my  first  intention  to  publish  these 
lectures  in  a  simple  and  general  form,  in  the  hope  of  showing  to 
wider  circles  how  the  varied  and  apparently  heterogeneous  cell- 
researches  of  the  past  twenty  years  have  grown  together  in  a 
coherent  group,  at  the  heart  of  which  are  a  few  elementary  phe- 
nomena, and  how  these  phenomena,  easily  intelligible  even  to  those 
having  no  special  knowledge  of  the  subject,  are  related  to  the 
problems  of  development.  Such  a  treatment  was  facilitated  by 
the  appearance,  in  1893,  of  Oscar  Hertwig's  invaluable  book  on 
the  cell,  which  brought  together,  in  a  form  well  designed  for  the 
use  of  special  students,  many  of  the  more  important  results  of 
modern  cell-research.  I  am  glad  to  acknowledge  my  debt  to  Hert- 
wig's book ;  but  it  is  proper  to  state  that  the  present  volume  was 
fully  sketched  in  its  main  outlines  at  the  time  the  Zelle  mid  Gcwcbc 
appeared.  Its  completion  was,  however,  long  delayed  by  investiga- 
tions which  I  undertook  in  order  to  re-examine  the  history  of  the 
centrosomes  in  the  fertilization  of  the  ^gg,  —  a  subject  which  had 
been  thrown  into  such  confusion  by  Fol's  extraordinary  account  of 
the  "  Quadrille  of  Centres  "  in  echinoderms  that  it  seemed  for  a  time 
impossible  to  form  any  definite  conception  of  the  cell  in  its  relation 
to  inheritance.  By  a  fortunate  coincidence  the  same  task  was  inde- 
pendently undertaken,  nearly  at  the  same  time,  by  several  other 
investigators.  The  concordant  results  of  these  researches  led  to  a 
decisive  overthrow  of  Fol's  conclusions,  and  the  way  was  thus  cleared 
for  a  return  to  the  earlier  and  juster  views  founded  by  Hertwig, 
Strasburger,  and  Van  Beneden,  and  so  lucidly  and  forcibly  developed 
by  Boveri. 

The  rapid  advance  of  discovery  in  the  mean  time  has  made  it 
seem  desirable  to  amplify  the  original  plan  of  the  work,  in  order  to 
render  it  useful  to  students  as  well  as  to  more  general  readers ;  and 
to  this  end  it  has  been  found  necessary  to  go  over  a  considerable 

vii 


Vlll  PREFACE 


part  of  the  ground  already  so  well  covered  by  Hertwig.i  This  book 
does  not,  however,  in  any  manner  aim  to  be  a  treatise  on  general 
histology,  or  to  give  an  exhaustive  account  of  the  cell.  It  has  rather 
been  my  endeavour  to  consider,  within  moderate  limits,  those  features 
of  the  cell  that  seem  more  important  and  suggestive  to  the  student 
of  development,  and  in  some  measure  to  trace  the  steps  by  which  our 
present  knowledge  has  been  acquired.  A  work  thus  limited  neces- 
sarily shows  many  gaps ;  and  some  of  these,  especially  on  the  botani- 
cal side,  are,  I  fear,  but  too  obvious.  On  its  historical  side,  too,  the 
subject  could  be  traced  only  in  its  main  outlines,  and  to  many 
investigators  of  whose  results  I  have  made  use  it  has  been  impossible 
to  do  full  justice. 

To  the  purely  speculative  side  of  the  subject  I  do  not  desire  to 
add  more  than  is  necessary  to  define  some  of  the  problems  still  to  be 
solved  ;  for  I  am  mindful  of  Blumenbach's  remark  that  while  Drelin- 
court  rejected  two  hundred  and  sixty-two  "groundless  hypotheses" 
of  development,  **  nothing  is  more  certain  than  that  Drelincourt's 
own  theory  formed  the  two  hundred  and  sixty-third."  ^  i  have  no 
wish  to  add  another  to  this  list.  And  yet,  even  in  a  field  where 
standpoints  are  so  rapidly  shifting  and  existing  views  are  still  so 
widely  opposed,  the  conclusions  of  the  individual  observer  may  have 
a  certain  value  if  they  point  the  way  to  further  investigation  of  the 
facts.  In  this  spirit  I  have  endeavoured  to  examine  some  of  the  more 
important  existing  views,  to  trace  them  to  their  sources,  and  in  some 
measure  to  give  a  critical  estimate  of  their  present  standing,  in  the 
hope  of  finding  suggestion  for  further  research. 

Every  writer  on  the  cell  must  find  himself  under  a  heavy  obliga- 
tion to  the  works  of  Van  Beneden,  Oscar  Hertwig,  Flemming,  Stras- 
burger,  and  Boveri ;  and  to  the  last-named  author  I  have  a  special 
sense  of  gratitude.  I  am  much  indebted  to  my  former  student, 
Mr.  A.  P.  Mathews,  for  calling  my  attention  to  the  importance  of 
the  recent  work  of  physiological  chemists  in  its  bearing  on  the 
problems  of  synthetic  metabolism.  The  views  developed  in  Chap- 
ter VII.  have  been  considerably  influenced  by  his  suggestions,  and 
this  subject  will  be  more  fully  treated  by  him  in  a  forthcoming  work  ; 
but  I  have  endeavoured  as  far  as  possible  to  avoid  anticipating  his  own 
special  conclusions.  Among  many  others  to  whom  I  am  indebted 
for  kindly  suggestion  and  advice,  I  must  particularly  mention  my 
ever  helpful  friend.  Professor  Henry  F.  Osborn,  and  Professors 
J.  E.  Humphrey,  T.  H.  Morgan,  and  F.  S.  Lee. 

In  copying  so  great  a  number  of  figures  from  the  papers  of  other 

1  Henneguy's  Le(;ons  sur  la  cellule  is  received,  too  late  for  further  notice,  as  this  volume 
is  going  through  the  press. 
^  Allen  Thomson. 


PREFACE  ix 

investigators,  I  must  make  a  virtue  of  necessity.  Many  of  the  facts 
could  not  possibly  have  been  illustrated  by  new  figures  equal  in  value 
to  those  of  special  workers  in  the  various  branches  of  cytological 
research,  even  had  the  necessary  material  and  time  been  available. 
But,  apart  from  this,  modern  cytology  extends  over  so  much  debatable 
ground  that  no  general  work  of  permanent  value  can  be  written  that 
does  not  aim  at  an  objective  historical  treatment  of  the  subject;  and 
I  believe  that  to  this  end  the  results  of  investigators  should  as  far  as 
practicable  be  set  forth  by  means  of  their  original  figures.  Those 
for  which  no  acknowledgment  is  made  are  original  or  taken  from 
my  own  earlier  papers. 

The  arrangement  of  the  literature  lists  is  as  follows.  A  general 
list  of  all  the  works  referred  to  in  the  text  is  given  at  the  end  of  the 
book  (p.  449).  These  are  arranged  in  alphabetical  order,  and  are 
referred  to  in  the  text  by  name  and  date,  according  to  Mark's  con- 
venient system.  In  order,  however,  to  indicate  to  students  the  more 
important  references  and  partially  to  classify  them,  a  short  separate 
list  is  given  at  the  end  of  each  chapter.  The  chapter-lists  include 
only  a  few  selections  from  the  general  list,  comprising  especially 
works  of  a  general  character  and  those  in  which  reviews  of  the 
special  literature  may  be  found. 

E.  B.  W. 

Columbia  University,  New  York, 
July,  1896. 


PREFACE    TO    THE    SECOND    EDITION 

Since  the  appearance  of  the  first  edition  of  this  work,  in  1896,  the 
aspect  of  some  of  the  most  important  questions  with  which  it  deals 
has  materially  changed,  most  notably  in  case  of  those  that  are 
f  ocussed  in  the  centrosome  and  involve  the  phenomena  of  cell-division 
and  fertilization.  This  has  necessitated  a  complete  revision  of  the 
book,  many  sections  having  been  entirely  rewritten,  while  minor 
changes  have  been  made  on  almost  every  page. 

In  its  first  form,  the  work  was  compressed  within  limits  too  nar- 
row for  a  sufficiently  critical  treatment  of  many  disputed  subjects.  It 
has  therefore  been  considerably  enlarged,  and  upwards  of  fifty  new 
illustrations  have  been  added.  The  endeavour  has,  however,  still  been 
made  to  keep  the  book  within  moderate  Hmits,  even  at  some  cost  of 
comprehensiveness  ;  and  the  present  edition  aims  no  more  than  did 
the  first  to  cover  the  whole  vast  field  of  cellular  biology.  Its  limita- 
tions are,  as  before,  especially  apparent  in  the  field  of  botanical 
cytology.  Here  progress  has  been  so  rapid  that,  apart  from  the  dif- 
ficulty experienced  by  a  zoologist  in  the  attempt  to  maintain  a  due 
sense  of  proportion  in  reviewing  the  subject,  an  adequate  treatment 
would  have  required  a  separate  volume.  I  have  therefore,  for  the 
most  part,  considered  the  cytology  of  plants  in  an  incidental  way, 
endeavouring  only  to  bring  the  more  important  phenomena  into  rela- 
tion with  those  more  fully  considered  in  the  case  of  animals. 

The  steady  and  rapid  expansion  of  the  literature  of  the  general 
subject  renders  increasingly  difficult  the  historical  form  of  treatment 
and  the  citation  of  specific  authority  in  matters  of  detail.  This  plan 
has  nevertheless  still  been  followed  as  far  as  possible,  despite  the 
increased  bulk  of  the  book  and  the  encumbrance  of  the  text  with 
references  thus  occasioned,  in  the  hope  that  these  disadvantages  will 
be  outweighed  by  increased  usefulness  of  the  work.  I  beg  the 
reader  to  remember,  however,  that  no  approach  to  a  complete  history 
of  cytology  and  experimental  embryology  could  be  attempted,  save 
in  a  work  of  far  greater  proportions,  and  that  it  has  been  necessary 


XI 


xii  PREFACE    TO    THE   SECOXD   EDITION 

to  pass  by,  or  dismiss  with  very  brief  mention,  many  works  to  which 
space  would  gladly  have  been  given. 

Recent  research  has  yielded  many  new  results  of  high  interest, 
conspicuous  among  them  the  outcome  of  experiments  on  cell-division, 
fertilization,  and  regeneration  ;  and  they  have  cleared  up  many  special 
problems.  Broadly  viewed,  however,  the  recent  advance  of  discovery 
has  not,  in  the  author's  opinion,  tended  to  simplify  our  conceptions 
of  cell-life,  but  has  rather  led  to  an  emphasized  sense  of  the  diversity 
and  complexity  of  its  problems.  "  One  is  sometimes  tempted  to  con- 
clude," was  recently  remarked  by  a  well-known  embryologist,  "  that 
every  <t%g  is  a  law  unto  itself  !  "  The  jest,  perhaps,  embodies  more 
of  the  truth  than  its  author  would  seriously  have  maintained,  express- 
ing, as  it  does,  a  growing  appreciation  of  the  intricacy  of  cell-phe- 
nomena, the  difficulty  of  formulating  their  general  aspects  in  simple 
terms,  and  the  inadequacy  of  some  of  the  working  hypotheses  that 
have  been  our  guides.  It  is  in  the  full  recognition  of  such  inade- 
quacy, when  it  exists,  and  of  the  danger  of  hasty  generalization  in  a 
subject  so  rapidly  moving  as  this,  that  our  best  hope  of  progress  lies. 

My  best  thanks  are  again  due  to  many  friends  for  helpful  criti- 
cism, suggestion,  and  other  aid ;  and  I  am  indebted  to  Professor  Ulric 
Dahlgren  for  the  beautiful  preparation  imperfectly  represented  by 
Fig.  59  (from  a  direct  photograph);  to  F.  Emil,  E.  M.  Van  Harlin- 
gen,  and  Dr.  G.  N.  Calkins,  for  aid  in  the  preparation  of  new  illus- 
trations ;  and  to  Messrs.  Ginn  &  Co.  for  the  electrotypes  of  Figs.  1 1, 
12,  and  1 88,  from  the  Wood's  Holl  Biological  Lectures  for  1899. 

Columbia  University, 
December  7,  1S99. 

Postscript.  —  Of  especial  importance  for  some  of  the  discussions  in  Chapters  I.,  V.,  and 
VII.  are  F'ischer's  extensive  work  on  protoplasm  (see  Literature,  I.)  and  Strasburger's  new 
researches  on  reduction  (see  Literature,  V.),  both  received  while  this  volume  was  in  press 
and  too  late  for  more  than  a  passing  mention  in  the  text. 

March,  1900. 


TABLE   OF   CONTENTS 


INTRODUCTION 

PAGE 

List  of  Figures xvii 

Historical  Sketch  of  the  Cell-theory;   its  Relation  to  the  Evolution-theory.     Earlier 
Views   of  Inheritance   and   Development.     Discovery  of  the   Germ-cells.     Cell- 
division,  Cleavage,  and  Development.    Modern  Theories  of  Inheritance.    Lamarck, 

Darwin,  and  Weismann          ...........  i 

Literature      ...............  14 

CHAPTER   I 
General  Sketch  of  the  Cell 

A.  General  Morphology  of  the  Cell     ..........  19 

B.  Structural  Basis  of  Protoplasm       ..........  23 

C.  The  Nucleus           .............  30 

1.  General  Structure         .         .         .         .         .         .         .         .         .         .         -31 

2.  Finer  Structure  of  the  Nucleus     .         .         .         .         .         .         .         ,         -37 

3.  Chemistry  of  the  Nucleus     ..........  41 

D.  The  Cytoplasm       .............  41 

E.  The  Centrosome    .............  50 

F.  Other  Cell-organs  .............  52 

G.  The  Cell-membrane        ............  53 

H.     Polarity  of  the  Cell 55 

I.     The  Cell  in  Relation  to  the  Multicellular  Body 58 

Literature,  I. 61 

CHAPTER   II 
Cell-division 


A.  Outline  of  Indirect  Division  or  Mitosis 

B.  Origin  of  the  Mitotic  Figure 

C.  Details  of  Mitosis 

1.  Varieties  of  the  Mitotic  Figure 

(a)  The  Achromatic  Figure 
(^)  The  Chromatic  Figure 

2.  Bivalent  and  Plurivalent  Chromosomes 

3.  Mitosis  in  the  Unicellular  Plants  and  Animals 

4.  Pathological  Mitoses     ..... 


65 
72 

77 

78 

78 
86 

87 
88 

97 


xni 


XIV 


TABLE    OF  COX  TEXTS 


D.     The  xMechanism  of  Mitosis     ..... 

1.  function  of  the  .Aniphiaster 

(/7)  Theory  of  Kil)rillar  Contractility 
{b)   Other  Facts  and  Theories 

2.  Division  of  the  Chromosomes 

E.  Direct  or  Amitotic  Division  ..... 

1.  General  Sketch     ...... 

2.  Centrosome  and  Attraction-sphere  in  .Xmitosis 

3.  Biological  Significance  of  .Vmitosis 

F.  Summary  and  Conclusion       ..... 
Literature,  II.        . 


PAGE 

100 

100 
100 
106 

I  \2 
114 
114 

'•5 
116 

119 

121 


CIIAPTKR    III 
The  Germ-cells 

A.     The  Ovum     .... 

1.  The  Nucleus 

2.  The  Cytoplasm     . 

3.  The  Egg-envelopes 
li.     The  Spermatozoon 

1.  The  Flagellate  Spermatozoon 

2.  Other  Forms  of  Spermatozoa 

3.  Paternal  Germ-cells  of  Plants 

C.  Origin  of  the  Germ-cells 

D.  Growth  and  Differentiation  of  the  Cierm-cells 

1.  The  Ovum    .... 

(a)   Growth  and  Nutrition 

{b)  Differentiation  of  the  Cytoplasm. 

{c)   Yolk-nucleus  . 

2.  Origin  of  the  Spermatozoon 

3.  Formation  of  the  Spermatozoids  in  Plants 

E.  *  Staining-reactions  of  the  Germ-nuclei 
Literature,  III.      ..... 


Deposit  of  Deutoplasm 


124 

125 

130 

132 

134 

135 

142 

142 

144 

150 

150 

.   150 

152 

•    155 

.  160 

172 

•   175 

•    177 

CHAPTER   IV 
Fertiliz.\tion  of  the  Ovi 

A.  General  Sketch      ....... 

1.  The  Germ-nuclei  in  Fertilization 

2.  The  Achromatic  Structures  in  Fertilization    . 

B.  Union  of  the  Germ-cells         ..... 

1.  Immediate  Results  of  Union 

2.  Paths  of  the  Germ-nuclei      .... 

3.  Union  of  the  Germ-nuclei.     The  Chromosomes 

C.  The  Centrosome  in  Pertilization     .... 

D.  Fertilization  in  Plants    ...... 

E.  Conjugation  in  Unicellular  Forms 

F.  Summary  and  Conclusion       ..... 
Literature,  IV.       ........ 


M 


180 

iSi 

185 

196 

200 

202 

204 

208 

215 
222 

229 
231 


TABLE    OF  CONTENTS 


XV 


CHAPTER   V 

Reduction  of  the  Chromosomes,  Oogenesis  and  Spermatogenesis 

PAGE 

A.  Genera'  Outline     .............     234 

1,  Reduction  in  the  Female.     The  Polar  Bodies       ......     236 

2,  Reduction  in  the  Male.     Spermatogenesis    .         .         .         .         .         .         .241 

3,  Weismann's  Interpretation  of  Maturation     .......     243 

B.  Origin  of  the  Tetrads 246 

1.  General  Sketch     ............     246 

2.  Detailed  Evidence        ...........     248 

C.  Reduction  without  Tetrad-formation      .........     258 

D.  Some  Peculiarities  of  Reduction  in  the  Insects      .         .         .         .         .         .         -271 

E.  The  Early  History  of  the  Germ-nuclei  .........     272 

F.  Reduction  in  Unicellular  Forms     .         .         .         .         .         .         .         .         .         -277 

G.  Maturation  of  Parthenogenetic  Eggs 2S0 

Appendix 

1.  Accessory  Cells  of  the  Testis         .........  284 

2.  Amitosis  in  the  Early  Sex-cells     .........  285 

H.     Summary  and  Conclusion       ...........  285 

J- iterature,  V.         ..............  287 

CHAPTER   VI 

t 

Some  Problems  of  Cell-organization 


A.  The  Nature  of  Cell-organs 

B.  Structural  Basis  of  the  Cell 

C.  Morphological  Composition  of  the  Nucleus 

I.  The  Chromatin     .         .         .         .         .         •     -    . 

{a)   Hypothesis  of  the  Individuality  of  the  Chromosomes 
(/;)   Composition  of  the  Chromosomes    .... 

D.  Chromatin,  Linin,  and  Cytoplasm 

E.  The  Centrosome    .         .■        . 

F.  The  Archoplasmic  Structures 

1.  Hypothesis  of  Fibrillar  Persistence 

2.  The  Archoplasm  Hypothesis 

3.  The  Attraction-sphere 

G.  Summary  and  Conclusion 
Literature,  VI 


291 

293 

294 

294 

294 

301 

302 

304 

31^ 
316 

3>S 

327 
328 


CHAPTER   VII 

Some  Aspects  of  C-iLL-cHEMisTRV  and  Cell-phvsiology 


A.     Chemical  Relations  of  Nucleus  ana  Cytoplasm 

1.  The  Proteids  and  their  Allies 

2.  The  Nuclein  Series       .... 

3.  Staining-reactions  of  the  Nuclein  Series 


330 

332 
334 


XVI 


TABLE    OF  COXTENTS 


B.  Physiological  Relations  of  Nucleus  and  Cytoplasm 

1.  Experiments  on  Unicellular  Organisms 

2.  Position  and  Movements  of  the  Nucleus 

3.  The  Nucleus  in  Mitosis 

4.  The  Nucleus  in  Fertilization 

5.  The  Nucleus  in  Maturation 

C.  The  Centrosome    ..... 

D.  Summary  and  Conclusion 
Literature.  VII 


PAGE 
340 


o:) 


I 


J3- 
353 
354 
358 
359 


CHArrr.R  viii 

Cell-division  and  Development 

A.  Geometrical  Relations  of  Cleavage-forms 

B.  Promorphological  Relations  of  Cleavage 

1.  Promorphology  of  the  Ovum 

(^/)   Polarity  and  the  Egg-axis 

(<^)   Axial  Relations  of  the  Primary  Cleavage-planes 

(r)  Other  Promorphological  Characters  of  the  Ovum 

2.  Meaning  of  the  Promorphology  of  the  Ovum 

C.  Cell-division  and  Growth       ....... 

Literature,  VIIL  .         . 


362 
37S 
378 
37S 

379 
382 

3S4 
394 


CHAPTER    IX 
Theories  of  Lmiekitance  and  Development 

A.  The  Theory  of  Germinal  Localization    . 

B.  The  Idioplasm  Theory  ..... 

C.  Union  of  the  Two  Theories    .... 

D.  The  Roux-Weismann  Theory  of  Development 

E.  Critique  of  the  Roux-Weismann  Theory 

F.  On  the  Nature  and  Causes  of  Differentiation 

G.  The  Nucleus  in  Later  Development 
H.  The  External  Conditions  of  Development 

I.     Development,  Inheritance,  and  Metabolism  . 
J.     Preformation  and  Epigenesis.     The  Unknown  Factor  in  Development 
Literature,  IX. 


Glossary 


General  Literature- list 
Lndex  of  Authors  . 
Index  of  Subjects  . 


397 
401 

403 
404 

407 

413 

425 
428 

430 
431 
434 

437 

449 

471 

477 


LIST    OF    FIGURES 


INTRODUCTION 


1.  Epidermis  of  larval  salamander 

2.  Section  of  growing  root-tip  of  the  onion 

3.  Avioeba  Proteus       .         .         .         .         . 

4.  Cleavage  of  the  ovum  in  Toxopneustes  . 

5.  Diagram  of  inheritance  .         . 


PAGE 

4 
1 1 


CHAPTER   I 

6.  Diagram  of  a  cell  ......••• 

7.  Spermatogonia  of  salamander  ...... 

Group  of  cells,  showing  cytoplasm,  nucleus,  and  centrosomes 
Living  cells  of  salamander  larva,  showing  fibrillar  structure    . 
Alveolar  or  foam-structure  of  protoplasm,  according  to  BiitschU 
Structure  of  protoplasm  in  the  echinoderm  e^ 
Aster-formation  in  alveolar  protoplasm  . 
Nuclei  from  the  crypts  of  Lieberkiihn    . 
Special  forms  of  nuclei  .... 

Scattered  nucleus  in  Trachelocerca 
Scattered  nucleus  in  Bacteria  and  Flagellata 
Ciliated  cells  . 
Cells  of  amphibian  pancreas 


8. 

9. 
10. 

II. 

12. 

13- 
14. 

15- 
16. 

17- 
18. 

19. 

20. 

21. 

22. 

2^. 


Nephridial  cell  of  Clepsine 

Nerve-cell  of  frog  . 

Diagram  of  dividing  cell 

Diagrams  of  cell-polarity 

Centrosomes  in  epithelium  and  in  blood-corpuscles 


18 
20 
21 

24 
26 

27 
28 

32 
35 
37 
39 
43 
44 
45 
47 
49 
56 
57 


CHAPTER   11 


24.  Remak's  scheme  of  cell-division 

25.  Diagram  of  the  prophases  of  mitosis 

26.  Diagram  of  later  phases  of  mitosis 

27.  Prophases  in  salamander-cells 

28.  Metaphase  and  anaphases  in  salamander-cells 

29.  Telophases  in  salamander-cells 

30.  Mid-body  and  cell-plate  in  cells  of  Umax 

31.  Middle  phases  of  mitosis  in  Ascaris-Qgg% 

32.  Mitosis  in  Stypocaulon   .... 

xvii 


64 
66 

69 

73 

75 
76 

79 
80 
Si 


XVlll 


LIST   OF  FIGURES 


FIG. 

2,2i-  Mitosis  in  Erysiphe         ....... 

34.  Mitosis  in  pollen-mother-cells  of  lily,  according  to  Guignard 

36.  Mitosis  in  spore-cells  of  Eqiiisetiim 

37.  Heterotypical  mitosis 

38.  Mitosis  in  Infusoria 

39.  Mitosis  in  Etiglypha 

40.  Mitosis  in  Euglena 

41.  Mitosis  in  Acanthocystis 

42.  Mitosis  in  Noctiliica 

43.  Mitosis  in  Paramaba 

44.  Mitosis  in  Actinospharium 

45.  Mitosis  in  Actinosphivriiim     . 

46.  Pathological  mitoses  in  cancer-cells 

47.  Pathological  mitosis  caused  by  poisons 

48.  \'an  Beneden's  account  of  astral  systems  in  Ascaris 

49.  Leucocytes     ..... 

50.  Pigment-cells  .... 

51.  Heidenhain's  model  of  mitosis 

52.  Mitosis  in  the  egg  of  Toxopneusies 

53.  Pathological  mitoses  in  polyspermy 

54.  Nuclei  in  the  spireme-stage    . 

55.  Early  division  of  chromatin  in  Ascaris 

56.  Amitotic  division    .... 


PAGE 

83 
84 

85 

87 

89 
90 

91 
92 

93 

95 
96 

97 
98 

99 
100 
102 
103 
104 
107 
109 
112 

"3 

"5 


CHAPTER   III 

57.  Volvox 

58.  Ovum  of  Toxopneustss   . 

59.  Ovum  of  the  cat     .... 

60.  Ovum  of  Nereis      .... 

61.  Germinal  vesicles  of  Unio  and  Epeira 

62.  Insect-egg      ..... 

63.  Micropyle  in  Argonauta 

64.  Germ-cells  of  Volvox 

65.  Diagram  of  the  flagellate  spermatozoon 

66.  Spermatozoa  of  fishes  and  amphibia 

67.  Spermatozoa  of  birds  and  other  animals 

68.  Spermatozoa  of  mammals 

69.  Unusual  forms  of  spermatozoa 

70.  Spermatozoids  of  Chara 

71.  Spermatozoids  of  various  plants 

72.  Germ-cells  of  Cladonema 

73.  Primordial  germ-cells  of  Ascaris    . 

74.  Primordial  germ-cells  of  Cyclops     . 

75.  Ovarian  ova  and  follicles  of  Helix 

76.  Egg  and  nurse-cells  in  Ophryotrocha 

77.  Ovarian  eggs  of  insects  . 

78.  Young  ovarian  eggs  of  various  animals 

79.  Young  ovarian  eggs  of  birds  and  mammals 

80.  Ovarian  eggs  of  spider,  earthworm,  ascidian,  showing  yolk-nucleus 


^^2, 
126 

127 

129 

130 

132 

134 
135 
136 
138 
140 
141 
142 

143 
146 

147 
149 

152 
153 
154 

155 
157 


LIST   OF^FIGURES 


XIX 


FIG. 


8i.  Ova.na.n  eggs  oi  Limti/tis  and  Po/yzoniuf/i    .... 

82.  Formation  of  the  spermatozoon  in  Ajiasa     .... 

83.  Transformation  of  the  spermatids  of  the  salamander 

84.  P'ormation  of  the  spermatozoon  in  Salamandra  and  Amphinma 

85.  The  same  in  Helix  and  in  elasmobranchs     .... 

86.  The  same  in  mammals  ....... 

87.  Formation  of  spermatozoids  in  cycads  ..... 

88.  Formation  of  spermatozoids  in  cryptogams  .... 


PAGE 


159 

162 

164 

166 

168 

169 

^IZ 

174 

CHAPTER   IV 

89.  Fertilization  of  Physa    . 

90.  Fertilization  of  Ascaris 

91.  Germ-nuclei  of  Nematodes  , 

92.  Fertilization  of  the  mouse     . 

93.  Fertilization  of  Pterotrachea 

94.  Entrance  and  rotation  of  sperm-head  in  Toxopneustes 

95.  Conjugation  of  the  germ-nuclei  in  Poxopnensies  . 

96.  Diagrams  of  fertilization        ..... 

97.  Fertilization  of  Nereis  ...... 

98.  Fertilization  of  Cyclops  ..... 

99.  Fertilization  and  persistence  of  centrosomes  in  Thalassema 

100.  Entrance  of  spermatozoon  into  the  egg 

101.  Pathological  polyspermy        .... 

102.  Polar  rings  of  Clepsine  .... 

103.  Paths  of  the  germ-nuclei  in  Toxopneustes 

104.  Fertilization  of  Myzostoma    .... 

105.  Fertilization  of  Pihilaria      .... 

106.  Penetration  of  the  pollen-tube  in  angiosperms 

107.  Fertilization  of  the  lily  .... 

108.  Fertilization  in  Zamia  .... 

109.  Diagram  of  conjugation  in  Infusoria 

110.  Qoxi]\xgzX\ox\  oi  Pa  ramie  ciiim 

111.  Conjugation  of  Vorticella     .... 

112.  Con]\xgz\.\on  o(  Noctiltcca       .... 

113.  Co\\]\xg2ii\on  oi  Spirogyra     .... 


180 

184 
185 
186 
187 
189 
190 
191 

193 
195 
197 
199 

201 
203 
209 
216 
217 
219 
220 
22  ? 
225 
226 
227 
228 


CHAPTER  V 


114.  Polar  bodies  in  Toxopneustes 

115.  Genesis  of  the  egg 

116.  Diagram  of  formation  of  polar  bodies 

117.  VoXaxhoCixtsm.  Ascaris 

118.  Genesis  of  the  spermatozoon 

119.  Diagram  of  reduction  in  the  male 

120.  Spermatogenesis  of  .'^5<r<7;-?5 

121.  Diagrams  illustrating  tetrad-formation 

122.  Tetrads  of  Gryllotalpa 

123.  Tetrads  and  polar  bodies  in  Cyclops 


o 
o 


234 
235 
237 
239 

240 
242 

244 

247 

249 

250 


XX 


LIST   OF  FIGURES 


FIG. 

124. 

125. 

126. 

127. 

128. 

129. 

130. 

132. 
134. 

135- 
136. 

137- 
138. 

139- 
140. 

141. 

142. 


Diagrams  of  tetrad-formation  in  arthropods 

Germinal  vesicles  and  tetrads 

Maturation  in  Auasa    .... 

Maturation  in  Anasa     .... 

Diagrams  of  reduction 

Maturation  in  Thalassema    . 

Maturation  in  Thalassema  and  Zirphiza 

Maturation  in  Salamamira  . 

The  maturation-divisions  in  angiosperms 

Maturation  in  I. ilium  .... 

Maturation  in  Liliiim  .... 

Diagrams  of  reduction  in  the  flowering  plants 

Ovary  of  Canthocamptus       .... 

Polar  spindles  without  centrosomes 

Polar  bodies  in  Actinophrys 

Polar  bodies  in  Acti)wsphiErinm  . 

Conjugation  and  reduction  in  Closteriiim 

First  type  of  parthenogenetic  maturation  in  Arlemia 

Second  type  of  parthenogenetic  maturation  in  Arlemia 


PAGE 

252 
254 
255 
259 
260 

261 
262 
264 
266 
268 
270 

273 
276 

278 

278 

279 

282 

28 -. 


CHAPTER  VI 

143.  Abnormalities  in  the  fertilization  of  ^j<r^;-7,f 

144.  G'xzni  Qvnhxyo  oi  A scaris       ...... 

145.  Individuality  ofchromosomes  in  Ascuris 

146.  Independence  of  chromosomes  in  fertilization  of  Cyclops 

147.  I  lybrid  fertilization  of /^5frt'r;.y      ..... 

148.  Mitosis  with  intranuclear  centrosome  in  Ascaris  . 

149.  Abnormal  mitoses  in  Plemerocallis        .... 

150.  Centrosomes  in  Chctlopteriis  and  Cerehratiiliis 

151.  Artificially  produced  asters  and  centrosomes  in  echinoderms 

152.  Diagram  of  different  types  of  centrosome  and  centrosphere 

153.  P(j]ar  mitoses  in  Diaiilula    ....... 

154.  Astral  systems  in  Ufiio  ....... 

155.  Astral  systems  in  Cerebratiilus  and  Thalassema    . 

156.  Structure  of  the  aster  in  spermatogonium  of  salamander 


295 
296 
297 
298 
300 

305 
306 

307 
308 

310 

^12 


3^3 
320 

326 


CHAPTER   VII 

157.  History  of  chromosomes  in  the  germinal  vesicle  of  sharks 

158.  Nucleated  and  enucleated  fragments  oi  Slylonychia 

159.  Regeneration  in  Slenlor        ...... 

160.  Nucleated  and  enucleated  fragments  of . -////«'/'<?    . 

161.  Nucleated  and  enucleated  fragments  of  plant-protoplasm 

162.  Position  of  nuclei  in  plant-cells     . 

163.  Ovzxy  o{  Forjictila         .... 

164.  Normal  and  dwarf  larvse  of  sea-urchins 

165.  Supernumerary  centrosome  in  Ascaris 

166.  Cleavage  of  dispermic  egg  of  Toxopneiisles 

167.  Centrosomes  and  cilia  .... 


339 
342 
343 
344 
345 
347 
349 
352 

355 
356 
357 


LIST   OF  FIGURES 


XXI 


FIG. 

i68. 
169. 
170. 
171. 
172. 

173- 
174. 

175- 

176. 

177. 
178. 
179. 
180. 
iSi. 


CHAPTER   VIII 

Geometrical  relations  of  cleavage-planes  in  plants 

Cleavage  of  Synapta     .... 

Cleavage  of  Polygordiiis 

Cleavage  of  Nereis        .... 

Variations  in  the  third  cleavage    . 

Meroblastic  cleavage  in  the  squid 

Rudimentary  cells  in  Aricia 

Teloblasts  of  the  earthworm 

Contradiction  of  Hertwig's  rule  in  Ascaris 

Bilateral  cleavage  in  tunicates 

Bilateral  cleavage  in  Loligo  . 

Eggs  of  Loligo      ..... 

Eggs  and  embryos  of  Corixa 

Variations  in  axial  relations  of  Cyclops 


PAGE 

365 
367 
369 
370 
11^ 

■\m  -% 
J/  J 

374 
376 
380 

381 
382 

383 
385 


CHAPTER  IX 

182.  Half-embryos  of  the  frog      .... 

183.  Half  and  whole  cleavage  in  sea-urchins 

184.  Normal  and  dwarf  gastrulas  of  .-iw//z/^jr«5  . 

185.  Dwarf  and  double  embryos  of  ^;;/////£'a-?^5'    . 

186.  Cleavage  of  sea-urchin  eggs  under  pressure  . 

187.  Cleavage  of  A^'^rm-eggs  under  pressure 

188.  Diagrams  of  cleavage  in  mollusks  and  polyclades 

189.  Partial  larva;  of  ctenophores 

190.  Partial  cleavage  in  Ilyanassa 

191.  Double  embryos  of  frog        .... 

192.  Cleavage  in  Crepidula  .... 

193.  Normal  and  modified  larvre  of  sea-urchins    . 

194.  Regeneration  in  coelenterates 


400 
407 
40S 
409 
411 
412 

414 

418 
420 
421 
424 
428 
429 


INTRODUCTION 


-OO^iO^OO- 


"  Jedes    Thier  erscheint    ah    eine   Sunwie    vitaler  Einheiten,    von   denen    jede   den    vollen 
Charakter  des  Lebens  an  sich  tragt."  \'lRCHO\v.i 

During  the  half-century  that  has  elapsed  since  the  enunciation  of 
the  cell-theory  by  Schleiden  and  Schwann,  in  1838-39,  it  has  become 
ever  more  clearly  apparent  that  the  key  to  all  ultimate  biological 
problems  must,  in  the  last  analysis,  be  sought  in  the  cell.  It  was  the 
cell-theory  that  first  brought  the  structure  of  plants  and  animals  under 
one  point  of  view,  by  revealing  their  common  plan  of  organization. 
It  was  through  the  cell-theory  that  Kolliker,  Remak,  Nageli,  and  Hof- 
meister  opened  the  way  to  an  understanding  of  the  nature  of  embryo- 
logical  development,  and  the  law  of  genetic  continuity  lying  at  the 
basis  of  inheritance.  It  was  the  cell-theory  again  which,  in  the  hands 
of  Goodsir,  Virchow,  and  Max  Schultze,  inaugurated  a  new  era  in  the 
history  of  physiology  and  pathology,  by  showing  that  all  the  various 
functions  of  the  body,  in  health  and  in  disease,  are  but  the  outward 
expression  of  cell-activities.  And  at  a  still  later  day  it  was  through 
the  cell-theory  that  Hertwig,  Fol,  Van  Beneden,  and  Strasburgcr 
solved  the  long-standing  riddle  of  the  fertilization  of  the  ^gg  and  the 
mechanism  of  hereditary  transmission.  No  other  biological  generali- 
zation, save  only  the  theory  of  organic  evolution,  has  brought  so  many 
apparently  diverse  phenomena  under  a  common  point  of  view  or  has 
accomplished  more  for  the  unification  of  knowledge.  The  cell-theory 
must  therefore  be  placed  beside  the  evolution-theory  as  one  of  the 
foundation  stones  of  modern  biology. 

And  yet  the  historian  of  latter-day  biology  cannot  fail  to  be  struck 
with  the  fact  that  these  two  great  generalizations,  nearly  related  as 
they  are,  have  been  developed  along  widely  different  lines  of  research, 
and  have  only  within  a  very  recent  period  met  upon  a  common  ground. 
The  theory  of  evolution  originally  grew  out  of  the  study  of  natural 
history,  and  it  took  definite  shape  long  before  the  ultimate  structure 
of  living  bodies  was  in  any  degree  comprehended.  The  evolutionists 
of  the  Lamarckian  period  gave  little  heed  to  the  finer  details  of 
internal  organization.     They  were  concerned  mainly  with  the  more 

^  Cellularpathologie,  p.  12,  1S5S. 


B  I 


2  IXTRODUCTION 

obvious  characters  of  plants  and  animals  —  their  forms,  colours, 
habits,  distribution,  their  anatom\'  and  embryonic  development  — 
and  with  the  systems  of  classification  based  upon  such  characters ; 
and  long  afterward  it  was,  in  the  main,  the  study  of  like  characters 
with  reference  to  their  historical  origin  that  led  Darwin  to  his  splen- 
did triumphs.  The  study  of  microscopical  anatomy,  on  which  the 
cell-theory  was  based,  lay  in  a  different  field.  It  was  begun  and  long 
carried  forward  with  no  thought  of  its  bearing  on  the  origin  of  living 
forms  ;  and  even  at  the  present  day  the  fundamental  problems  of 
organization,  with  which  the  cell-theory  deals,  are  far  less  accessible 
to  historical  inquiry  than  those  suggested  by  the  more  obvious 
external  characters  of  ])lants  and  animals.  Only  within  a  few  years, 
indeed,  has  the  ground  been  cleared  for  that  close  alliance  between 
students  of  organic  evolution  and  students  of  the  cell,  which  forms  so 
striking  a  feature  of  latter-day  biology  and  is  exerting  so  great  an  influ- 
ence on  the  direction  of  research.  It  has,  therefore,  only  recently 
become  possible  adequately  to  formulate  the  great  problems  of  devel- 
opment and  heredity  in  the  terms  of  cellular  biology  —  indeed,  we  can 
as  vet  do  little  more  than  so  formulate  them.  Yet  the  fact  that  these 
two  great  lines  of  research,  both  concerned  with  the  deeper  problems 
of  life,  yet  having  their  beginnings  so  far  apart,  have  at  length 
converged  to  a  meeting-point,  is  one  of  the  more  striking  evidences  of 
progress  that  modern  biology  has  to  show ;  and  it  sufficiently  justifies 
an  attempt  to  treat  the  cell  from  the  standpoint  of  the  general  student 
of  development. 

Let  us  at  the  outset  briefly  outline  the  cell-theory  as  thus  regarded, 
and  indicate  the  manner  of  its  historical  connection  with  the  general 
problems  of  evolution.^ 

^  Schleiden  and  Schwann  are  universally  and  justly  recognized  as  the  founders  of  the  cell- 
theory;  hut  like  every  other  great  generalization  the  theory  was  based  on  a  long  series  of 
earlier  investigations  l)eginniiig  with  the  memorable  microscopical  researches  of  l.eeuwen- 
hoek,  Mali)ighi,  lIo(>ke,  and  (jrew  in  the  second  lialf  of  the  seventeenth  century. 

Wolff,  in  the  Theoria  Cenerationis  (1759),  clearly  recognized  the  "spheres"  and  "vesi- 
cles" composing  the  embryonic  parts  both  of  animals  and  of  plants,  though  without  grasping 
iheir  real  nature  or  mode  of  origin,  and  his  conclusions  were  developed  by  the  botanist 
Mirbel  at  the  beginning  of  the  i^resent  century.  Nearly  at  the  same  time  (1805)  Oken  fore- 
shadowed the  cell-theory  in  the  form  that  it  assumed  with  Schleiden  and  Schwann;  but  his 
conception  of  "  Urschleim  "  and  "  Hlaschen  "  can  hardly  be  regarded  as  more  than  a  lucky 
guess.  A  still  closer  approximation  to  the  truth  is  fuuntl  in  the  works  of  'ruri)in  (1826), 
Meyen  (1830),  Raspail  (1831),  and  Dutrochet  (1837);  '^"^^  these,  like  others  of  the  same 
period,  only  paved  the  way  for  the  real  founders  of  the  cell-theory.  Among  other  immedi- 
ate predecessors  f)r  contemporaries  of  Schleiden  and  Schwann  should  be  especially  mentioned 
Robert  Brown,  Dujardin,  Johannes  Miiller,  I'urkinje,  Hugo  von  Mohl,  Valentin,  Unger, 
Nageli,  and  Henle.  The  significance  of  Schleiden's,  and  especially  of  Schwann's,  work  lies 
in  the  thorough  and  comprehensive  way  in  which  the  problem  was  studied,  the  philosophic 
breadth  with  which  the  conclusions  were  developed,  and  the  far-reaching  influence  which 
they  exercised  upon  subsequent  research.  In  this  respect  it  is  hardly  too  much  to  com- 
pare the  Mikroikopische  L'ntersiichnngcn  with  the  Origin  of  Species. 


INTR  OD  UC  TION  3 

During  the  past  thirty  years  the  theory  of  organic  descent  has 
been  shown,  by  an  overwhelming  mass  of  evidence,  to  be  the  only 
tenable  conception  of  the  origin  of  diverse  living  forms,  however  we 
mav  conceive  the  causes  of  the  process.  While  the  study  of  general 
zoology  and  botany  has  systematically  set  forth  the  results,  and  in  a 
measure  the  method,  of  organic  evolution,  the  study  of  microscopical 

a 


X 

Fig  i._  A  portion  of  the  epidermis  of  a  larval  salamander  {Amblystoma)  as  seen  in  slightly 
oblique  horizontal  section,  enlarged  550  diameters.  Most  of  the  cells  are  polygonal  m  form,  con- 
tain large  nuclei,  and  are  connected  by  delicate  protoplasmic  bridges.  Above  v  is  a  branched, 
dark  pigment-cell  that  has  crept  up  from  the  deeper  layers  and  lies  between  the  epidermal  ceLs. 
Three  of  the  latter  are  undergoing  division,  the  earliest  stage  {sp,rcme)  at  a,  ^J  l-';»«^r/»-'f  ("^"^"^ 
figure  in  the  anaphase)  at  b,  showing  the  chromosomes,  and  a  final  stage  {telophase),  showing 
fission  of  the  cell-body,  to  the  right. 

anatomy  has  shown  us  the  nature  of  the  material  on  which  it  has 
operated,  demonstrating  that  the  obvious  characters  ot  plants  and 
animals  are  but  varving  expressions  of  a  subtle  interior  organization 
common  to  all.  In  its  broader  outlines  the  nature  of  this  organiza- 
tion is  now  accurately  determined;  and  the  ''cell-theory,"  by  which 
it  is  formulated,  is,  therefore,  no  longer  of  an  inferential  or  hypo- 


^  INTRODUCriOy 

thetical  character,  but  a  generalized  statement  of  observed  fact  which 
may  be  outlined  as  follows  :  —  * 

In  all  the  higher  forms  of  life,  whether  plants  or  animals,  the 
body  may  be  resolved  into  a  vast  host  of  minute  structural  units 
known  as  cells,  out  of  which,  directly  or  indirectly,  every  part  is 
built  (Figs.  1,2).  The  substance  of  the  skin,  of  the  brain,  of  the  blood, 
of  the  bones  or  muscles  or  any  other  tissue,  is  not  homogeneous,  as  it 
appears  to  the  unaided  eye,  but  is  shown  by  the  microscope  to  be  an 
aercrretrate  composed  of  innumerable  minute  bodies,  as  if  it  were  a 


Fig.  2.  —  General  view  of  cells  in  the  growing  root-tip  of  the  onion,  from  a  longitudinal  section, 
enlarged  800  diameters. 

a.  non-dividing  cells,  with  chromatin-network  and  deeply  stained  nucleoli ;  b.  nuclei  preparing 
for  division  (spireme-stage)  ;  <r.  dividing  cells  showing  mitotic  figures;  e.  pair  of  daughter-cells 
shortly  after  division. 


colony  or  congeries  of  organisms  more  elementary  than  itself.  The 
name  cells  given  to  these  bodies  by  the  early  botanists,  and  ulti- 
mately adopted  by  nearly  all  students  of  microscopical  anatomy, 
was  not  happily  chosen  ;  for  modern  studies  have  shown  that  although 
the  cell  may  assume  the  form  of  a  hollow  chamber,  as  the  name 
indicates,  this  is  not  one  of  its  characteristic  or  even  usual  features. 
Essentially  the  cell  is  a  minute  mass  of  protoplasm,  a  substance  long 
since  identified  by  Cohn,  Leydig,  Max  Schultze,  and  De  Bary  as  the 
essential  active  basis  of  the  organism,  afterward  happily  characterized 


INTRODUCTION 


5 


by  Huxley  as  the  *'  physical  basis  of  life,"  and  at  the  present  time 
universally  recognized  as  the  immediate  substratum  of  all  vital 
activity.^  Endlessly  diversified  in  the  details  of  their  form  and  struc- 
ture, these  protoplasmic  masses  nevertheless  possess  a  characteristic 
type  of  organization  common  to  them  all;  hence,  in  a  certain  sense, 
they  may  be  regarded  as  elementary  organic  units  out  of  which  the 
body  is  compounded.    This  composite  structure  is,  however,  character- 


3     ''.>;; 


Fig.  3.  —  Amcela  Proteus,  an  animal  consisting  of  a  single  naked  cell,  x  280.    (From  Sedgwick 
and  Wilson's  Biology.) 

n.  The  nucleus;  iv.v.  water-vacuoles ;  c.v.  contractile  vacuole ;  f.v.  food-vacuole, 

istic  of  only  the  higher  forms  of  life.  Among  the  lowest  forms  at  the 
base  of  the  series  are  an  immense  number  of  microscopic  plants  and 
animals,  famiUar  examples  of  which  are  the  bacteria,  diatoms,  rhizo- 
pods,  and  Infusoria,  in  which  the  entire  body  consists  of  a  single  cell 
(Fig.  3),  of  the  same  general  type  as  those  which  in  the  higher  multi- 
cellular forms  are  associated  to  form  one  organic  whole.  Structurally, 
therefore,  the  multicellular  body  is  in  a  certain  sense  comparable  with 
a  colony  or  aggregation  of  the  lower  one-celled  forms.-    This  com- 

1  The  word  protoplasm  is  due  to  Purkinje  (1840),  who  applied  it  to  the  formative  sub- 
stance of  the  animal  embryo  and  compared  it  with  the  granular  material  of  vegetable 
"cambium."  It  was  afterward  independently  used  by  \\.  von  Mohl  (1846)  to  designate 
the  contents  of  the  plant-cell.  The  full  physiological  signiticance  of  protoplasm,  its  identity 
with  the  "sarcode"  (Dujardin)  of  the  unicellular  forms,  and  its  essential  similarity  in 
plants  and  animals,  was  first  clearly  placed  in  evidence  through  the  classical  works  of  Max 
Schultze  and  De  Bary,  beside  which  should  be  placed  the  earlier  works  of  Dujardin,  L  nger, 
Nageli,  and  Mohl,  and  that  of  Cohn,  Huxley,  Virchow,  Leydig,  Brucke,  Kuhne,  and  Beale. 

2  This  comparison  must  be  taken  with  some  reservation,  as  will  appear  beyond. 


6  IXTRODUCTIOX 

parison  is  not  less  suggestive  to  the  physiologist  than  to  the  mor- 
phologist.  In  the  one-celled  forms  all  of  the  vital  functions  are 
performed  by  a  single  cell.  In  the  multicellular  forms,  on  the  other 
hand,  these  functions  are  not  ecjualh-  i)erformed  b)-  all  the  cells,  but 
are  in  varving  degree  distributed  among  them,  the  cells  thus  falling 
into  physiological  groups  or  tissues,  each  ot  which  is  especially  de- 
voted to  the  performance  of  a  specific  function.  Thus  arises  the 
"physiological  division  of  labour"  through  which  alone  the  highest 
development  of  vital  activity  becomes  possible  ;  and  thus  the  cell 
becomes  a  unit,  not  merely  of  structure,  but  also  of  function.  luich 
bodilv  function,  and  even  the  life  of  the  organism  as  a  whole,  may 
thus  in  one  sense  be  regarded  as  a  resultant  arising  through  the  inte- 
gration of  a  vast  number  of  cell-activities  ;  and  it  cannot  be  adequately 
investigated  without  the  study  of  the  individual  cell-activities  that  lie 
at  its  root.^ 

The  foregoing  conceptions,  founded  by  Schwann,  and  skilfully 
developed  by  Kolliker,  Siebold,  Virchow,  and  Haeckel,  gave  an  im- 
pulse to  anatomical  and  physiological  investigation  the  force  of  which 
could  hardly  be  overestimated;  yet  they  did  not  for  many  years 
measurably  affect  the  more  speculative  side  of  biological  inquiry. 
The  Origin  of  Species,  published  in  1859,  scarcely  mentions  it;  nor, 
with  the  important  exception  of  the  theory  of  pangenesis,  did  Darwin 
attempt  at  any  later  period,  to  bring  it  into  any  very  definite  relation 
to  his  views.  The  initial  impulse  to  the  investigations  that  brought 
the  cell-theory  into  definite  contact  with  the  evolution-theory  was 
given  nearly  twenty  years  after  the  Origin  of  Species,  by  researches 
on  the  early  history  of  the  germ-cells  and  the  fertilization  of  the 
ovum.  Begun  in  1873-74  by  Auerbach,  Fol,  and  Butschli,  and 
eagerly  followed  up  b\-  Oscar  Hertwig,  Van  Beneden,  Strasburger, 
and  a  host  of  later  workers,  these  investigations  raised  wholly  new 
questions  regarding  the  mechanism  of  development  and  the  role  of 
the  cell  in  hereditary  transmission.  Through  them  it  became  for  the 
first  time  clearly  apparent  that  the  general  problems  of  embryology, 
heredity,  and  evolution  are  indissolubly  bound  up  with  those  of  cell- 
structure,  and  can  only  be  fully  apprehended  in  the  light  of  cytologi- 
cal  research.  As  the  most  significant  step  in  this  direction,  we  may 
re£:ard  the  identification  of  the  cell-nucleus  as  the  vehicle  of  inheri- 

1  Cf.  pp.  58-61.  "  It  is  to  the  cell  that  the  study  of  every  bodily  function  sooner  or  later 
drives  us.  In  the  muscle-cell  lies  the  problem  of  the  heart-beat  and  that  of  muscular  con- 
traction ;  in  the  gland-cell  reside  the  causes  of  secretion  ;  in  the  epithelial  cell,  in  the 
white  blood-cell,  lies  the  problem  of  the  absorption  of  food,  and  the  secrets  of  the  mind  are 
hidden  in  the  ganglion-cell.  ...  If  then  physiology  is  not  to  rest  content  with  the 
mere  extension  of  our  knowledge  regarding  the  gross  activities  of  the  human  body,  if  it 
would  seek  a  real  explanation  of  the  fundamental  phenomena  of  life,  it  can  only  attain  its 
end  through  the  study  of  cell-physiology''''  (Verworn,  Alkemeine  Fhysiologie,  p.  53,  1895). 


INTRODUCTION  y 

tance,  made  independently  and  almost  simultaneously  in  18S4-85  by 
-Oscar  Hertwig,  Strasburger,  Kolliker,  and  Wcismann/  while  nearly 
at  the  same  time  (1883)  the  splendid  researches  of  Van  Beneden  on 
the  early  history  of  the  animal  Qgg  opened  possibilities  of  research 
into  the  finer  details  of  cell-phenomena  of  which  the  early  workers 
could  hardly  have  dreamed. 

We  can  only  appreciate  the  full  historical  significance  of  the  new 
period  thus  inaugurated  by  a  glance  at  the  earlier  history  of  opinion 
regarding  embryological  development  and  inheritance.  To  the  modern 
student  the  germ  is,  in  Huxley's  words,  simply  a  detached  living  por- 
tion of  the  substance  of  a  preexisting  living  body  ^  carrying  with  it  a 
definite  structural  organization  characteristic  of  the  species.  By  the 
earlier  embryologists,  however,  the  matter  was  very  differently  re- 
garded ;  for  their  views  in  regard  to  inheritance  were  vitiated  by  their 
acceptance  of  the  Greek  doctrine  of  the  equivocal  or  spontaneous 
generation  of  life ;  and  even  Harvey  did  not  escape  this  pitfall,  near 
as  he  came  to  the  modern  point  of  view.  "  The  Qgg,''  he  savs,  "  is 
the  mid-passage  or  transition  stage  between  parents  and  offspring, 
between  those  who  are,  or  were,  and  those  who  are  about  to  be  ; 
it  is  the  hinge  or  pivot  upon  which  the  whole  generation  of  the 
bird  revolves.  The  Qgg  is  the  terminus  from  which  all  fowls,  male 
and  female,  have  sprung,  and  to  which  all  their  lives  tend  —  it  is  the 
result  which  nature  has  proposed  to  herself  in  their  being.  And 
thus  it  comes  that  individuals  in  procreating  their  like  for  the  sake 
of  their  species,  endure  forever.  The  egg,  I  say,  is  a  period  or  por- 
tion of  this  eternity."  ^ 

This  passage  appears  at  first  sight  to  be  a  close  approximation  to 
the  modern  doctrine  of  s^erminal  continuitv  about  which  all  theories 
of  heredity  are  revolving.  In  point  of  fact,  however,  Harvey's 
view  is  only  superficially  similar  to  this  doctrine  ;  for,  as  Huxley 
pointed  out,  it  was  obscured  by  his  belief  that  the  germ  might  arise 
*' spontaneously,"  or  through  the  influence  of  a  mysterious  ''  calidiun 
innaUmi,''  out  of  not-living  matter."*  Neither  could  Harvey,  great 
physiologist  and  embryologist  as  he  w^as,  have  had  any  adequate  con- 
ception of  the  real  nature  of  the  ^gg  and  its  morphological  relation  to 

1  It  must  not  be  forgotten  that  Haeckel  expressed  the  same  view  in  1866 — only,  how- 
ever, as  a  speculation,  since  the  data  necessary  to  an  inductive  conclusion  were  not  obtained 
until  long  afterward.  "The  internal  nucleus  provides  for  the  transmission  of  hereditary 
characters,  the  external  plasma  on  the  other  hand  for  accommodation  i)«  adaptation  to  the 
external  world"  {Gen.  MorpJi.,  pp.  2S7-289). 

2  Evolution  in  Biology,  1878;    Science  and  Culture,  p.  291. 
^  De  Generatione,  1651;   Trans.,  p.  271. 

^  Whitman,  too,  in  a  brilliant  essay,  has  shown  how  far  Harvey  was  from  any  real  grasp 
of  the  law  of  cenetic  continuitv.  which  is  well  characterized  as  the  central  fact  of  modern 
biology.     Evolution  and  Epigenesis,  Wood's  HoU  Biological  Lectures,  1894. 


8  JNTR  OD  UC  TION 

the  body  of  which  it  forms  a  part,  since  the  cclkilar  structure  of  Uving 
things  was  not  comprehended  until  nearly  two  centuries  later,  the 
spermatozoon  was  still  undiscovered,  and  the  nature  of  fertilization 
was  a  subject  of  fantastic  and  baseless  speculation.  For  a  hundred 
years  after  Harvey's  time  embryologists  sought  in  vain  t(^  penetrate 
the  mysteries  enveloping  the  beginning  of  the  individual  life,  and 
despite  their  failure  the  controversial  writings  of  this  period  form  one 
of  the  most  interesting  chapters  in  the  history  of  biology.  By  the 
extreme  "  evolutionists  "  or  "  prceformationists  "  the  egg  was  believed 
to  contain  an  embryo  fully  formed  in  miniature,  as  the  bud  contains 
the  flower  or  the  chrysalis  the  butterfly.  Development  was  to  them 
merely  the  unfolding  of  that  which  already  existed  ;  inheritance,  the 
handing  down  from  parent  to  child  of  an  infinitesimal  re])roduction 
of  its  own  body.  It  was  the  service  of  Bonnet  to  push  this  concep- 
tion to  its  logical  consequence,  the  theory  of  eiJiboitciiicjit  or  encase- 
ment, and  thus  to  demonstrate  the  absurdity  of  its  grosser  forms, 
pointing  out  that  if  the  egg  contains  a  complete  embryo,  this  must 
itself  contain  eggs  for  the  next  generation,  these  other  eggs  in  their 
turn,  and  so  ad  infinitum,  like  an  infinite  series  of  boxes,  one  within 
another  —  hence  the  term  cniboitemcnt.  Bonnet  himself  renounced 
this  doctrine  in  his  later  writings,  and  Caspar  Friedrich  Wolff  ( 1759) 
led  the  way  in  a  return  to  the  teachings  of  Harvey,  showing  by  pre- 
cise actual  observation  that  the  egg  does  not  at  first  contain  any 
formed  embryo  whatever ;  that  its  structure  is  wholly  different 
from  that  of  the  adult;  that  development  is  not  a  mere  process 
of  unfolding,  but  involves  the  continual  formation,  one  after  an- 
other, of  new  parts,  previously  non-existent  as  such.  This  is  some- 
what as  Harvey,  himself  following  Aristotle,  had  conceived  it  — 
a  process  of  cpigcncsis  as  opposed  to  evolution.  Later  researches 
established  this  conclusion  as  the  very  foundation  of  embryological 
science. 

But  although  the  external  nature  of  development  was  thus  deter- 
mined, the  actual  structure  of  the  egg  and  the  mechanism  of  inheri- 
tance remained  for  nearly  a  century  in  the  dark.  It  was  reserved 
for  Schwann  (1839)  and  his  immediate  followers  to  recognize  the 
fact,  conclusively  demonstrated  by  all  later  researches,  that  tJic  egg 
is  a  cell  having  the  same  essential  structure  as  other  cells  of  the 
body.  And  thus  the  wonderful  truth  became  manifest  that  a  single 
cell  may  contain  within  its  microscopic  compass  the  sum-total  of 
the  heritage  of  the  species.  This  conclusion  first  reached  in  the 
case  of  the  female  sex  was  soon  afterward  extended  to  the  male 
as  well.  Since  the  time  of  Leeuwenhoek  (1677)  it  had  been  known 
that  the  sperm  or  fertilizing  fluid  contained  innumerable  minute 
bodies  endowed  in  nearly  all  cases  with  the  power  of  active  move- 


INTRODUCTION 


ment,  and  therefore  regarded  by  the  early  observers  as  parasitic 
animalcules  or  infusoria,  a  view  which  gave  rise  to  the  name  sperma- 
tozoa (sperm-animals)  by  which  they  are  still  generally  known. ^  As 
long  ago  as  1786,  however,  it  was  shown  by  Spallanzani  that  the 
fertilizing  power  must  lie  in  the  spermatozoa,  not  in  the  liquid  in 
which  they  swim,  because  the  spermatic  fluid  loses  its  power  when 
filtered.  Two  years  after  the  appearance  of  Schwann's  epoch-mak- 
ino-  work  Kolliker  demonstrated  (1841)  that  the  spermatozoa  arise 
directly  from  cells  in  the  testis,  and  hence  cannot  be  regarded  as 
parasites,  but  are,  like  the  ovum,  derived  from  the  parent-body.  Not 
until  1865,  however,  was  the  final  proof  attained  by  Schweigger- 
Seidel  and  La  Valette  St.  George  that  the  spermatozoon  contains 
not  only  a  nucleus,  as  Kolliker  believed,  but  also  cytoplasm.  It 
was  thus  shown  to  be,  like  the  ^^,g,  a  single  cell,  peculiarly  modified 
in  structure,  it  is  true,  and  of  extraordinary  minuteness,  yet  on  the 
whole  morphologically  equivalent  to  other  cells.  A  final  step  was 
taken  ten  years  later  (1875),  when  Oscar  Hertwig  established  the 
all-important  fact  that  fertilization  of  the  egg  is  accomplished  by 
its  union  with  one  spermatozoon,  and  one  only.  In  sexual  repro- 
duction, therefore,  each  sex  contributes  a  single  cell  of  its  own  body 
to  the  formation  of  the  offspring,  a  fact  which  beautifully  tallies 
with  the  conclusion  of  Darwin  and  Galton  that  the  sexes  play,  on 
the  whole,  equal,  though  not  identical  parts  in  hereditary  trans- 
mission. The  ultimate  problems  of  sex,  fertilization,  inheritance, 
and  development  were  thus  shown  to  be  cell-problems. 

Meanwhile,  during  the  years  immediately  following  the  announce- 
ment of  the  cell-theory,  the  attention  of  investigators  was  especially 
focussed  upon  the  question  :  How  do  the  cells  of  the  body  arise  .? 
The  origin  of  cells  by  the  division  of  preexisting  cells  was  clearly 
recognized  by  Hugo  von  Mohl  in  1835,  though  the  full  significance 
of  this  epoch-making  discovery  was  so  obscured  by  the  errrors  of 
Schleiden  and  Schwann  that  its  full  significance  was  only  perceived 
long  afterward.  The  founders  of  the  cell-theory  were  unfortunately 
led'to  the  conclusion  that  cells  might  arise  in  two  different  ways,  viz. 
either  by  division  or  fission  of  a  preexisting  mother-cell,  or  by  "Iree 
cell-formation,"  new  cells  arising  in  the  latter  case  not  from  pre- 
existing ones,  but  by  crystallizing,  as  it  were,  out  of  a  formative  or 
nutritive  substance,  termed  the  "  cytoblastema "  ;  and  they  even 
beheved  the  latter  process  to  be  the  usual  and  typical  one.  It 
was  only  after  many  years  of  painstaking  research  that  "  free  cell- 
formation  "  was    absolutely  proved  to  be  a  myth,  though  many  of 

iThe  discovery  of  the  spermatozoa  is  generally  accredited  to  Ludwig  Hamm.  a  pupil 
of  Leeuwenhoek  (1677).  though  Ilartsoeker  afterward  claimed  the  ment  of  havmg  seen 
them  as  early  as  1674  (Dr.  Allen  Thomson). 


10  IXTRODUC  TION 

Schwann's  immediate  followers  threw  doubts  upon  it,^  and  as  early 
as  1855  Virchow  positively  maintained  the  universality  of  cell-divi- 
sion, contending  that  ever}-  cell  is  the  offs})ring  of  a  preexisting 
parent-cell,  and  summing  up  in  the  since  famous  aphorism,  "  oniuis 
celliila  c  Cillula.^''^  At  the  ]:)resent  day  this  conclusion  rests  upon  a 
foundation  so  firm  that  we  arc  justified  in  regarding  it  as  a  universal 
law  of  development. 

Now,  if  the  cells  of  the  body  always  arise  by  the  division  of  pre- 
existing cells,  all  must  be  traceable  back  to  the  fertilized  egg-cell  as 
their  common  ancestor.  Such  is,  in  fact,  the  case  in  every  plant  and 
animal  whose  development  is  accurately  known.  The  first  step  in 
development  consists  in  the  division  of  the  <:.^^  into  two  j^arts,  each 
of  which  is  a  cell,  like  the  <t^^^  itself.  The  two  then  divide  in  turn  to 
form  four,  eight,  sixteen,  and  so  on  in  more  or  less  regular  progres- 
sion (Fig.  4.)  until  step  by  step  the  f^gg  has  split  up  into  the  multitude 
of  cells  which  build  the  body  of  the  embryo,  and  finally  of  the  adult. 
This  process,  known  as  the  cleavage  or  segmentatioji  of  the  ^^^^ 
was  observed  long  before  its  meaning  was  understood.  It  seems  to 
have  been  first  definitely  described  in  the  case  of  the  frog's  ^^,^,,  by 
Prevost  and  Dumas  ( 1824),  though  earlier  observers  had  seen  it;  but 
at  this  time  neither  the  ^g'g  nor  its  descendants  were  known  to  be 
cells,  and  its  true  meaning  was  first  clearly  perceived  by  Bergmann, 
Kolliker,  Reichert,  Von  Baer,  and  Remak,  some  twenty  years  later. 
The  interpretation  of  cleavage  as  a  process  of  cell-division  was  fol- 
lowed by  the  demonstration  that  cell-division  does  not  begin  with 
cleavage,  but  can  be  traced  back  into  the  foregoing  goieration  ;  for  the 
egg-cell,  as  well  as  the  sperm-cell,  arises  by  the  division  of  a  cell  pre- 
existing in  the  parent-body.  //  is  therefore  derived  by  direct  descent 
from  an  egg-cell  of  the  foregoing  generation,  and  so  on  ad  infinitnni. 
Embryologists  thus  arrived  at  the  conception  so  vividly  set  forth  by 
Virchow  in  1858-'^  of  an  uninterrupted  series  of  cell-divisions  extend- 
ing backward  from  existing  plants  and  animals  to  that  remote  and 
unknown  period  when  vital  organization  assumed  its  present  form. 
Life  is  a  continuous  stream.  The  death  of  the  individual  involves  no 
breach  of  continuitv  in  the  series  of  cell-divisions  bv  which  the  life 
of  the  race  flows  onwards.  The  individual  body  dies,  it  is  true,  but 
the  germ-cells  live  on,  carrying  with  them,  as  it  were,  the  traditions 
of  the  race  from  which  they  have  sprung,  and  handing  them  on  to 
their  descendants. 

1  Among  these  may  be  especially  mentiDncd  Mohl,  L'ngcr,  Nageli,  Martin  liarry,  Goodsir, 
and  Remak. 

2  Arch,  fur  Path.  Anat.,  VIII..  p.  23,  1851;. 

3  See  the  quotation  from  the  original  edition  of  the   Celhdarpathologie   at  the  head   of 
Chapter  II.,  p.  63. 


INTRODUCTION' 


II 


We  have  thus  arrived  at  the  form  in  which  the  problems  of  heredity 
and  development  confront  the  investigator  of  the  present  day.  It 
remains  to  point  out  more  clearly  how  they  are  related  to  the  general 
problems  of  evolution  and  to  those  post-Darwinian  discussions  in 
which  Weismann  has  taken  so  active  a  part.     All  theories  of  evolu- 


B 


C 


D 


F 


G  ^  ^ 

Fig  4.  — Cleavage  of  the  ovum  of  the  sea-urchin  Toxopncustes,  X  33°.  ^''oni  ''f^^-  ^''^^  suc- 
cessive divisions  up  to  the  i6-cell  stage  (//)  occupy  about  two  hours.  /  is  a  section  of  the  embryo 
(blastula)  of  three  hours,  consisting  of  approximately  128  cells  surrounding  a  central  cavity  or 
blastocoel. 

tion  take  the  facts  of  variation  and  heredity  as  fundamental  postulates, 
for  it  is  by  variation  that  new  characters  arise  and  by  heredity  that 
they  are  perpetuated.  Darwin  recognized  two  kinds  of  variation, 
both  of  which,  being  inherited  and  maintained  through  the  conserving 
action  of  natural  selection,  might  give  rise  to  a  permanent  transfor- 
mation of  species.     The  first  of  these  includes  congenital  or  mborn 


12  INTR  OD  UCTIOJV 

variations,  i.e.  such  as  appear  at  birth  or  are  developed  "spontane- 
ously," without  discoverable  connection  with  the  activities  of  the 
organism  itself  or  the  direct  effect  of  the  environment  upon  it,  though 
Darwin  clearly  recognized  the  fact  that  even  such  variations  must 
indirectly  be  due  to  changed  conditions  acting  upon  the  parental 
organism  or  on  the  germ.  In  a  second  class  of  variations  were 
placed  the  so-called  acquired  characters,  i.e.  definite  effects  directly 
produced  in  the  course  of  the  individual  life  as  the  result  of  use  and 
disuse,  or  of  food,  climate,  and  the  like.  The  inheritance  of  congen- 
ital characters  is  now  universally  admitted,  but  it  is  otherwise  with 
acquired  characters.  The  inheritance  of  the  latter,  now  the  most 
debated  question  of  biology,  had  been  taken  for  granted  by  Lamarck 
a  half-century  before  Darwin  ;  but  he  made  no  attempt  to  show  how 
such  transmission  is  possible.  Darwin,  on  the  other  hand,  squarely 
faced  the  physiological  requirements  of  the  problem,  recognizing  that 
the  transmission  of  acquired  characters  can  only  be  possible  under  the 
assumption  that  the  germ-cell  definitely  reacts  to  all  other  cells  of  the 
body  in  such  wise  as  to  register  the  changes  taking  place  in  them.  In 
his  ingenious  and  carefully  elaborated  theory  of  pangenesis,^  Darwin 
framed  a  provisional  physiological  hypothesis  of  inheritance  in  ac- 
cordance with  this  assumption,  suggesting  that  the  germ-cells  are 
reservoirs  of  minute  germs  or  gemmules  derived  from  every  part  of 
the  body  ;  and  on  this  basis  he  endeavoured  to  explain  the  trans- 
mission both  of  acquired  and  of  congenital  variations,  reviewing  the 
facts  of  variation  and  inheritance  with  wonderful  skill,  and  buildinc: 
up  a  theory  which,  although  it  forms  the  most  speculative  and  hypo- 
thetical portion  of  his  writings,  must  always  be  reckoned  one  of  his 
most  interesting  contributions  to  science. 

In  the  form  advocated  by  Darwin  the  theory  of  pangenesis  has 
been  generally  abandoned  in  spite  of  the  ingenious  attempt  to  remodel 
it  made  by  Brooks  in  1883.-  In  the  same  year  the  whole  aspect  of 
the  problem  was  changed,  and  a  new'period  of  discussion  inaugurated 
by  Weismann,  who  put  forth  a  bold  challenge  of  the  entire  Lamarckian 
principle.'^  "  I  do  not  propose  to  treat  of  the  whole  problem  of  hered- 
ity, but  only  of  a  certain  aspect  of  it,  —  the  transmission  of  acquired 
characters,  which  has  been  hitherto  assumed  to  occur.  In  taking  this 
course  I  may  say  that  it  was  impossible  to  avoid  going  back  to  the 
foundation  of  all  phenomena  of  heredity,  and  to  determine  the  sub- 
stance with  which  they  must  be  connected.  In  my  opinion  this  can 
only  be  the  substance  of  the  germ-cells  ;    and  this  substance  trans- 

1  Variation  of  Animals  and  Plants,  Chapter  XXVII. 

2  The  Law  of  Heredity,  Baltimore,  1883. 

3  Ueber  Vererbtmg,  1883.  See  Essays  upon  Heredity,  I.,  by  A.  Weismann,  Clarendon 
Press,  Oxford,  1889. 


INTRODUCTION 


fers  its  hereditary  tendencies  from  generation  to  generation,  at  first 
unchanged,  and  always  uninfluenced  in  any  corresponding  manner, 
by  that  which  happens  during  the  life  of  the  individual  which  bears 
it.  If  these  views  be  correct,  all  our  ideas  upon  the  transformation 
of  species  require  thorough  modification,  for  the  whole  princij^le  of 
evolution  by  means  of  exercise  (use  and  disuse)  as  professed  by  La- 
marck, and  accepted  in  some  cases  by  Darwin,  entirely  collapses" 
{I.e.,  p.  69). 

It  is  impossible,  he  continues,  that  acquired  traits  should  be  trans- 
mitted, for  it  is  inconceivable  that  definite  changes  in  the  body,  or 
"soma,"  should  so  affect  the  protoplasm  of  the  germ-cells  as  to  cause 
corresponding  changes  to  appear  in  the  offspring.  How,  he  asks, 
can  the  increased  dexterity  and  power  in  the  hand  of  a  trained  piano- 
player  so  affect  the  molecular  structure  of  the  germ-cells  as  to  pro- 
duce a  corresponding  development  in  the  hand  of  the  child  'i  It  is 
a  physiological  impossibility.  If  we  turn  to  the  facts,  we  find,  W'eis- 
mann  affirms,  that  not  one  of  the  asserted  cases  of  transmission  of 
acquired  characters  will  stand  the  test  of  rigid  scientific  scrutiny.  It 
is  a  reversal  of  the  true  point  of  view  to  regard  inheritance  as  taking 
place  from  the  body  of  the  parent  to  that  of  the  child.  The  child 
inherits  from  the  parent  germ-cell,  not  from  the  parent-body,  and  the 
germ-cell  owes  its  characteristics  not  to  the  body  which  bears  it,  but 
to  its  descent  from  a  preexisting  germ-cell  of  the  same  kind.  Thus 
the  body  is,  as  it  were,  an  offshoot  from  the  germ-cell  (Fig.  5).     As 


Line  of  succession. 


\£)  Line  of  inheritance. 

G 

Fig.  5.  —  Diagram  illustrating  Weismann's  theory  of  inheritance. 

G.   The  germ-cell,  which  by  division  gives  rise  to  the  body  or  soma  (5)  and  to  new  germ-cells 
(G)  which  separate  from  the  soma  and  repeat  the  process  in  each  successive  generation. 


far  as  inheritance  is  concerned,  the  body  is  merely  the  carrier  of  the 
germ-cells,  which  are  held  in  trust  for  coming  generations. 

Weismann's  subsequent  theories,  built  on  this  foundation,  have 
given  rise  to  the  most  eagerly  contested  controversies  of  the  post- 
Darwinian  period,  and,  whether  they  are  to  stand  or  fall,  have  played 
a  most  important  part  in  the  progress  of  science.  For^aside_fromjhe 
truth  or  error  of  his  special  theories,  it  has  been  Weismann's  great 
service  to  place  the  keystone  between  the  work  of  the  evolutionists 
and  that  of  the  cytologists,  and  thus  to  bring  the  cell-theory  and  the 


14  I.\TK  OD  UC  TION 

evolution-theory  into  organic  connection.  It  is  from  the  point  of  view 
thus  suggested  that  the  present  volume  has  been  written.  It  has 
accordingly  not  been  my  primary  object  to  dwell  on  the  Diiuiitice  of 
histology,  still  less  to  undertake  an  exhaustive  description  of  all  the 
modifications  of  cell-structure  and  cell-action  ;  and  for  these  the  stu- 
dent must  refer  to  other  and  more  extended  treatises.  Yet  the  broader 
questions  with  which  we  have  to  deal  cannot  ])rofitably  be  discussed 
apart  from  the  concrete  phenomena  by  which  they  are  suggested,  and 
hence  a  considerable  part  of  the  text  is  necessarily  given  over  to 
descriptive  detail ;  but  I  hope  that  the  reader  will  not  lose  sight  of 
the  relation  of  the  part  to  the  whole,  or  forget  the  primary  intention 
of  the  work. 

We  shall  follow  a  convenient,  rather  than  a  strictly  logical,  order 
of  treatment,  beginning  in  the  first  two  chapters  with  a  general  sketch 
of  cell-structure  and  cell-division.  The  following  three  chapters  deal 
with  the  germ-cells,  —  the  third  with  their  structure  and  mode*  of 
origin,  the  fourth  with  their  union  in  fertilization,  the  fifth  with  the 
phenomena  of  maturation  by  which  they  are  prepared  for  their  union. 
The  sixth  chapter  contains  a  critical  discussion  of  cell-organization, 
completing  the  morphological  analysis  of  the  cell.  In  the  seventh 
cha])ter  the  cell  is  considered  with  reference  to  its  more  fundamental 
chemical  and  physiological  properties  as  a  prelude  to  the  examination 
of  development  which  follows.  The  succeeding  chapter  approaches 
the  objective  point  of  the  book  by  considering  the  cleavage  of  the 
ovum  and  the  general  laws  of  cell-division  of  which  it  is  an  expression. 
The  ninth  chapter,  finally,  deals  with  the  elementary  operations  of 
development  considered  as  cell-functions  and  with  the  theories  of 
inheritance  and  development  based  upon  them. 


SOME   GENERAL   WORKS    OX    THE    CELL-THEORY  ^ 

Bergh.  R.  S.  —  Vork'sungen  liber  die  Zelle  und  die  einfachen  Gewebe  :  Wiesbaden^ 
1894. 

Carnoy.  J.  B.  —  La  Biologie  Cellulaire  :  IJcrrc,  1884. 

Delage,  Yves.  —  La  Structure  du  Protoplasma  et  les  Theories  sur  THdrdditd  et  les 
grands  Problemes  de  la  Biologie  Gcnerale :  Paris,  1895. 

Geddes  &  Thompson.  —  The  Evolution  of  Sex  :  A'cw  ]'ork,  1890. 

Hacker.  V.  —  Pra.xis  und  Theorie  der  Zellen-  und  Befruchtungslehre  :  Jena.  1899. 

Henneguy.  L.  F.  —  Legons  sur  la  Cellule  :  /^a?/s.  1896. 

Hertwig.  0.  —  Die  Zelle  und  die  Gewebe:  Fischer,  Jeua,  L.  1893.  II.,  1898.  Trans- 
lation, published  by  Mactnillan,  London  and  Neiv  York^  1895. 

Hofmeister.       Lehre  von  der  Pflanzenzelle  :  Leipzii^,  1867. 

Huxley.  T.  H.  —  Review  of  the  Cell-theory:  British  and  Foreign  Medico-Chiriirgical 
Review,  XIL,  1853. 

^  See  also  Literature,  T.,  p.  61. 


INTRODUCTION 


15 


Minot.  C.  S.  —  Human  Embryology:  New  York,  1892. 

Remak.  R.  —  Untersuchimgen  iiber  die  Entwicklung  der  Wirbelthiere  :  Berlin, 
1850-55. 

Sachs,  J.  V.       History  of  Botany.     Translation:   Oxford,  \%<^q. 

Schleiden,  M.  J. —  Beitrage  zur  Phytogenesis  :  M'uller's  Arclih\  1838.  Translation 
in  Sydenham  Soc,  XII.     Loudon,  1847. 

Schwann.  Th.  —  Mikroscopische  Untersuchungen  liber  die  Uebereinstimmung  in  der 
Structur  und  dem  Wachsthum  der  Thiere  und  Pflanzen  :  Berlin,  1839.  Trans- 
lation in  Sydenham  Soc.  XII.     London,  1847. 

Tyson.  James.  —  The  Cell-doctrine,  2d  ed.     PJiiladelpJiia,  1878. 

Virchow,  R.  —  Die  Cellularpathologie  in  ihrer  Begriindung  auf  physiologische  und 
pathologische  Gewebelehre :  Berlin^  1858. 

Weismann,  A.  —  Essays  on  Heredity.  Translation:  First  series,  Oxford,  1891  ; 
Second  series,  Oxford,  1892.  • 

Id.  —  The  Germ-plasm:  Ne^u  York,  1893. 


CHAPTER  I 


GENERAL  SKETCH  OF  THE  CELL 

"  Wir  haben  gesehen,  dass  alle  Organismen  aus  wesentlich  gleichen  Thcilen,  namlich  aus 
Zellen  zusammengesetzt  sind,  dass  diese  Zellen  nach  wesentlich  densellKTi  Cieset/en  sich 
bilden  und  wachsen,  dass  also  diese  Prozesse  iiberall  auch  durch  dieselben  Krafte  hervorge- 
bracht  warden  miissen."  Schwann.^ 

In  the  passage  quoted  above  Schwann  expressed  a  truth  which 
subsequent  research  has  estabhshed  on  an  ever  widening  basis ;  and 
we  have  now^  more  than  ever  reason  to  believe  that  despite  unending 
diversity  of  form  and  function  all  cells  may  be  brought  into  definite 
relation  to  a  common  morphological  and  physiological  type.  We  are, 
it  is  true,  still  unable  to  specify  all  its  essential  features,  and  hence 
can  give  no  adequate  brief  definition  of  the  cell.  For  practical  pur- 
poses, however,  no  such  definition  is  needed,  and  we  may  be  content 
with  the  simple  type  that  has  been  familiar  to  histologists  since  the 
time  of  Leydig  and  Max  Schultze. 

It  should  from  the  outset  be  clearly  recognized  that  the  term 
"cell"  is  a  biological  misnomer;  for  cells  only  rarely  assume  the 
form  implied  by  the  word  of  hollow  chambers  surrounded  by  solid 
walls.  The  term  is  merely  an  historical  survival  of  a  word  casually 
employed  by  the  botanists  of  the  seventeenth  century  to  designate 
the  cells  of  certain  plant-tissues  which,  when  viewed  in  section,  give 
somewhat  the  appearance  of  a  honeycomb.^  The  cells  of  these  tis- 
sues are,  in  fact,  separated  by  conspicuous  solid  walls  which  were 
mistaken  by  Schleiden,  followed  by  Schwann,  for  their  essential  i)art. 
The  living  substance  contained  within  the  walls,  to  which  Hugo  von 
Mohl  gave  the  r\2imQ  protoplasm^  (1846),  was  at  first  overlooked  or 
was  regarded  as  a  waste-product,  a  view  based  upon  the  fact  that  m 
many  important  plant-tissues  such  as  cork  ox  wood  it  may  wholly 
disappear,  leaving  only  the  lifeless  walls.  The  researches  of  Herg- 
mann,    Kolliker,    Bischoff,   Cohn,    Max   Schultze,   and    many   others 

1  Uniersuchungen,  p.  227,  1839. 

2  The  word  seems  to  have  been  first  employed  by  Robert  Hooke,  in  1665,  to  designate 
the  minute  cavities  observed  in  cork,  a  tissue  which  he  describcl  as  made  up  of  •' httle 
boxes  or  cells  distinct  from  one  another  "  and  separated  In-  solid  walls. 

3  The  same  word  had  been  used  by  Purkinje  some  years  before  (1840)  to  designate  the 
formative  material  of  young  animal  embryos. 

c  17 

D.  H.  HILL  LIBRARY 


i8 


GENERAL   SKETCH  OF   THE    CELL 


showed,  however,  that  most  living  cells  are  nut  hollow  but  solid 
bodies,  and  that  in  many  cases  —  for  example,  the  colourless  corpuscles 
of  blood  and  lymph  —  they  are  naked  masses  of  protoplasm  not  sur- 
rounded by  definite  walls.  Thus  it  was  proved  that  neither  the 
vesicular  form  nor  the  presence  of  surrounding  walls  is  an  essential 
character,  and  that  the  cell-contents,  i.e.  the  protoplasui,  must  be  the 
seat  of  vital  activity. 

Within   the  protoplasm  (Figs.  6  <S)  lies  a  body,  usually  of  definite 
rounded  form,  known  as  the  nnclcus,'^  and  this  in  turn  often  contains 


Attraction-sphere  enclosing  two  ccntrosomes 


Nucleus  - 


r  Plasmosome  or 

true 

nucleolus 

Chromatin- 

nctwork 

Linin-network 

I   Karyosome, 
net-knot,  or 
chromatin- 
nucleolus 


Plastids  lying  in  the 
cytoplasm 


Vacuole 


Passive  bodies  fmeta- 
plasm  or  paraplasm) 
suspended  in  the  cy- 
toplasmic mesh  work 


Fig.  6. —  Diagram  of  a  cell.     Its  basis  consists  of  a  meshwork  containing  numerous  minute 
granules  {microsomes)  and  traversing  a  transparent  ground-substance.  a 


one  or  more  smaller  bodies  or  nucleoli.  By  some  of  the  earlier 
workers  the  nucleus  was  supposed  to  be,  like  the  cell-wall,  of  sec- 
ondary im])ortance,  and  many  forms  of  cells  were  described  as  being 
devoid  of  a  nucleus  ("cytodes"  of  Haeckel).  Nearly  all  later  re- 
searches have  indicated,  however,  that  the  characteristic  nuclear 
material,  whether  forming  a  single  body  or  scattered  in  smaller 
masses,  is  always  present,  and  that  it  plays  an  essential  part  in  the 
life  of  the  cell. 

Besides  the  presence  of   protoplasm  and  nucleus,  no  other  struc- 
tural features  of  the  cell  are  yet  known  to  be  of  universal  occurrence. 

1  First  described  by  Fontana  in  1 781,  and  recognized  as  a  normal  element  of  the  cell  by 
Robert  Brown  in  1S33. 


GENERAL  MORPHOLOGY   OF   THE    CELL 


19 


We  may  therefore  still  accept  as  valid  the  definition  ^qven  more  than 
thirty  years  ago  by  Leydig  and  Max  Schultzc,  that  a  cell  is  a  mass 
of  protoplasm  containing  a  nucleus}  to  which  we  may  add  Schultze's 
statement  that  both  nucleus  and  protoplasm  arise  tlirougli  the  division 
of  the  corresponding  elements  of  a  preexisting  cell.  Nothing  could  be 
less  appropriate  than  to  call  such  a  body  a  ''cell  "  ;  yet  the  word  has 
become  so  firmly  established  that  every  effort  to  replace  it  bv  a 
better  has  failed,  and  it  probably  must  be  accepted  as  part  of  the 
established  nomenclature  of  science.^ 


A.  General  Morphology  of  the  Cell 

The  cell  is  a  rounded  mass  of  protoplasm  which  in  its  simplest 
form  is  approximately  spherical.  The  form  is,  however,  seldom 
realized  save  in  isolated  cells  such  as  the  unicellular  plants  and  ani- 
mals or  the  egg-cells  of  the  higher  forms.  In  vastly  the  greater 
number  of  cases  the  typical  spherical  form  is  modified  by  unequal 
growth  and  differentiation,  by  active  movements  of  the  cell-substance, 
or  by  the  mechanical  pressure  of  surrounding  structures,  but  true 
angular  forms  are  rarely  if  ever  assumed  save  by  cells  surrounded  by 
hard  walls.  The  protoplasm  which  forms  its  active  basis  is  a  viscid, 
translucent  substance,  sometimes  apparently  homogeneous,  more  fre- 
quently finely  granular,  and  as  a  rule  giving  the  appearance  of  a 
meshwork,  which  is  often  described  as  a  spongelike  or  netlike  ''  reticu- 
lum." ^  Besides  the  active  substance  or  protoplasm  proper  the  cell 
almost  invariably  contains  various  lifeless  bodies  suspended  in  the 
meshes  of  the  network;  examples  of  these  are  food-granules,  pig- 
ment-bodies, drops  of  oil  or  w^ater,  and  excretory  matters.  These 
bodies  play  a  relatively  passive  part  in  the  activities  of  the  cell, 
being  either  reserve  food-matters  destined  to  be  absorbed  and  built 
up  into  the  living  substance,  or  by-products  formed  from  the  proto- 
plasm as  waste-matters  or  in  order  to  play  some  role  subsidiary  to 
the  actions  of  the  protoplasm  itself.  The  passive  inclusions  in  the 
protoplasm  maybe  collectively  designated  as  metaplasm  (Hanstein) 
ox  paraplasm  {Yi\\^^^^x\  in  contradistinction  to  the  -AQixxc  protoplas}n. 

1  Leydig,  Lchrlmch  der  Hisiologie,  p.  9,  1857;    Schultze,  .7;r//.  AiiaL  u.  IViys.,\^.  11,  1S61. 

2  Sachs  has  proposed  the  convenient  word  cnergid  {flora,  '92,  p.  57)  to  dcsijjnatc  the 
essential  living  part  of  the  cell,  i.e.  the  nucleus  with  that  portion  of  the  active  cyt«»plasm 
that  falls  within  its  sphere  of  influence,  the  two  forming  an  organic  unit  both  in  a  morpho- 
logical and  in  a  physiological  sense.  It  is  to  be  regretted  that  this  convenient  and  appro- 
priate term  has  not  come  into  general  use.  (See  also  Flora,  '95,  p.  405.  and  cf.  Kupfter 
('96),  Meyer  ('96),  and  Kolliker  ('97).) 

3  Such  meshworks  are  sometimes  plainly  visible  in  the  living  protoplasm  (p.  44).  It  is 
always  more  or  less  an  open  question  how  far  the  appearances  seen  in  hxed  (coagulated) 
material  correspond  with  the  conditions  existing  in  life.     See  pp.  42-46. 


20 


GENERAL   SKETCH   OE   THE  CELL 


It  is  often  difficult  to  distinguish  between  such  metaplasmic  bodies 
and  the  granules  commonly  supposed  to  be  elements  of  the  active 
protoplasm;  indeed,  as  will  appear  beyond  (p.  29),  there  is  reason 
to  believe  that  "protoplasmic"  and  "metaplasmic"  granules  cannot 
be  separated  by  any  definite  limit,  but  are  connected  by  various 
gradations.  Among  the  lifeless  products  of  the  protoplasm  must  be 
reckoned  also  the  ctli-ica!/  or  lucnibrauc  bv  which  the  cell-body  may 


Fig.  7  —  Spermatogonia  of  the  salamander.     [Meves.] 
Above,  two  cells  showing  large  nuclei,  with  linin-threads  and  scattered  chromatin-graniiles ;  in 
each  cell   an  attraction-sphere  with  two  centrosomes.     Below,  three  contiguous   spermatogonia, 
showing  chromatin-reticulum,  centrosomes  and  spheres,  and  sphere-bridges. 

be  surrounded  ;  but  it  must  be  remembered  that  the  cell-wall  in  some 
cases  arises  by  a  direct  transformation  of  the  protoplasmic  substance, 
and  that  it  often  retains  the  power  of  growth  by  intussusception  like 
living  matter. 

It  is  unfortunate  that  some  confusion  has  arisen  in  the  use  of  the 
word  protoplasm.  When  Leydig,  Schultze,  Brlicke,  De  Bary,  and 
other  earlier  writers  spoke  of  "protoplasm,"  they  had  in  mind  only 
the  substance  of  the  cell-body,  not  that  of  the  nucleus.     Strasburger, 


GENERAL   MORPHOLOGY   OF   TLIE    CELL 


21 


J.  ♦il?.v-'.v.  -.  ,  . 


^ 


c 


z> 


.  ^^^-^-^'^lioiis  cells  showing  the  typical  parts. 

N.c1;or2XrreS'„:,'f  lli^e,,:^ 'o^J  sa,an,a„de...,arva.     T.o   ce„,.oso„,es   a,  ,he   rish. 

f?.'l'  kT'  ''T  ''"'"  ''''°P'=''  ''>'  ™'^"y'  b"t  not  all.  later  writers 
eestio'n  h  ""I'  "'"'t^'''"-  '^-•'"S,  however,  at  Flen,n,i„,'s  u" 
gestion,  been  changed  to  ta,yop/as„r  At  the  present  time  there 
fore,  the  word /;...^/.„,„  is  used  by  some  authors'(Hutsch  i,  H^  tw  '" 


22  GENERAL   SKETCH  OE   THE    CELL 

Kolliker,  etc.)  in  its  orif^inul  narrower  sense  (equivalent  to  Stras-. 
burger's  cytoplasm),  while  perhaps  the  majority  of  writers  have 
accepted  the  terminology  of  Strasburger  and  Flemming.  On  the 
whole,  the  terms  cytoplasin  and  kiDyop/asui  seem  too  useful  to  be 
rejected,  and,  without  attaching  too  much  importance  to  them,  they 
will  be  employed  throughout  the  present  work.  It  must  not,  how- 
ever, be  supposed  that  either  of  the  words  denotes  a  single  homo- 
geneous substance;  for,  as  will  soon  appear,  both  cytoplasm  and 
karyoplasm  consist  of  several  distinct  elements. 

The  nucleus  is  usually  bounded  by  a  definite  membrane,  and  often 
appears  to  be  a  perfectly  distinct  vesicular  body  suspended  in  the 
cvtoplasm  —  a  conclusion  sustained  by  the  fact  that  it  may  move 
actively  through  the  latter,  as  often  occurs  in  both  vegetable  and 
animal  cells.  Careful  study  of  the  nucleus  during  all  its  phases  gives, 
however,  reason  to  believe  that  its  structural  basis  is  similar  to  that 
of  the  cell-body  ;  and  that  during  the  course  of  cell-division,  when 
the  nuclear  membrane  usually  disappears,  cytoplasm  and  karyoplasm 
come  into  direct  contmuity.  Even  in  the  resting  cell  there  is  good 
evidence  that  both  the  intranuclear  and  the  extranuclear  material  may 
be  structurally  continuous  with  the  nuclear  membrane^  and  among  the 
Protozoa  there  are  forms  (some  of  the  flagellates)  in  which  no  nuclear 
membrane  can  at  any  period  be  seen.  For  these  and  other  reasons 
t/ic  tcrtns  ''nucleus^'  and  '' ccU-bQ,dy''  sJioiild  probably  be  regarded  as 
only  topographical  expressions  denoting  tzuo  differentiated  areas  in  a 
common  structural  basis.  The  terms  karyoplasm  and  cytoplasm  possess, 
however,  a  specific  significance  owing  to  the  fact  that  there  is  on 
the  whole  a  definite  chemical  contrast  between  the  nuclear  substance 
and  that  of  the  cell-body,  the  former  being  characterized  by  the 
abundance  of  a  substance  rich  in  phosphorus  known  as  nuclein,  while 
the  latter  contains  no  true  nuclein  and  is  especially  rich  in  albuminous 
substances  such  as  nucleo-albumins,  albumins,  globulins,  and  the  like, 
which  contain  little  or  no  phosphorus. 

Both  morphologically  and  physiologically  the  differentiation  of  the 
active  cell-substance  into  nucleus  and  cell-body  must  be  regarded  as  a 
fundamental  character  of  the  cell  because  of  its  universal,  or  all  but 
universal,  occurrence,  and  because  there  is  reason  to  believe  that  it  is 
in  some  manner  an  expression  of  the  dual  aspect  of  the  fundamental 
process  of  metabolism,  constructive  and  destructive,  that  lies  at  the 
basis  of  cell  life.  The  view  has  been  widely  held  that  a  third  essen- 
tial element  is  the  centrosome,  discovered  by  Flemming  and  Van 
Beneden  in  1875-76,  and  since  shown  to  exist  in  a  large  number  of 
other  cells  (Figs.   7,  8).     This  is  an  extremely  minute   body  which 

1  Conklin  ('97,  i).  Obst  ('99),  and  some  others  have  described  a  direct  continuity  in  the 
resting  cell  between  the  intranuclear  and  extranuclear  ineshworks. 


STRUCTURAL   BASIS   OF  PROTOPLASM  23 

is  concerned  in  the  process  of  cell-division  and  in  the  fertilization  of 
the  G.gg,  and  has  been  characterized  as  the  "  dynamic  centre  "  of  the 
cell.  Whether  it  has  such  a  significance,  and  whether  it  is  in  point 
of  morphological  persistence  comparable  with  the  nucleus,  are  ques- 
tions still  sub  judicCy  which  will  be  discussed  elsewhere.^ 


B.     Structural  Basis  of  Protoplasm 

As  ordinarily  seen  under  moderate  powers  of  the  microscope,  proto- 
plasm  appears  as  a  more  or  less  vague    granular   substance  which 
shows  as  a  rule   no   definite   structure   organization.      More   precise 
examination  under  high  powers,  especially  after  treatment  by  suitable 
fixing  and  staining  reagents,  often  reveals  a  highly  complex  structure 
in  both  nucleus  and  cytoplasm.     Since  the  fundamental  activities  of 
protoplasm  are  everywhere  of   the   same   nature,  investigators  have 
naturally  sought  to  discover  a  corresponding  fundamental  morpho- 
logical organization  common  to  all  forms  of   protoplasm  and  under- 
lying all  of  its  special  modifications.     Up  to  the  present  time,  however, 
these  attempts  have  not  resulted  in  any  consensus  of  opinion  as  to 
whether  such  a  common  organization  exists.     In  many  forms  of  proto- 
plasm, both  in  hfe  and  after  fixation  by  reagents,  the  basis  of  the 
structure  is  a  more  or  less  regular  framework  or  niesJiwork,  consisting 
of  at  least  two  substances.     One  of  these  forms  the  substance  of  the 
meshwork  proper;    the  other,  often  called  the  ground-substance  (also 
cell-sap,  enchylema,    hyaloplasma,   paramitome,   interfilar  substance, 
etc.), 2  occupies  the  intervening  spaces.     To  these  two  elements  must 
be  added  minute,  deeply  staining  granules  or  "  microsomes  "  scattered 
along  the   branches  of   the  meshwork,  sometimes   quite  irregularly, 
sometimes  with  such  regularity  that  the  meshwork  seems  to  be  built 
of  them.     Besides  the  foregoing  three  elements,  which  we  may  pro- 
visionally regard  as  constituting  the  active  substance,  the  protoplasm 
almost  invariably  contains  various  passive  or  metaplasmic  substances 
in  the  form  of  larger  granules,  drops  of  liquid,  crystalloid  bodies,  and 
the  like.     These  bodies,  which  usually  lie  in  the  spaces  of  the  mesh- 
work, are  often  difficult  to  distinguish  from  the  microsomes  lying  in 
the  meshw^ork  proper —  indeed,  it  is  by  no  means  certain  that  any 
adequate  ground  of  distinction  exists.^ 

From  the  time  of  Frommann  and  Arnold  ('65-'67)  onwards,  most 
of  the  earlier  observers  regarded  the  meshwork  as  a  fibrillar  structure, 
either  forming  a  continuous  network  or  reticulum  somewhat  like  the 
fibrous  network  of  a  sponge  ("reticular  theory  "  of  Klein,  Van  Bene- 
den,    Carnoy,    Heitzmann),   or    consisting    of    disconnected    threads, 

1  Cf.  pp.  304.  354.  '  Q'-  ^'l^^ssary.  ^  Cf.  p.  29. 


24 


GENERAL    SKETCH   OF   THE    CELL 


••^ 


jD 


/ 


[       TK 


IS  ^ 

Fig-  9-  —  Living  cells  of  salamander-larva.     [Flemminc^..] 

A.  Group  of  epidermal  cells  at  different  foci,  showing  protoplasmic  bridges,  nuclei,  and  cyto- 
plasmic fibrillae;   the  central  cell  with  nucleus  in  the  spireme-stage.     B.  Connective  tissue  cell. 

C.  Epidermal  cell   in  early  mitosis   (segmented   spireme)   surrounded  by  protoplasmic   bridges. 

D.  Dividing  cell.     E.F.  Cartilage-ceils  with  cytoplasmic  fibrillas  (the  latter  somewhat  exaggerated 
in  the  plate). 


STRUCTURAL   BASIS   OF  PROTOPLASM  2$ 

whether  simple  or  branching  (''filar  theory"  of  Flcmming),  and  the 
same  view  is  widely  held  at  the  present  time.  The  meshwork  has 
received  various  names  in  accordance  with  this  conception,  among 
which  may  be  mentioned  reticulum,  thrcad-ivork,  spongioplasm,  mitomc, 
filar  subslance}  all  of  which  are  still  in  use.  Under  this  view  the 
"  granules  "  described  by  Schultze,  Virchow  and  still  earHer  observers 
have  been  variously  regarded  as  nodes  of  the  network,  optical  sec- 
tions of  the  threads,  or  as  actual  granules  ("  microsomes  ")  suspended 
in  the  network  as  described  above. 

Widely  opposed  to  these  views  is  the  "  alveolar  theory  "  of  Butschli, 
which  has  won  an  increasing  number  of  adherents.     Butschli  regards 
protoplasm    as    having    a    foam-like    alveolar    structure   ("W'aben- 
struktur"),  nearly  similar  to  that  of  an  emulsion  (Fig.   lo),  and  he 
has  shown  in  a  series  of  beautiful  experiments  that  artificial  emul- 
sions, variously  prepared,  may  show  under  the  microscope  a  marvel- 
lously close  resemblance  to  living  protoplasm,  and  further  that  drops 
of  oil-emulsion  suspended  in  water  may  even  exhibit  amoeboid  changes 
of  form.     To  restate  Biitschli's  view,  protoplasm  consists  of  separate, 
closely  crowded  minute  drops^  of  a  liquid  alveolar  substance  suspended 
in  a  continuous  interalvcolar  substance,  likewise  liquid,  but  of  different 
physical  nature.     The  latter  thus  forms  the  walls  of  closed  chambers 
or  alveoli  in  which  the  alveolar  drops  lie,  just  as  in  a  fine  emulsion 
the  emulsifying  liquid  surrounds  the  emulsified  drops.     The  appear- 
ance of  a  network  in  protoplasm  is  illusory,  being  due  to  optical  sec- 
tion of  the  interalvcolar  walls  or  partitions  as  viewed  at  any  given 
focus  of  the  microscope.     As  thus  seen,  the  walls  themselves  appear 
as  fibres,  while  the  "spaces  of  the  network"  are  in  like  manner  oi)ti- 
cal   sections   of    the   alveoli,   the   alveolar  substance   that   fills   them 
corresponding  to  the   ''ground  substance."     As  explained   beyond/^ 
Butschli  interprets  in  like  manner  the  radiating  systems   or   asters 
formed   during   cell-divison,   the   astral   rays  (usually  considered    as 
fibres)  being  regarded  as  merely  the  septa  between  radially  arranged 

alveoH  (Fig.  lo). 

The  two  (three)  general  views  above  outlined  may  be  designated 
respectively  as  thQ  fibrillar  (reticular  or  filar)  and  alveolar  \.\\cox\c^ 
of  protoplasmic  structure  ;  and  each  of  them  has  been  believed  by 
some  of  its  adherents  to  be  universally  applicable  to  all  forms  of 
protoplasm.  Beside  them  may  be  placed,  as  a  third  general  view, 
Xh^  granular  theory  especially  associated  with  the  name  of  Altmann, 
by  whom  it  has  been  most  fully  developed,  though  a  number  of 
earlier  writers  have  held  similar  views.  According  to  Altmann's 
view,  which   apart  from   its   theoretical   development   approaches   in 

1  See  Glossary. 

2  Measuring  on  an  average  about  .ooi  mm.  in  diameter.  ^  Cf.  p.  no. 


26 


GENERAL   SKETCH  OF   THE    CELL 


some  respects  that  of  Biitschli,  protoplasm  is  compounded  of  innu- 
merable minute  granules  which  alone  form  its  essential  active  basis ; 
and  while  fibrillar  or  alveolar  structures  may  occur,  these  are  of  only 
secondary  importance. 


r 


1 


Fig.  10. — Alveolar  or  foam-structure  of  protoplasm,  according  to  Biitschli.     [BuTSCllLl.] 

A.  Epidermal  cell  of  the  earthworm.  B.  Aster,  attraction-sphere,  and  centrosome  from  sea- 
urchin  egg.  C.  Intracapsular  protoplasm  of  a  radiolarian  ( Thalassicolla)  with  vacuoles. 
D.  Peripheral  cytoplasm  of  sea-urchin  egg.  E.  Artificial  emulsion  of  olive-oil,  sodium  chloride, 
and  water. 


It  is  impossible  here  adequately  to  review  the  many  combinations 
and  modifications  of  these  views  which  different  investigators  have 


STRUCTURAL   BASIS  OF  PROTOPLASM 


27 


made.^  On  the  whole,  the  present  drift  of  opinion  is  toward  the 
conclusion  that  none  of  the  above  interpretations  has  succeeded  in 
the  attempt  to  give  a  universal  formula  for  protoplasmic  structure  ; 
and  many  recent  observers  have  reached  the  conclusion,  earlier  advo- 
cated by  Kolliker  ('89),  that  the  various  types  described  above  are 
connected  by  intermediate  gradations  and  may  be  transformed  one 
into  another,  in  different  phases  of  cell-activity.  Unna  ('95),  for 
example,  endeavours  to  show  how  an  alveolar  structure  may  pass  into 
a  sponge-like  or  reticular  one  by  the   breaking  down  of   the  inter- 


a 


.-.   .' "■•.#^.     i      ''.     .'*^.      zer,      "^  -•"t»*'/*'w"*-v'**-**        ^>       '*^m'l       '. 


005;^ 


^.; 


^ 


o:°Ao:„-D, 


o 


0°b-,-?.-lio°voQ, 


00. -o-.... 
o'o  •""•" 
o" 


■?.?o 


Fig.  II.— ('?)  Protoplasm  of  the  egg  of  the  sea-urchin  {Toxopneustes)  in  section  showing 
meshwork  of  microsomes;  {b)  protoplasm  from  a  living  star-fish  egg  {Astcrias)  showing  alveolar 
spheres  with  microsomes  scattered  between  them  ;  {c)  the  same  in  a  dying  condition  atler  crush- 
ing the  egg ;  alveolar  spheres  fusing  to  form  larger  spheres ;  {d)  protoplasm  from  a  young  ovarian 
egg  of  the  same.     (All  the  figures  magnified  1200  diameters.) 

alveolar  walls.  Flemming,  for  many  years  the  foremost  and  most 
consistent  advocate  of  the  fibrillar  theory,  now  admits  that  protoplasm 
may  be  fibrillar,  alveolar,  granular,  or  sensibly  homogeneous,^  and 
that  we  cannot,  therefore,  regard  any  one  of  these  types  of  structure 
as  absolutely  diagnostic  of  the  living  substance.  In  plant-cells 
Strasburger^  and  a  number  of  his  pupils  maintain  that  the  "kino- 
plasm"  (p.  322)  or  filar  plasm,  from  which  the  spindle-fibres  and 
astral  rays  are  formed,  is  fibrillar,  while  the  "  trophoplasm "  or 
alveolar  plasm  forming  the  main  body  of  the  cell  is  alveolar,  the 
former,  however,  assuming  the  fibrillar  structure,  as  a  rule,  only 
during  the  mitotic  activity  of  the  cell.  My  own  long-continued 
studies  on  various  forms  of  protoplasm  have  likewise  led  to  the  con- 
clusion that  no  universal  formula  for  protoplasmic  structure  can  be 

1  For  full  discussion,  with  literature  list,  see  Flemming,  '82,  '97.  ^  '97.  2.  and  Butschli. 


'92,  2,  '99. 


2  '97,  I,  p.  260. 


3  '95.  '97,  3.  '^8. 


28 


GENERAL   SKETCH   OF  THE    CELL 


given. ^  In  that  classical  object,  the  echinoderm-egg,  for  example, 
it  is  easy  to  satisfy  oneself,  both  in  the  living  cell  and  in  sections, 
that  the  protoplasm  has  a  beautiful  alveolar  structure,  exactly  as 
described  by  Hutschli  in  the  same  object  (Fig.  1 1 ).  This  structure 
is  here,  however,  entirely  of  secondary  origin ;  for  its  genesis  can 
be  traced  step  by  step  during  the  growth  of  the  ovarian  eggs  through 
the  deposit  of  minute  drops  in  a  homogeneous  basis,  which  ultimately 
gives  rise  to  the  interalveolar  walls.  In  these  same  eggs  the  astral 
systems    formed   during   their   subsequent   division   (Fig.    12)  are,   I 


..  .■•.,•••  .»••      ''/   '■.  /'  V<i      *■•.•.''.     >"•• 


:     "  vr^t'-.-   :••••.    >-./•;     *>'••-'"•.•<    :••-..••..••-> 


Fig.  12.  —  Section  of  sea-urchin  egg  (Toxopficustcs),  li  minutes  after  entrance  of  the  sperma- 
tozoon, showing  alveoli  and  microsomes,  sperm-nucleus,  middle  piece,  and  aster  (about  2000 
diameters). 

believe,  no  less  certainly  fibrillar ;  and  thus  we  see  the  protoplasm 
of  the  same  cell  passing  successively  through  homogeneous,  alveolar, 
and  fibrillar  phases,  at  different  periods  of  growth  and  in  different 
conditions  of  physiological  activity.  There  is  good  reason  to  regard 
this  as  typical  of  protoplasm  in  general.  BiJtschli's  conclusions, 
based  on  researches  so  thorough,  j^rolonged,  and  ingenious,  are 
entitled  to  great  weight ;  yet  it  is  impossible  to  resist  the  evidence 
that  fibrillar  and  granular  as  well  as  alveolar  structures  are  of  wide 
occurrence ;  and  while  each  may  be  characteristic  of  certain  kinds  of 

1  Wilson,  '99. 


STRUCTURAL  BASIS   OF  PROTOPLASM 


29 


cells,  or  of  certain  physiological  conditions,^  none  is  common  to  all 
forms  of  protoplasm.  If  this  position  be  well  grounded,  we  must 
admit  that  the  attempt  to  find  in  visible  protoplasmic  structure  any 
adequate  insight  into  its  fundamental  modes  of  physiological  activity 
has  thus  far  proved  fruitless.  We  must  rather  seek  the  source  of 
these  activities  in  the  ultramicroscopical  organization,  accepting  the 
probability  that  apparently  homogeneous  protoplasm  is  a  complex 
mixture  of  substances  which  may  assume  various  forms  of  visible 
structure  according  to  its  modes  of  activity. 

Some  of  the  theoretical  speculations  regarding  the  essential  nature 
of  that  organization  are  discussed  in  Chapter  VI.,  but  one  q2iasi-X\\(to- ' 
retical  point  must  be  here  considered.  Much  discussion  has  been 
given  to  the  (question  as  to  which  of  the  visible  elements  of  the  proto- 
plasm should  be  regarded  as  the  "living"  substance  proper;  and  the 
diversity  of  opinion  on  this  subject  may  be  judged  by  the  fact  that 
although  many  of  the  earlier  observers  identified  the  "reticulum  "  as 
the  living  element,  and  the  ground-substance  as  Ufeless,  others,  such 
as  Leydig  and  Schafer,  held  exactly  the  reverse  view,  while  Altmann 
insisted  that  only  the  "  granules  "  were  alive.  Later  discussions  have 
shown  the  futility  of  this  discussion,  which  is  indeed  largely  a  verbal 
one,  turning  as  it  does  on  the  sense  of  the  word  "living."  In  practice 
we  continually  use  the  word  "living"  to  denote  various  degrees  of 
vital  activity.  Protoplasm  deprived  of  nuclear  matter  has  lost,  wholly 
or  in  part,  one  of  the  most  characteristic  vital  properties,  namely,  the 
power  of  synthetic  metaboHsm ;  yet  we  still  speak  of  it  as  "  living," 
since  it  still  retains  for  a  longer  or  shorter  period  such  properties 
as  irritability  and  the  power  of  coordinated  movement ;  and,  in  like 
manner,  various  special  elements  of  protoplasm  may  be  termed  "  liv- 
ing "  in  a  still  more  restricted  sense.  In  its  fullest  meaning,  however, 
the  word  "living"  implies  the  existence  of  a  group  of  cooperating 
activities  more  complex  than  those  manifested  by  any  one  substance 
or  structural  element.  I  am  therefore  entirely  in  accord  with  the 
view  urged  by  Sachs,  Kolliker,  Verworn,  and  other  recent  writers, 
that  life  can  'only  be  properly  regarded  as  a  property  of  the  cell- 
system  as  a  whole ;  and  the  separate  elements  of  the  system  would, 
with  Sachs,  better  be  designated  as  "active"  or  "passive,"  rather 
than    as    "living"    or    "lifeless."       Thus    regarded,    the    distinction 

1  Thus  the  alveolar  structure  seems  to  be  characteristic  of  Protozoa  in  general,  and  of 
the  protoplasm  of  plant-cells  when  in  the  vegetative  state,  the  fibrillar  of  nerve-cells  and 
muscle-cells.  The  granular  type  is  characteristic  of  some  forms  of  leucocytes  and  gland- 
cells;  but  many  of  the  granules  in  these  cells  are  no  doubt  metaplasmic,  and  it  is  further 
very  doubtful  whether  such  a  granular  or  "pseudo-alveolar"  structure  can  be  logically  dis- 
tinguished from  an  alveolar  (c/.  Wilson,  '99).  In  the  pancreas-cell  granular  and  hbr.llar 
structures  alternate  with  the  varying  phases  of  secretory  activity  {r/.  Mathews,  '99). 


30  GEXERAL   SKETCH  OF   THE   CELL 

between  "protoplasmic"  and  "  nictai:)lasmic "  substances,  while  a 
real  and  necessary  one,  becomes  after  all  one  of  degree.  I  believe 
that  we  are  probably  justified  in  regarding  the  continuous  substance 
as  the  most  constant  and  active  element,  and  that  which  forms  the 
fundamental  basis  of  the  system,  transforming  itself  into  granules, 
drops,  fibrilla?,  or  networks  in  accordance  with  varying  physiological 
needs. ^ 

Thus  stated,  the  question  as  to  the  relative  activity  of  the  various 
elements  becomes  a  real  and  important  one.  It  now  seems  probable 
that  the  substance  of  the  meshwork  (fibrillar  or  interalveolar  structure) 
is  most  active  in  the  processes  of  cell-division,  in  contractile  organs 
such  as  cilia  and  muscle-fibres,  and  in  nerve-cells  ;  but  the  ground- 
substance,  while  apparently  the  most  frequent  seat  of  metaplasmic 
deposits,  is  certainly  also  the  seat  of  active  chemical  changes.  This 
subject  has,  however,  not  yet  been  sufficiently  investigated. 


C.     The  Nucleus 

A  fragment  of  a  cell  deprived  of  its  nucleus  may  live  for  a  consid- 
erable time  and  manifest  the  power  of  coordinated  movement  without 
perceptible  impairment.  Such  a  mass  of  protoplasm  is,  however, 
devoid  of  the  powers  of  assimilation,  growth,  and  repair,  and  sooner 
or  later  dies.  In  other  words,  those  functions  that  involve  destructive 
metabolism  may  continue  for  a  time  in  the  absence  of  the  nucleus ; 
those  that  involve  constructive  metabolism  cease  with  its  removal. 
There  is,  therefore,  strong  reason  to  believe  that  the  nucleus  plays  an 
essential  part  in  the  constructive  metabolism  of  the  cell,  and  through 
this  is  especially  concerned  with  the  formative  processes  involved  in 
growth  and  development.  For  these  and  many  other  reasons,  to  be 
discussed  hereafter,  the  nucleus  is  generally  regarded  as  a  controlling 

1  Wilson,  '99.  Cf.  Sachs  ('92.  '95),  Kulliker  ('97),  Meyer  ('96),  and  Kupffcr  ('96)  on 
energids.  Sachs  sharply  distinguishes  between  the  energid  {r^\x(^^\x%  and  protoplasm),  which 
forms  a  living  unit,  and  the  passive  ^x^^ix^xA-prodticts,  placing  in  the  former  the  nucleus, 
nucleolus,  general  cytoplasm,  centrosome  and  plastids  (chloroplasts  and  leucoplasts),  and  in 
the  latter  the  starch-grains,  aleurone-crystals,  and  membrane.  Meyer  carries  the  analysis 
further,  classifying  the  active  energid-elemcnts  m\.o  protoplasmatic  and  alloplasmatic  organs, 
the  former  (nucleus  cytoplasm,  chromatophores,  and  perhaps  the  centrosomes)  arising  only 
by  division,  the  latter  (cilia,  and  according  to  Kolliker,  also  the  muscle- and  nerve-fibriiiee) 
formed  by  differentiation  from  the  protoplasmatic  elements.  The  passive  energid-products 
{ergastic  structures  or  "  formed  material  "  of  Beale)  are  formed  as  enclosures  (starch-grains, 
etc.),  or  excretions  (membranes).  These  general  views  arc  accejited  by  Kolliker;  but 
none  of  these  writers  has  undertaken  to  show  how  "  alloplasmatic "'  structures  are  to  be 
distinguished  from  metaplasmic  or  ergastic.  I  believe  Sachs'  view  to  be  in  principle  not 
only  true  but  of  high  utility.  Practically,  however,  it  involves  us  in  considerable  difficulty, 
unless  the  terminology  adopted  above  —  itself  directly  suggested  by  and  nearly  agreeing  with 
the  usage  of  Sachs  and  Kolliker  —  be  employed. 


THE  NUCLEUS 


31 


centre  of  cell-activity,  and  hence  a  primary  factor  in  growth,  develop- 
ment, and  the  transmission  of  specific  qualities  from  cell  to  cell,  and 
so  from  one  generation  to  another. 

I,    General  Strncticre 

The  cell-nucleus  passes  through  two  widely  different  phases,  one  of 
which  is  characteristic  of  cells  in  their  ordinary  or  vegetative  condi- 
tion, while  the  other  only  occurs  during  the  complicated  changes 
involved  in  cell-division.  In  the  first  phase,  falsely  characterized  as 
the  *' resting  state,"  the  nucleus  usually  appears  as  a  rounded  sac-like 
body  surrounded  by  a  distinct  membrane  and  containing  a  conspicu- 
ous irregular  network  (Figs.  6,  7,  13),  which  is  in  some  cases  plainly 
visible  in  the  living  cell  (Fig.  9).  The  form  of  the  nucleus,  though 
subject  to  variation,  is  on  the  whole  singularly  constant,  and  as  a  rule 
shows  no  very  definite  relation  to  that  of  the  cell-body,  though  in  elon- 
gated cells  such  as  muscle-cells,  in  certain  forms  of  parenchyma, 
and  in  epithelial  cells  (Fig.  49),  the  nucleus  is  itself  often  elongated. 
Typically  spherical,  it  may,  in  certain  cases,  assume  an  irregular  or 
amoeboid  form,  may  break  up  into  a  group  of  more  or  less  completely 
separated  lobes  (polymorphic  nuclei.  Fig.  49),  sometimes  forming  an 
irregular  ring  ("  ring-nuclei  "  of  leucocytes,  giant-cells,  etc.,  Fig.  14.  J)). 
It  is  usually  very  large  in  gland-cells  and  others  that  show  a  very 
active  metabolism,  and  in  such  cases  its  surface  is  sometimes  increased 
by  the  formation  of  complex  branches  ramifying  through  the  cell 

(Fig.  14,  E).  ^  ^ 

These  forms  seem  in  general  to  be  fairly  constant  in  a  given  species 
of  cell,  but  in  a  large  number  of  cases  the  nucleus  has  been  seen  in 
the  living  cell  (cartilage-cells,  leucocytes,  ova)  to  undergo  more  or  less 
active  changes  of  form,  sometimes  so  marked  as  to  merit  the  name  of 
amoeboid  (Fig.  77).  Perhaps  the  most  remarkable  deviations  from  the 
usual  type  of  nucleus  occur  among  the  unicellular  forms.  In  the  dil- 
ate Infusoria  the  nuclei  are  massive  bodies  of  two  kinds,  viz.  a  large 
macrormcleiis  and  one  or  more  smaller  viicroujiclci,  both  of  which  arc 
present  in  the  same  cell,  the  former  kind  being  generally  regarded  as 
the  active  nucleus,  the  latter  as  a  reserve  nucleus  from  which  at  cer- 
tain periods  new  macronuclei  arise  (p.  224).  The  macronuclei  show  a 
remarkable  diversity  of  form  and  structure  in  different  species.  Still 
more  interesting  are  the  so-called  scattered  or  distributed  nuclei,  de- 
scribed by  Butschli  in  flagellates  and  Bacteria,  by  Gruber  in  certain 
rhizopods  and  Infusoria,  and  by  several  authors  in  the  Cyanophyccx 
(Figs.  15,  16).  The  nuclear  material  is  here  apparently  scattered 
through  the  cell  in  the  form  of  numerous  minute,  deeply  stained  gran- 
ules, which,  if  this  identification  is  correct,  represent  the  most  primi- 


32 


GENERAL   SKETCH   OF   THE    CELL 


tive  known  types  of  nucleus ;  but  this  subject  is  still  sub  jiidice 
(p.  39).  A  transition  from  this  condition  to  nuclei  of  the  ordinary 
type  appears  to  be  L^iven  in  the  nuclei  of  certain  flagellates  (e.g.  CJii- 
Uwionas  and  Tracluhnotias),  where  the  chromatin-granules  are  aggre- 
gated about  a  nucleolus-like  body,  but  are  not  enclosed  by  a  membrane.^ 
In  considering  the  structure  of  the  nucleus,  as  seen  in  sections,  we 
must,  as  in  the  case  of  the  cytoplasm,  bear  in  mind  the  possibility,  or 

rather  probability,  that  some  of 
the  elements  described  may  be 
coagulation  -  products  ;  for  the 
nucleus  is  in  life  comj^osed  of 
liquid  or  semi-liquid  substance, 
and  Albrecht  ('99)  has  recently 
shown  that  nuclei  isolated  in  the 
fresh  condition  will  flow  together 
to  form  a  single  body.  Most  of 
the  main  features  of  the  nucleus, 
both  in  the  resting  and  in  the 
dividing  phases,  have,  however, 
been  seen  in  life  (Fig.  9),  and  the 
principal  danger  of  mistaking 
artifacts  for  normal  structures  re- 
lates to  the  finer  elements,  con- 
sidered beyond. 

In  the  ordinary  forms  of  nuclei 
in  their  resting  state  the  follow- 
ing structural  elements  may  as  a 
rule  be  distinguished  (Figs.  6,  7, 
10):  — 

Fig.  13.  — Two    nuclei    from    the    crypts   of  ^^       q^^e      liuclcav     DUmbraUC,     a 

Lieberkiihn  in  the  salamander.    [Heidenhain.]  ^^    -^     r         i       i    t       ^  n         i  •    i 

well-defined    delicate   wall  which 

The    character     of    the    chromatin-network        •  ,  i  i 

(^aj/r/4r^wa//«)  is  accurately  shown.     The  upper  glVCS  the  UUClCUS  a  Sharp  COUtOUr 

nucleus    contains    three    plasmosomes   or    true  -^^-^^   differentiates    it    clcarly  from 

nucleoli;  the  lower,  one.   A  few  fine  linin-threads  .  t  .        •>  -r-^' 

{oxychromattn)   are  seen  in  the  upper  nucleus  the  SUrrOUuding  Cytoplasm.      ThlS 

running  off  from  the  chromatin-masses.  The  wall  SOmctimCS  StaiuS  but  VCiy 
clear  spaces  are  occupied  by  the  ground-sub-  ^|i n]  ^,^,|  ^aU  SCarCcly  be  dif- 
stance.  n        ./  '  j 

ferentiated  from  the  outlying 
cytoplasm.  In  other  and  perhaps  more  frequent  cases,  it  approaches 
in  staining  capacity  the  chromatin. 

b.  The  nuclear  rcticuliiDi.  This,  the  most  essential  part  of  the 
nucleus,  forms  an  irregular  branching  network  or  reticulum  which  con- 
sists of  two  very  different  constituents.  The  first  of  these,  forming  the 
general  protoplasmic  basis  of  the  nucleus,  is  a  substance  known  as  lini7i 

1  Calkins,  '98,  i. 


THE  NUCLEUS 


33 


(Schwarz),  invisible  until  after  treatment  by  reagents,  which  in  sections 
shows  a  finely  granular  structure  and  stains  like  the  cytoplasmic  sub- 
stance, to  which  it  is  nearly  related  chemically  (Figs.  7,  49).  The 
second  constituent,  a  deeply  staining  substance  known  as  chromatiji 
(Flemming),  is  the  nuclear  substance /(^r  excellence,  for  in  many  cases 
it  appears  to  be  the  only  element  of  the  nucleus  that  is  directly  handed 
on  by  division  from  cell  to  cell,  and  it  seems  to  have  the  power  to  pro- 
duce all  the  other  elements.  The  chromatin  often  appears  in  the  form 
of  scattered  granules  and  masses  of  differing  size  and  form,  which  are 
embedded  in  and  supported  by  the  linin-substance  (Figs.  7,  19).  In 
some  cases  the  entire  chromatin-content  of  the  nucleus  appears  to  be 
condensed  into  a  single  mass  which  simulates  a  nucleolus  ;  for  exam- 
ple, in  Spirogyra  and  in  various  flagellates  and  rhizopods  (e.g.  Acti- 
jiospJicBviuni,  Ai'cella) ;  or  there  may  be  several  such  chromatin-masses, 
as  in  some  of  the  Foraminifera  and  in  Noctihica.  More  commonly  the 
chromatin  forms  a  more  or  less  regular  network  intermingled  with  and 
more  or  less  embedded  in  the  linin,  from  which  it  is  often  hardly  dis- 
tinguishable until  the  approach  of  mitosis,  when  a  condensation  of  the 
chromatin-substance  occurs. 

In  contradistinction  to  the  other  nuclear  elements,  chromatin  is  nut 
acted  upon,  or  is  but  slowly  affected,  by  peptic  digestion.  It  may  thus 
be  easily  isolated  for  chemical  analysis,  which  shows  it  to  consist 
mainly  of  luiclein,  i.e.  a  compound  in  varying  proportions  of  a  complex 
phosphorus-containing  acid  known  as  inicleinic  acid,  with  albumi- 
nous bodies  such  as  histon,  protamin,  or  in  some  cases  albumin  itself.' 
Upon  this,  as  will  be  show^n  in  Chapter  VL,  probably  depends  the  pro- 
nounced staining  capacity  when  treated  with  the  so-called  *'  nuclear 
stains  "  {e.g.  hsematoxylin,  methyl-green,  and  the  basic  tar-colours  gen- 
erally) from  which  chromatin  takes  its  name.  This  capacity  always 
increases  as  the  nucleus  prepares  for  division,  reaching  a  climax  in  the 
spireme-  and  chromosome-stages,  and  it  is  also  very  marked  in  con- 
densed nuclei  such  as  those  of  spermatozoa.  These  variations  are 
almost  certainly  due  to  varying  proportions  in  the  constituents  of  the 
nuclein,  the  staining  capacity  standing  in  direct  ratio  to  the  amount  of 
nucleinic  acid. 

c.  The  nucleoli,  one  or  more  larger  rounded  or  irregular  bodies, 
suspended  in  the  network,  and  staining  intensely  with  many  dyes. 
In  some  nuclei  they  are  entirely  absent.  When  present  the  nucleoli 
vary  in  number  from  one  to  five  or  more;  and  the  number  is  otten 
inconstant  in  the  same  species  of  cell,  and  even  varies  in  the  same 
cell  with  varying  physiological  conditions.  In  the  eggs  of  some 
animals,  at  certain  periods  of  growth  {e.g.  lower  vertebrates),  the 
nucleus  may  contain  hundreds  of  nucleoli.     An  interesting  case  is 

1  See  p.  334- 
D 


^^  GENERAL   SKETCH  OF   THE    CELL 

that  of  the  subcutaneous  gland-cells  of  Pisciola,  the  nuclei  of  which 
contain  in  early  phases  of  secretion  but  a  single  nucleolus.  During 
growth  of  the  cell  the  nucleolus  fragments,  finally  giving  rise  to 
several  hundred  nucleoli  which  then  appear  to  migrate  out  into  the 
cytoplasm,  leaving  but  a  single  nucleolus  to  repeat  the  cycle. ^ 

The  bodies  known  as  nucleoli  are  of  at  least  two  different  kinds. 
The  first  of  these,  the  so-called  true  nucleoli  or  phuinosomcs  (Figs.  6, 
8,  B,  13),  are  of  spherical  form,  and  are  shown  by  the  staining 
reactions  to  differ  widely  from  chromatin,  being  in  general  sharply 
stained  by  dyes  which,  like  eosin,  orange  or  acid  fuchsin,  stain  the 
linin  and  the  general  cytoplasm.  The  plasmosomes  sometimes  seem 
to  have  no  envelope,  but  in  many  cases  {e.g.  in  leucocytes)  are 
surrounded  by  a  thin  layer  that  stains  Hke  chromatin.  Nucleoli  of  a 
quite  different  type  are  the  *' net-knots "  (Netzknoten),  chromatin- 
nucleoli,  or  karyosojucs,  which  agree  in  staining  reaction  with  chro- 
matin and  are  doubtless  to  be  regarded  as  merely  a  portion  of  the 
chromatin-network  (Figs.  8,  49).  These  are  sometimes  spherical, 
more  often  irregular  (Fig.  8),  and  often  are  hardly  to  be  distinguished, 
except  in  size,  from  nodes  of  the  chromatin-reticulum.''^  The  relations 
between  these  two  forms  of  nucleoli  are  far  from  certain,  and  the 
variations  in  staining  reaction  shown  by  true  nucleoli  render  it  not 
improbable  that  intermediate  forms  exist  which  may  represent  an 
actual  transition  from  one  to  the  other.-*^  In  many  of  the  Protozoa, 
as  described  beyond,  the  ''nucleolus"  is  shown  by  its  behaviour 
during  mitosis  to  be  comparable  with  an  attraction-sphere  or  centro- 
some  ('Muicleolo-centrosome,"  Keuten);  and  even  in  higher  forms 
there    are    some    cells    in    which    the    centrosome    is    intranuclear 

(Fig.  148). 

There  is  good  reason  to  believe  that  the  chromatin-nucleoli  are 
merely  more  condensed  portions  of  the  chromatin-network.  since 
during  cell-division  they  have  the  same  history  as  the  remaining 
portion  of  the  chromatin-substance.*  The  nature  of  the  true  nucleoli 
is  still  imperfectly  known.  By  some  observers,  including  Flemming, 
O.  and  R.  Hertwig,  and  Carnoy,  they  have  been  regarded  as  store- 
houses of  material  (para-nuclein,   plastin)  which    contributes  to    the 

^  Montgomery,  '98,  2. 

2  Flemining  first  called  attention  to  the  chemical  difference  between  the  true  nucleoli  and 
the  chromatic  reticulum  ('82,  pp.  138,  163)  in  animal-cells,  and  Zacharias  soon  afterward 
studied  more  closely  the  difference  of  staining  reaction  in  plant-cells,  showing  that  the 
former  are  especially  coloured  by  alkaline  carmine  solutions,  the  latter  by  acid  solutions. 
Other  studies  by  Carnoy,  Zacharias,  Ogata,  Rosen,  Schwarz,  Heidenhain,  and  many  others 
show  that  the  medullary  substance  (pyrenin)  of  true  nuclei  is  coloured  by  acid  tar-colours  and 
other  plasma  stains,  while  the  chromatin  has  a  special  affinity  for  basic  dyes.      Cf.  p.  337. 

3  For  very  full  review  of  the  literature  of  the  nucleoli  see  Montgomery  (  '98,  2). 

4  Cf.  p.  67. 


THE  NUCLEUS 


35 


formation  of  chromosomes  during  division,  and  hence  may  play  an 
active  role  in  the  nuclear  activity.  Strasburger  (  '95)  likewise  be- 
lieves them  to  contain  a  store  of  active  material  which,  however,  has 
no  direct  relation  to  the  chromosomes  but  consists  of  "  kinoplasm  " 


Fig.  14.  — Special  forms  of  nuclei. 

A.  Permanent  spireme-nucleus,  salivary  gland  of  Chirowmus  larva.  Chromatin  in  a  single 
thread,  composed  of  chromatin-discs  (chromomeres),  terminating  at  each  end  m  a  true  nucleolus 
or  plasmosonie.     [Balkianl] 

B.  Permanent   spireme-nuclei.   intestinal   epithelium   of  dipterous   larva   Ptychcptfra.     [\  AN 

Gehuchten.]     C.  The  same,  side  view. 

D.  Polymorphic  ring-nucleus,  giant-cell  of  bone-marrow  of  the  rabbit;  c.  a  group  of  ccntro- 

somes  or  centrioles.     [Heidenhain.] 

E.  Branching  nucleus,  spinning  gland  of  butterfly-larva  {Picris) .     [KORSCHELT.J 


(p.  322),  from  which  arises  the  achromatic  part  of  the  division- 
figure  (p.  82).  On  the  other  hand,  Hacker  (  '95,  '99)  ^^^^^  o^^er 
observers  regard  the  nucleolar  material  as  a  passive  by-product  of 
the    chromatin-activity   destined  to  be   absorbed   by  the  active    sub- 


36  GENERAL   SKETCH  OF   THE    CELL 

Stances.  This  is  supported  by  the  fact  that  in  some  forms  of  mitosis 
the  nucleokis  is  at  the  time  of  division  actually  cast  out  of  the 
nucleus  into  the  cytoplasm,  where  it  degenerates  without  further 
apparent  function.  This  seems  to  constitute  decisive  evidence  in 
support  of  Hacker's  view  as  api)lied  to  certain  cases ;  but  without 
further   evidence    it    must    remain    doubtful    whether    it    applies    to 

all.' 

d.  The  groiiiid-siihstancc,  nuclear  sap,  or  kavyolympJi,  a  clear  sub- 
stance occupying  the  interspaces  of  the  network  and  left  unstained 
by  most  of  the  dyes  that  colour  the  chromatin,  the  linin,  or  the  plas- 
mosomes.  By  most  observers  the  ground-substance  is  regarded  as  a 
liquid  filling  a  more  or  less  completely  continuous  space  traversed  by 
the  nuclear  network.  By  Biitschli,  however,  and  some  of  his  fol- 
lowers the  nucleus  is  regarded  as  an  alveolar  structure,  the  walls  of 
which  represent  the  "netw^ork,"  while  the  ground-substance  corre- 
sponds to  the  alveolar  material.  Nearly  related  with  this  is  the  view 
of  Reinke  (  94)  that  the  ground-substance  consists  of  large  pale 
frranules  of  'Manthanin  "  or  **  oedematin." 

The  configuration  of  the  chromatic  network  varies  greatly  in  dif- 
ferent cases.  It  is  sometimes  of  a  very  loose  and  open  character, 
as  in  many  epithelial  cells  (Fig.  i);  sometimes  extremely  coarse  and 
irregular,  as  in  leucocytes  (Fig.  49);  sometimes  so  compact  as  to 
appear  nearly  or  quite  homogeneous,  as  in  the  nuclei  of  spermatozoa 
and  in  many  Protozoa.  In  some  cases  the  chromatin  does  not  form 
a  network,  but  appears  in  the  form  of  a  thread  closely  similar  to  the 
spireme-stage  of  dividing  nuclei  (r/.  p.  65).  The  most  striking  case 
of  this  kind  occurs  in  the  salivary  glands  of  dipterous  \?.x\'^  {^Chiron 0- 
viHs\  where,  as  described  by  Balbiani,  the  chromatin  has  the  form  of 
a  single  convoluted  thread,  composed  of  transverse  discs  and  termi- 
nating at  each  end  in  a  large  nucleolus  (Fig.  14,  A).  Somewhat  simi- 
lar nuclei  (Fig.  14,  B)  occur  in  various  epithelial  cells  of  other  insects 
(Van  Gehuchten,  Gilson),  and  also  in  the  young  ovarian  eggs  of  cer- 
tain animals  (r/.  p.  273).  In  certain  gland-cells  of  the  marine  isopod 
Anilocra  it  is  arranged  in  regular  rosettes  (Vom  Rath).  Rabl,  fol- 
lowed bv  Van  Gehuchten,  Heidenhain,  and  others,  has  endeavoured 
to  show  that  the  nuclear  network  shows  a  distinct  polarity,  the 
nucleus  having  a  *'  pole "  toward  which  the  principal  chromatin- 
threads  converge,  and  near  which  the  centrosome  lies.-  In  many 
nuclei,  however,  no  trace  of  such  polarity  can  be  discerned. 

The  network  may  undergo  great  changes  both  in  physical  con- 
figuration and  in  staining  capacity  at  different  periods  in  the  life 
of  the  same  cell,  and  the  actual  amount  of  chromatin  fluctuates, 
sometimes  to  an  enormous  extent.     Embryonic  cells  are  in  general 

1  Cf.  pp.  126-130.  2  cf.  the  polarity  of  the  cell,  p.  55. 


THE  NUCLEUS 


17 


characterized  by  the  large  size  of  the  nucleus;  and  Zacharias  has 
shown  in  the  case  of  plants  that  the  nuclei  of  meristem  and  other 
embryonic  tissues  are  not  only  relatively  large,  but  contain  a  larger 
percentage  of  chromatin  than  in  later  stages.  The  relation  of  these 
changes  to  the  physiological  activity  of  the  nucleus  is  still  imperfectly 
understood.^ 

2.    Finer  Stnictiire  of  tJie  Nucleus 

A  considerable  number  of  observers 
have  raised  the  question  whether  the 
nuclear  structures  may  not  be  regarded 
as  aggregates  of  more  elementary 
morphological  bodies,  though  there  is 
still  no  general  agreement  regarding 
their  nature  and  relationships.  The 
most  definite  evidence  in  this  direction 
relates  to  the  chromatic  network.  In 
the  stages  preparatory  to  division  this 
network  resolves  itself  into  a  definite 
number  of  rod-shaped  bodies  known 
as  chroniosoines  (Fig.  21),  which  split 
lengthwise  as  the  cell  divides.  These 
bodies  arise  as  aggregations  of  minute 
rounded  bodies  or  microsomes  to  which 
various  names  have  been  g\vQx\{cJiroi)io- 
mcres,  Fol ;  ids,  Weismann).  They 
are  as  a  rule  most  clearly  visible  and 
most  regularly  arranged  during  cell- 
division,  when  the  chromatin  is  ar- 
ranged in  a  thread  {spireme),  or  in 
separate  cJironiosoines  (Figs.  8,  D,  53, 
B)\  but  in  many  cases  they  are  dis- 
tinctly visible   in   the    reticulum    of   the     scatteredchromatin-graniiles.  [GRUBKK.] 

"resting"    nucleus  ,(Fig.   54).      It  is, 

however,  an  open  question  whether  the  chromatin-granules  of  the 
reticulum  are  individually  identical  with  those  forming  the  chromo- 
somes  or   the    spireme-thread.       The   larger    masses    of    the    reticu- 


Fig.  15.  —  An    infusorian.     Trachelo- 
cerca,  with  diffused  nucleus  consisting  of 


1  Both  chromatin-granules  and  nucleoli  have  been  seen  in  a  considerable  number  of  living 
cells  (Fig.  9).  Favourable  objects  for  this  purpose  are  according  to  Korschelt  ('96)  the  silk- 
glands  of  caterpillars,  where  the  whole  nucleus  may  be  seen  to  be  filled  with  fine  granules 
("microsomes"),  among  which  are  scattered  many  larger  granules  ("  macrosomes  ").  'I  he 
later  studies  of  Meves  ('97,  i)  make  it  probable  that  the  latter  are  true  nucleoli  and  the  for- 
?r  chromatin-granules.  Korschelt,  however,  regards  the  "macrosomes"  as  composed  of 
romatin  and  the  "microsomes"  as  representing  the  so-called  "achromatic  sulistance." 


me 
ch 


38  GENERAL   SKETCH  OE   THE    CELL 

lum  undoubtedly  represent  aggregations  of  such  granules,  but  whether 
the  latter  completely  fuse  or  remain  always  distinct  is  unknown. 
Even  the  chromosomes  at  certain  stages  appear  perfectly  homoge- 
neous, and  the  same  is  sometimes  true  of  the  entire  nucleus,  as  in  the 
spermatozoon.  It  is  nevertheless  possible  that  the  chromatin-gran- 
ules  have  a  persistent  identity  and  are  to  be  regarded  as  morpho- 
logical units  of  which  the  chromatin  is  built  up.^ 

Heidenhain  ('93,  94),  whose  views  have  been  accepted  by  Rcinke, 
Waldeyer,  and  others,  has  shown  that  the  "achromatic"  nuclear  net- 
work is  likewise  composed  of  granules,  which  he  distinguishes  as 
/tint/ianiu-  or  <u;jr//;v;;/^?//;/-granules  from  the  has ic/iroiULi t i fi-gr3.\\\i\Q.s> 
of  the  chromatic  network.  Like  the  latter,  the  oxychromatin-granules 
are  suspended  in  a  non-staining  clear  substance,  for  which  he  reserves 
the  term  //;////.  Both  forms  of  granules  occur  in  the  chromatic 
network,  while  the  achromatic  network  contains  only  oxychromatin. 
Thev  are  sharply  differentiated  by  dyes,  the  basichromatin  being 
coloured  by  the  basic  tar-colours  (methyl-green,  saffranin,  etc.)  and 
other  true  "nuclear  stains";  while  the  oxychromatin-granules,  like 
many  cytoplasmic  structures,  and  like  the  substance  of  true  nucleoli 
(pyrenin),  are  coloured  by  acid  tar-colours  (rubin,  eosin,  etc.)  and 
other  "plasma  stains."  This  distinction,  as  will  appear  in  Chapter 
VI I., is  possibly  one  of  great  physiological  significance. 

Still  other  forms  of  granules  have  been  distinguished  in  the  nucleus 
by  Reinke  ('94)  and  Schloter  ('94).  Of  these  the  most  important 
are  the  "  oedematin-granules,"  which  according  to  the  first  of  these 
authors  form  the  principal  mass  of  the  ground-substance  or  "  nuclear 
sap  "  of  Hertwig  and  other  authors.  These  granules  are  identified 
by  both  observers  with  the  "  cyanophilous  granules,"  which  Altmann 
regarded  as  the  essential  elements  of  the  nucleus.  It  is  at  present 
impossible  to  give  a  consistent  interpretation  of  the  morphological 
value  and  physiological  relations  of  these  various  forms  of  granules. 
The  most  that  can  be  said  is  that  the  basichromatin-granules  are 
probably  normal  structures ;  that  they  play  a  principal  role  in  the 
life  of  the  nucleus  ;  that  the  oxychromatin-granules  are  nearly  related 
to  them  ;  and  that  not  improbably  the  one  form  may  be  transformed 
into  the  other  in  the  manner  suggested  in  Chapter  VII. 

The  nuclear  membrane  is  not  yet  thoroughly  understood,  and 
much  discussion  has  been  devoted  to  the  question  of  its  origin  and 
structure.  The  most  probable  view  is  that  long  since  advocated  by 
Klein  i^yd))  and  Van  Beneden  ('83)  that  the  membrane  arises  as  a 
condensation  of  the  general  protoplasmic  substance,  and  is  part  of 
the  same  structure  as  the  linin-network  and  the  cytoplasmic  mesh- 
w^ork.     Like  these,  it  is  in  some  cases  "achromatic,"  but  in  other  cases 

1  Cf.  Chapter  VI. 


THE   NUCLEUS 


39 


it  shows  the  same  staining  reactions  as  chromatin,  or  may  be  double, 
consisting  of  an  outer  achromatic  and  an  inner  chromatic  layer.  Ac- 
cording to  Reinke,  it  consists  of  oxychromatin-granules  like  those  of 
the  linin-network. 

Interesting  questions  are  raised  by  a  comparison  of  these  facts 
with  the  conditions  observed  in  some  of  the  lowest  organisms,  such 
as   the    flagellates    and    lower    rhizopods    among   animals    and    the 


A 


D 


«,     •    / 


B 


H 


O 


F 


% 


'%) 


7 


E 


or  distrib 


Fig  i6.- Forms  of  Cyanophyce^,  Bacteria,  and  Flagellates  showing  the  so-called  scattered 

distributed  nuclei.     [.4-C  BuTSCHLi;  Z?-/^  Schewiakoff;   G-y.  Calkin^.] 

A.   Oscillaria.     B.  Chromafmrn,     C.  Bacterium  lineola.     D.  Achrowatiuw.     E.  The  same  m 

division.     /;  Fission  of  the  granules.     G.    7l'//-</w////.^  with  central  sphereandscatteredgninu.es. 

H.  Aggregation  of  the  granules.     /.  Division  of  the  sphere.     J.  Fission  of  the  cell. 

Cyanophyce^  and  Bacteria  among  plants.  In  many  ol  these  forms 
(Fig.  i6)  no  distinct  nucleus  can  be  demonstrated,  the  cell  consistmg 
of  a  mass  of  protoplasm  in  which  are  scattered  numerous  deeply 
staining  granules.  Many  of  these  granules  stam  mtensely  with 
hematoxylin  and  other  " nuclear"  dyes;  like  chromatm,  they  resist 
the  action  of  peptic  digestion,  and  in  at  least  one  case  (the  bacterium- 
like  AckromatiHm,  according  to  Schewiakoff,  '93)  they  have  the  power 
of  division  like  the  chromatin-granules  of  higher  forms.     For  these 


40 


GENERAL   SKETCH   OF   THE    CELL 


reasons  most  observers  (Biitschli,  Gruber,  Schewiakoff,  Nadson,  etc.) 
recrard  them  as  true  chromatin-f(ranules  which  rein-esent  a  scattered  or 
distributed  nucleus  not  differentiated  as  a  definite  morphological  body. 
If  this  identification  is  correct,  such  forms  probably  give  us  the  most 
primitive  condition  of  the  nuclear  substance,  which  only  in  higher 
forms  is  collected  into  a  distinct  mass  enclosed  by  a  membrane;  and 
the  scattered  granules  are  comparable  to  those  forming  the  chro- 
matin-reticulum and  chromosomes  in  the  higher  types.  The  identi- 
fication is,  however,  difficult,  owing  to  the  impossibility  of  actual 
chemical  analysis;  and  Fischer  (97)  has  shown  in  the  case  of  the 
Bacteria  and  Cyanophycene  that  we  cannot  safely  trust  either  the 
staining  reactions  or  the  digestion  test,  since  the  former  are  variable, 
while  the  latter  does  not  differentiate  the  granules  from  some  other 
cytoplasmic  constituents.^  It  is,  however,  certain  that  the  staining 
power  of  chromatin  in  the  higher  forms  varies  with  different  condi- 
tions, and  furthermore  there  is  reason  to  believe  that  these  granules 
may  divide  by  fission.  Besides  these  observations  of  Schewiakoff  on 
AcJiromatiuin  (see  above),  we  have  those  of  several  authors  on 
Infusoria,  and  more  recently  those  of  Calkins  on  flagellates, 
both  pointing  to  the  same  conclusion.  Balbiani,  Gruber,  Maupas, 
and  others  have  described  various  Infusoria  (^/r^i-Zj/Z^',  Trac/icloccrca, 
HolosticJia,  Urolcptns),  as  well  as  some  rhizopods  {Pcloniyxa),  in 
which  the  body  contains  very  numerous  minute  chromatin-granules 
of  "nuclei"  (Fig.  15),  which  Gruber  {"^'J)  showed  to  multiply  by 
division.  Balbiani  ('61)  long  since  showed  that  in  Urostyla  these 
bodies  become  concentrated  toward  the  centre  of  the  cell  at  the  time 
of  division,  and  Bergh  ('89)  demonstrated  that  they  then  fuse  to  form 
a  macronucleus  of  the  usual  type,  that  elongates,  assumes  a  fibrillar 
structure,  and  divides  by  fission.  After  division  of  the  cell-body 
the  macronucleus  again  fragments  into  minute  scattered  granules, 
which  in  this  case  certainly  represent  a  distributed  nucleus.  In  the 
flagellate  Tctramitus  Calkins  ('98,  i)  likewise  finds  numerous  scat- 
tered chromatin-granules,  which  at  the  time  of  division  become  aggre- 
gated into  a  single  dividing  mass  (p.  92);  while  in  other  forms  the 
mass  (nucleus)  persists  as  such  without  (yTracJiclomouas,  Lagcnclla, 
CJiiloDionas)  or  with  {Euglcna,  Synura)  a  surrounding  membrane. 

Taken  together,  the  foregoing  facts,  while  certainly  not  conclusive, 
give  good  ground  for  the  provisional  acceptance  of  Biitschli's  con- 
ception of  the  distributed  nucleus,  and  indicate  that  nucleus  and 
cytoplasm  have  arisen  through  the  differentiation  of  a  common 
protoplasmic  mass.     The  nucleus,  as  Carnoy  has  well  said,-  is  like  a 


^  It  should  be  remembered  that  we  have  no  unerring  "  chromatin-stain."      Cf.  p.  335. 
=^'84,  p.  251. 


THE    CYTOPLASM  ^I 

house  built  to  contain  the  chromatic  elements,  and  its  achromatic  ele- 
ments (Unin,  etc.)  were  originally  a  part  of  the  general  cell-substance. 
Moreover,  as  Carnoy  points  out,  the  house  periodically  goes  to  pieces 
in  the  process  of  mitotic  division,  the  chromatin  afterward  "buildin"- 
for  itself  a  new  dwelling." 

3.    Chemist)')' of  tJie  Nucleus 

The  chemical  nature  of  the  various  nuclear  elements  will  be  considered  in  Chapter 
VII.,  and  a  brief  statement  will  here  suffice.  The  following  classification  of  the 
nuclear  substances,  proposed  by  Schwarz  in  1887,  has  been  widely  accepted,  thou'di 
open  to  criticism  on  various  grounds. 

1.  Chromatin.     The  chromatic  substance  (basichromatin)  of  the  network  and  of 

those  nucleoli  known  as  net-knots  or  karyosomes. 

2.  Linin.     The  achromatic  network  and  the  spindle  fibres  arising  from  it. 

3.  Paratiniii.     The  ground-substance. 

4.  Pyrcniii  or  Parachroniatiii.     The  inner  mass  of  true  nucleoli. 

5.  Ainphipyrenin.     The  substance  of  the  nuclear  membrane. 

Chromatin  is  probably  identical  with  imclein  (p.  332).  which  is  a  compound  of 
nucleiiiic  acid  (a  complex  organic  acid,  rich  in  phosphorus)  and  albuminous  sub- 
stances. In  certain  cases  (nuclei  of  spermatozoa,  and  probably  also  the  chromo- 
somes at  the  time  of  mitosis)  the  percentage  of  nucleinic  acid  is  very  large  (p.  'Ji'}^'},). 
The  /////;/  is  supposed  to  be  composed  of  "plastin''  —  a  substance  identified  -by 
Reinke  and  Rodewald  ('81)  and  probably  a  nucleo-albumin  or  a  related  substance. 
'•  Pyrenin  "  is  related  to  plastin  ;  and  Carnoy  and  Zacharias  apply  the  latter  word  to 
the  nucleolar  substance,  while  O.  Hertw'ig  calls  it  paranuclein.  "  Amphipyrenin"" 
has  no  very  definite  meaning ;  for  the  nuclear  membrane  sometimes  appears  to  be  of 
the  same  nature  as  the  linin,  w'hile  in  other  cases  it  stains  like  chromatin.  For  cri- 
tique of  the  staining  reactions  see  page  334. 


D.     The  Cytoplasm 

It  has  long  been  recognized  that  in  the  unicellular  forms  the 
cytoplasmic  substance  is  often  differentiated  into  two  well-marked 
zones :  viz.  an  inner  medullary  substance  or  cjuioplasui  in  which  the 
nucleus  lies,  and  an  outer  cortical  substance  or  exoplasm  (ectoplasm) 
from  which  the  more  differentiated  products  of  the  cytoplasm,  such 
as  cilia,  trichocysts,  and  membrane,  take  their  origin.  Indications  of 
a  similar  differentiation  are  often  shown  in  the  tissue-cells  of  higher 
plants  and  animals, ^  though  it  may  take  the  form  of  a  polar  differen- 
tiation of  the  cell-substance,  or  may  be  wholly  wanting.  Whether 
the  distinction  is  of  fundamental  importance  remains  to  be  seen  ;  but 
it  appears  to  be  a  general  rule  that  the  nucleus  is  surrounded  by 

1  This  fact  was  first  pointed  out  in  the  tissue-cells  of  animals  liy  Kupffer  ('75).  and  its 
importance  has  since  been  urged  by  Waldeyer,  Reinke,  and  others.  The  cortical  layer  is 
by  Kupffer  termed  paraplasm,  by  p'feffer  hyaloplasm,  by  Tringsheim  the  Haiitschiclit.  The 
medullary  zone  is  termed  by  Y.^x^^'^^x  protoplasm,  sensii  strictu;  by  Strasburger,  K'dmer- 
plasma  ;  by  ^'■k<g€(\,  polioplasm. 


42  GENERAL    SKETCH   OF   THE    CELL 

protoplasm  of  relatively  slight  differentiation,  while  the  more  highly 
differentiated  products  of  cell-activity  are  laid  down  in  the  more 
peripheral  region  of  the  cell,  either  in  the  cortical  zone  or  at  one 
end  of  the  cell.^  This  fact  is  full  of  meaning,  not  only  because  it  is 
an  expression  of  the  adaptation  of  the  cell  to  its  external  environment, 
but  also  because  of  its  bearing  on  the  problems  of  nutrition. ^  For  if, 
as  we  shall  see  reason  to  conclude  in  Chapter  VI  I., the  nucleus  be 
immediately  concerned  with  synthetic  metabolism,  we  should  expect 
to  find  the  immediate  and  less  differentiated  products  of  its  action  in 
its  neighbourhood,  and  on  the  whole  the  facts  bear  out  this  view. 

The  most  pressing  of  all  questions  regarding  the  cytoplasmic 
structure  is  whether  the  sponge-like,  fibrillar,  or  alveolar  appearance 
is  a  normal  condition  existing  during  life.  There  are  many  cases, 
especially  among  plant-cells,  in  which  the  most  careful  examination 
has  thus  far  failed  to  reveal  the  presence  of  a  reticulum,  the  cyto- 
plasm appearing,  even  under  the  highest  powers  and  after  the  most 
careful  treatment,  merely  as  a  finely  granular  substance.  This  and 
the  additional  fact  that  the  cytoplasm  may  show  active  streaming  and 
flowing  movements,  has  led  some  authors,  especially  among  bota- 
nists, to  regard  the  reticulum  as  non-essential  and  as  being,  when 
present,  either  a  secondary  differentiation  of  the  cytoplasmic  sub- 
stance specially  developed  for  the  performance  of  particular  functions 
or  a  mere  coagulation-product  due  to  the  action  of  fixatives.  It  has 
been  shown  that  structureless  proteids,  such  as  egg-albumin  and 
other  substances,  when  coagulated  by  various  reagents,  often  show  a 
structure  closely  similar  to  that  of  protoplasm  as  observed  in  micro- 
scopical sections.  Flemming  ('82)  long  since  called  attention  to  the 
danger  of  mistaking  such  coagulation-products  for  normal  structures 
as  seen  in  fixed  and  stained  material,  and  his  warning  has  been 
emphasized  by  the  later  experiments  of  Berthold  {"^6\  Schwarz  i^^jX 
and  especially  of  Butschli  ('92,  '98),  Fischer  ('94,  '95,  '99),  and 
Hardy  ('99).  Butschli's  extensive  studies  of  such  coagulation-phe- 
nomena show  that  coagulated  or  dried  albumin,  starch-solutions,  gela- 
tin, gum  arable,  and  other  substances  show  a  fine  alveolar  structure 
scarcely  to  be  distinguished  from  that  which  he  believes  to  be  the 
normal  and  typical  structure  of  protoplasm.  Fischer  and  Hardy 
have  likewise  made  extensive  tests  of  solutions  of  albumin,  peptone, 
and  related  substances,  in  various  degrees  of  concentration,  fixed  and 
stained  by  a  great  variety  of  the  reagents  ordinarily  used  for  the 
demonstration  of  cell-structures.  The  result  was  to  produce  a  mar- 
vellously close  simnlacniui  of  the  appearances  observed  in  the  cell, 
alveolar,  reticulated,  and  fibrillar  structures  being  produced  that  often 
contain  granules  closely  similar  in  every  respect  to  those  described  as 

1  Cf.  p.  55.  2  See  Kupfter  ('90),  pp.  473-476- 


THE    CYTOPLASM 


43 


"microsomes  "  in  sections  of  actual  protoplasm.  After  impregnating 
pith  with  peptone-solution  and  then  hardening,  sectioning,  and  stain- 
ing, the  cells  may  even  contain  a  central  nucleus-like  mass  suspended 
in  a  network  of  anastomosing  threads  that  extend  in  every  direction 
outward  to  the  walls,  and  give  a  remarkable  likeness  of  a  normal  cell. 
These  facts  show  how  cautious  we  must  be  in  judging  the  appear- 
ances seen  in  preserved  cells,  and  justify  in  some  measure  the  hesita- 


¥s 


miMi!! 


mm 


ajEiR»i>JXiin:nnia«>uiiftuiniff 


B 


i\u 


w,u 


iviV^*i»»«»»»«»>»«»»?«»jv'!*»«*»<' 


::.«.-V".»..*"'„^ti.  ■ 


y^ 


D 


Fig.  17. —  Ciliated  cells,  showing  cytoplasmic  fibriilcTe  terminating  in  a  zone  of  peripheral 
microsomes  to  which  the  cilia  are  attached.     [Engelmann.] 

A.  From  intestinal  epithelium  of  Anodonta.  B.  From  gill  of  Anodonta.  CD.  Intestinal  epi- 
thelium of  Cyclas. 

tion  with  which  many  existing  accounts  of  cell-structure  are  received. 
The  evidence  is  nevertheless  overwhelmingly  strong,  as  I  believe, 
that  not  only  the  fibrillar  and  alveolar  formations,  but  also  the  micro- 
somes observed  in  cell-structures,  are  in  part  normal  structures.  This 
evidence  is  derived  partly  from  a  study  of  the  living  cell,  partly  from 
the  resfular  and  characteristic  arrangement  of  the  thread-work  and 


44 


GENERAL   SKETCH  OF   THE    CELL 


microsomes  in  certain  cases.  In  many  Protozoa,  for  example,  a  fine 
alveolar  structure  may  be  seen  in  the  living  jirotoplasm  ;  and  Flem- 
ming  as  well  as  manv  later  observers  has  clearly  seen  fibrillar  struc- 
tures in  the  living  cells  of  cartilage,  epithelium  connective-tissue,  and 
some  other  animal  cells  (  Fig.  9).  Mikosch,  also,  has  recently  described 
^;v7;///A7/' threads  in  living  plant-cells. 

Almost  equally  conclusive  is  the  beautifully  regular  arrangement 
of  the  fibrillct'  in  ciliated  cells  ( Fig.  17,  Fngelmann),  in  muscle-fibres 
and  ncrve-hbres,  and  especially  in  the  mitotic  figure  of  dividing  cells 


B 


C 


D 


Fig.  18.  —  Cells  of  the  pancreas  in  Amphibia.     [Mathfavs.] 

A-C.  Nectiiriis ;  D.  Rami.  A  and  H  represent  two  stages  of  the  "  loaded "  cell,  showing 
zymogen-granules  in  the-peripheral  and  fibrillar  structures  in  tlie  basal  part  of  the  cell.  C  shows 
cells  after  discharge  of  the  granule-material  and  invasion  of  the  entire  cell  by  fibrillie.  In  D  por- 
tions of  the  fibrillar  material  are  coiled  to  form  the  mitosome  ("  paranucleus  "  or  "  Nebenkern  "}. 


(Figs.  2  1,  31),  where  they  are  likewise  more  or  less  clearly  visible 
in  life.  A  very  convincing  case  is  afforded  by  the  pancreas-cells 
of  Nccturiis,  which  Mathews  has  carefully  studied  in  my  laboratory. 
Here  the  thread-work  consists  of  long,  conspicuous,  defmite  fibrillae, 
some  of  which  may  under  certain  conditions  be  wound  up  more  or 
less  closely  in  a  spiral  mass  to  form  the  so-called  Nebcjikern.  In  all 
these  cases  it  is  impossible  to  regard  the  thread-work  as  an  accidental 
coagulation-product.  In  the  case  of  echinoderm  eggs,  I  have  made 
('99)  a  critical  comparison  of  the  living  structure,  as  seen  under  powers 


THE    CYTOPLASM 


45 


of  a  thousand  diameters  and  upwards,  with  the  same  ohject  stained 
in  thin  sections  after  fixation    by  picro-acetic,   subHmate-acetic,  and 


*•/.*••■;    ■    ,.'-•,■ 


«I^- 


^*^;s^SsyiiJ^i^/(^j^J^ 


Fig.  19.  —  Section  through  a  nephridia!  cell  of  the  leech,  Clepstite  (drawn  by  Arnold  Graf  from 

one  of  his  own  preparations). 

The  centre  of  the  cell  is  occupied  by  a  large  vacuole,  filled  with  a  watery  licinid.  The  cyto- 
plasm forms  a  very  regular  and  distinct  reticulum  with  scattered  microsomes  which  become  very 
large  in  the  peripheral  zone.  The  larger  pale  bodies,  lying  in  the  ground-substance,  are  cvcretory 
granules  {i.e.  metaplasm).  The  nucleus,  at  the  right,  is  surrounded  by  a  thick  chromatic  mem- 
brane, is  traversed  by  a  very  distinct  linin-network,  contains  numerous  scattered  chromatin- 
granules,  and  a  single  large  nucleolus  within  which  is  a  vacuole.  Above  are  two  isolated  nuclei 
showing  nucleoli  and  chromatin-granules  suspended  in  the  linin-threads. 


Other  reagents.  The  comparison  leaves  no  doubt  that  the  normal 
structures  are  in  this  case  very  perfectly  preserved,  thoui^h  the  sec- 
tions give  at  first  sight  an  appearance  somewhat  different  from  that 


46  GEXERAL   SKE7CII  OF   THE    CELL 

of  the  living  object,  owing  to  differences  of  staining  capacity.  In 
these  eggs  the  microsomes,  thickly  scattered  through  the  alveolar 
walls,  stain  deeply  (Figs.  ii.  i  J ),  while  the  alveolar  spheres  hardly 
stain  at  all.  When,  therefore,  the  stained  sections  are  cleared  in 
balsam,  the  contours  of  the  alveolar  spheres  almost  disappear,  and  the 
eye  is  caught  by  the  walls,  which  give  at  first  sight  quite  the  appear- 
ance of  a  granular  reticulum,  as  it  has  been  in  fact  described  by  many 
observers.  Careful  study  of  the  sections  shows,  however,  that  t  lie  form 
and  arraui^cjuiut  of  all  the  elements  is  almost  idcntieally  t/ie  same  as 
in  life. 

This  result  shows  that  careful  treatment  by  reagents  in  some  cases 
at  least  gives  a  very  faithful  picture  of  the  normal  structure  ;  and 
while  it  should  never  be  forgotten  that  in  sections  we  are  viewing 
coagulated  material,  much  of  which  is  liquid  or  semi-liquid  in  life,  w^ 
should  not  adopt  too  pessimistic  a  view  of  the  results  based  on  fixed 
material,  as  I  think  some  of  the  experimenters  referred  to  above  have 
done.  Wherever  possible,  the  structures  observed  in  sections  should 
be  compared  with  those  in  the  living  material.  When  this  is  imprac- 
ticable we  must  rely  on  indirect  evidence;  but  this  is  in  many  cases 
hardly  less  convincing  than  the  direct. 

It  is  a  very  interesting  and  important  question  whether  living 
protoplasm  that  appears  to  the  eye  to  be  homogeneous  does  not  really 
possess  a  structure  that  is  invisible,  owing  to  the  extreme  tenuity  of 
the  fibrillar  or  alveolar  walls  (as  was  long  since  suggested  by  Heitz- 
mann  and  Biitschli),^  or  to  uniformity  of  refractive  index  in  the 
structural  elements.  It  is  highly  prc^bable  that  such  is  often  the  case; 
indeed,  Butschli  has  shown  that  such  *'  homogeneous  "  protoplasm  in 
Protozoa  may  show  a  typical  alveolar  structure  after  fixation  and 
staining.  This  explanation  will  not,  however,  apply  to  the  young 
echinoderm  eggs  (already  referred  to  at  p.  28),  where  the  genesis  of 
the  alveolar  structure  may  be  follow'ed  step  by  step  in  the  li\ing  cell. 
The  protoplasm  here  appears  at  first  almost  like  glass,  showing  at 
most  a  sparse  and  fine  granulation  ;  but  after  fixing  and  staining  it 
appears  as  a  mass  of  fine,  closely  crowded  granules.  This  may  indi- 
cate the  existence  of  an  extremely  fine  alveolar  structure  in  life;  but 
on  the  whole  I  believe  that  these  granules  are  for  the  most  part  coagu- 
lation-products, since  they  cannot  be  demonstrated  by  staining  ijitra 
vitam,  and  they  very  closely  resemble  the  coagulation-granules  found 
in  structureless  proteids  like  egg-albumin  after  treatment  by  the  same 
reagents.  In  common  with  many  other  investigators,  therefore,  I 
believe  that  protoplasm  may  in  fact  be  homogeneous  dowji  to  the 
present  limits  of  microscopical  vision. 

One  of  the  must  beautiful  forms  of  cyto-reticulum  with  which  I 

1  Cf  Butschli,  '92,  2,  p.  169. 


THE    CYTOPLASM 


A7 


am  acquainted  has  been  described  by  Bolsius  and  Graf  in  the  ncnh 
ridial  cells  of  leeches  as  shown  in  Fig.  19  (from   a   preparation  by 
Dr.  Arnold  Graf).     The  meshwork  is  here  of  great  di.stinctness  and 
regularity,  and  scattered  microsomes  are  found  along  its  threads.      It 


Fig.  20.  —  Spinal  ganglion-cell  of  the  frog.  [Lknhossek.] 
The  nucleus  contains  a  single  intensely  chromatic  nucleolus,  and  a  paler  linin-network  with 
rounded  chromatin-granules.  The  cytoplasmic  fibrillae  are  faintly  shown  passing  out  into  the 
nerve-process  below.  (They  are  figured  as  far  more  distinct  by  Flemming.)  Tlie  dark  cyto- 
plasmic masses  are  the  deeply  staining  "  chromophiiic  granules"  (.N'issi)  of  unknown  function. 
(The  centrosome,  which  lies  near  the  centre  of  the  cell,  is  shown  in  Fig.  8,  C.)  At  the  left,  two 
connective  tissue-cells. 


appears  with  equal  clearness,  though  in  a  somewhat  different  form, 
in  many  eggs,  where  the  meshes  are  rounded  and  often  contain  food- 
matters  or  deutoplasm  in  the  inter-spaces  (Figs.  59,  60).  In  cartilage- 
cells  and  connective  tissue-cells,  where  the  threads  can  be  plainly  seen 


48  GENERAL   SKETCH  OF   THE    CELL 

in  life,  the  network  is  loose  and  open,  and  appears  to  consist  of  more 
or  less  completely  separate  threads  (F'ig.  9).  In  the  cells  of  colum- 
nar epithelium,  the  threads  in  the  peripheral  part  of  the  cell  often 
assume  a  more  or  less  parallel  course,  passing  outwards  from  the 
central  region,  and  giving  the  outer  zone  of  the  cell  a  striated  appear- 
ance. This  is  very  conspicuously  shown  in  ciliated  epithelium,  the 
fibrill:^  corresponding  in  number  with  the  cilia  as  if  continuous  with 
their  bases  (Fig.  17).^  In  nerve-fibres  the  threads  form  closely  set 
parallel  fibrillce  which  may  be  traced  into  the  body  of  the  nerve-cell; 
here,  according  to  most  authors,  they  break  up  into  a  network  in 
which  are  suspended  numerous  deeply  staining  masses,  the  "chromo- 
philic  granules"  of  Nissl  (Fig.  20).-  In  the  contractile  tissues  the 
threads  are  in  most  cases  very  conspicuous  and  have  a  parallel  course. 
This  is  clearly  shown  in  smooth  muscle-fibres  and  also,  as  Ballowite 
has  shown,  in  the  tails  of  spermatozoa.  This  arrangement  is  most 
striking  in  striped  muscle-fibres  where  the  fibrillae  are  extremely  well 
marked.  According  to  Retzius,  Carnoy,  Van  Gehuchten,  and  others, 
the  meshes  have  here  a  rectangular  form,  the  principal  fibrillar  having 
a  longitudinal  course  and  being  connected  at  regular  intervals  by 
transverse  threads  ;  but  the  structure  of  the  muscle-fibre  is  probably 
far  more  complicated  than  this  account  would  lead  one  to  suppo.se, 
and  opinion  is  still  divided  as  to  whether  the  contractile  substance 
is  represented  by  the  reticulum  proper  or  by  the  ground-substance. 

Nowhere,  perhaps,  is  a  fibrillar  structure  shown  with  such  beauty  as 
in  dividing  cells,  where  (Figs.  21,  31)  the  fibrillae  group  themselves 
in  two  radiating  systems  or  asters,  which  are  in  some  manner  the 
immediate  agents  of  cell-division.  Similar  radiating  systems  of  fibres 
occur  in  amoeboid  cells,  such  as  leucocytes  (Fig.  49)  and  pigment- 
cells  (Fig.  50),  where  they  probably  form  a  contractile  system  by 
means  of  which  the  movements  of  the  cell  are  performed. 

The  views  of  Biitschli  and  his  followers,  which  have  been  touched 
on  at  p.  25,  differ  considerably  from  the  foregoing,  the  fibrillae  being 
regarded  as  the  optical  sections  of  thin  plates  or  lamelte  which  form 
the  walls  of  closed  chambers  filled  by  a  more  liquid  substance. 
Butschli,  followed  by  Rcinke,  Eismond,  Erlanger,  and  others,  inter- 
prets in  the  same  sense  the  astral  systems  of  dividing  cells  which 
are  regarded  as  a  radial  configuration  of  the  lamellae  about  a  central 
point  (Fig.  10,  B).     Strong  evidence  against  this  view  is,  I  believe, 

1  The  structure  of  the  ciliated  cell,  as  described  by  Engelmann,  may  be  beautifully  demon- 
strated in  the  funnel-cells  of  the  nephridia  and  sperm-ducts  of  the  earthworm. 

2  The  remarkable  researches  of  Apathy  ('97)  on  the  nerve-cells  of  leeches  have  revealed 
the  existence  within  the  nerve-cell  of  networks  far  more  complex  and  definite  than  was 
formerly  supposed,  and  showing  definite  relations  to  incoming  and  outgoing  fibrillae  within 
the  substance  of  the  nerve-fibres. 


THE    CYTOPLASM 


49 


afforded  by  the  appearance  of  the  spindle  and  asters  in  cross-section. 
In  the  early  stages  of  the  ^g^  of  Nereis,  for  example,  the  astral  rays 
are  coarse  anastomosing  fibres  that  stain  intensely  and  are  therefore 
very  favourable  for  observation  (Fig.  60).  That  they  are  actual  fibres 
is,  I  think,  proved  by  sagittal  sections  of  the  asters  in  which  the  rays 
are  cut  at  various  angles.  The  cut  ends  of  the  branching  rays  appear 
in  the  clearest  manner,  not  as  plates  but  as  distinct  dots,  from  which 
in  obHque  sections  the  ray  may  be  traced  inwards  toward  the  centro- 
sphere.  Druner,  too,  figures  the  spindle  in  cross-section  as  consisting 
of  rounded  dots,  like  the  end  of  a  bundle  of  wires,  thou^-h  these  are 
connected  by  cross-branches  (Fig.   28,  F).     Again,  the  crossin^r  of 


Centrospherc  con- 
taining the  cen- 
trosome. 


Aster. 


Spindle. 


Chromosomes  forming  the  equatorial  plate. 

Fig.  21.  —  Diagram  of  the  dividing  cell,  showing  the  mitotic  figure  and  its  relation  to  the  cyto- 
plasmic meshwork. 

the  rays  proceeding  from  the  asters  (Fig.  128),  and  their  beha^-iour  in 
certain  phases  of  cell-division,  is  difficult  to  explain  under  any  other 
than  the  fibrillar  theory. 

We  must  admit,  however,  that  the  meshwork  varies  greatly  in  differ- 
ent cells  and  even  in  different  physiological  phases  of  the  same  cell ; 
and  that  it  is  impossible  at  present  to  bring  it  under  any  rule  of  uni- 
versal application.  It  is  possible,  nay  probable,  that  in  one  and  the 
same  cell  a  portion  of  the  meshwork  may  form  a  true  alveolar  structure 
such  as  is  described  by  Biitschli,  while  other  portions  may.  at  the 
same  time,  be  differentiated  into  actual  fibres.  If  this  be  true  the 
fibrillar  or  alveolar  structure  is  a  matter  of  secondary  moment,  and 
the  essential  features  of  protoplasmic  organization  must  be  sought  in 
a  more  subtle  underlying  structure.^ 

1  See  Chapter  VI. 
E 


50  GENERAL   SKETCH  OF  THE    CELL 

Space  would  not  sufifice  for  a  comparative  account  of  the  endless 
modifications  shown  by  the  cytoplasmic  substance  in  different  forms 
of  cells.  Many  of  these  arise  through  special  differentiations  of  the 
active  substance,  the  character  of  the  structure  thus  being  some- 
times so  highly  modified,  as  in  the  striated  muscle-fibre,  that  it  is 
difficult  to  trace  its  exact  relation  to  the  more  usual  forms.  More 
commonly  the  cytoplasm  is  modified  through  the  formation  of  passive 
or  metaplasmic  substances  which  often  completely  transform  the 
original  appearance  of  the  cell.  The  most  frequent  of  such  modifi- 
cations arise  through  the  deposit  of  liquid  drops  and  *' granules " 
(many  of  the  latter,  however,  being  no  doubt  liquid  in  life).  When 
the  liquid  drops  arc  of  watery  nature  the  cavities  in  which  they  lie 
are  known  as  vacuoles^  which  are  especially  characteristic  of  the  pro- 
toplasm of  plant-cells  and  of  Protozoa.  These  may  enlarge  or  run 
together  to  form  extensive  cavities  in  the  cell,  the  protoplasm  becom- 
ing reduced  to  a  peripheral  layer,  or  to  strands  and  networks  travers- 
ing the  spaces ;  while  in  some  forms  of  unicellular  glands  the  spaces 
may  form  branching  canals  traversing  the  protoplasm. 

The  vacuolization  or  meshlike  appearance  arising  through  the 
formation  of  larger  vacuoles  or  the  deposit  of  other  metaplasmic 
material  is  not  to  be  confounded  with  the  primary  protoplasmic  struc- 
ture. When,  however,  smaller  vacuoles  or  metaplasmic  granules  are 
evenly  distributed  through  the  protoplasm,  a  "  pseudo-alveolar  "  struc- 
ture (Reinke)  arises  that  can  often  hardly  be  distinguished  from  the 
"  true  "  alveolar  structure  of  Blitschli.^  Comparative  study  shows 
that  all  gradations  exist  between  the  "false  "  and  the  "true  "  alveolar 
structures  and  that  no  logical  ground  of  distinction  between  the  two 
exists.^  We  thus  reach  ground  for  the  conclusion  that  the  coarser 
secondary  alveolar  or  reticular  formations  are  to  be  regarded  as  only 
an  exaggeration  of  the  primary  structure,  and  that  the  alveolar  mate- 
rial of  *Biitschli's  structure  belongs  in  the  same  general  category  with 
the  passive  or  metaplasmic  substance.^ 


E.     The  Centrosome 

The  centrosome^  is  usually  an  extremely  minute  body,  or  more 
commonly  a  pair  of  bodies,  staining  intensely  with  haematoxylin  and 

^  In  the  latter  the  alveolar  spheres  are,  according  to  Biitschli,  not  more  than  one  or  two 
microns  in  diameter. 

^  This  has  been  demonstrated  in  the  cells  of  plants  by  Craio  ('96),  and  more  recently 
by  the  writer  ('99),  in  the  case  of  echinoderm  and  other  eggs. 

^  QC  p.  29. 

*  The  centrosome  was  apparently  first  seen  and  described  by  Flemming  in  1875,  ^"  ^^^ 
egg  of  the  fresh-water  mussel  Anodonta,  and  independently  discovered  by  Van  Beneden,  in 


THE    CENTROSOME  5  I 

some  other  reagents,  and  surrounded  by  a  cytoplasmic  radiating  aster 
or  by  a  rounded  mass  known  as  the  attr'actio}i-spJicrc  (Figs.  8,  49,  etc.). 
As  a  rule  it  lies  in  the  cytoplasm,  not  far  from  the  nucleus,  and 
usually  opposite  an  indentation  or  bay  in  the  latter ;  but  in  a  few 
cases  it  lies  inside  the  nucleus  (Fig.  148).  In  epithelia  the  centro- 
somes  (usually  double)  lie  as  a  rule  near  the  free  end  of  the  cell 
(Fig.   21)} 

There  is  still  much  confusion  regarding  the  relation  of  the  centro- 
some  to  the  surrounding  structures,  and  this  has  involved  a  corre- 
sponding ambiguity  in  the  terminology.  We  w^ill  therefore  only 
consider  it  briefly  at  this  point,  deferring  a  more  critical  account  to 
Chapter  VI.  In  its  simplest  form  it  is  a  single  minute  granule,  which 
may,  however,  become  double  or  triple  (leucocytes,  connective  tissue- 
cells,  some  epithelial  cells)  or  even  multiple,  as  in  certain  giant-cells 
(Fig.  14,  D\  and  as  also  occurs  in  some  forms  of  cell-division  (Fig. 
52).  In  some  cases  (Figs.  8,  C,  120,  148)  the  **  centrosome "  is  a 
larger  body  containing  one  or  more  central  granules  or  '*  centrioles  " 
(Boveri);  but  it  is  probable  that  in  some  of  these  cases  the  central 
granule  is  itself  the  true  centrosome,  and  the  surrounding  body  is  part 
of  the  attraction-sphere.  During  the  formation  of  the  spermatozoon 
the  centrosome  undergoes  some  remarkable  morphological  changes 
(p.  171),  and  is  closely  involved  in  the  formation  of  the  contractile 
structures  of  the  tail. 

The  nature  and  functions  of  the  centrosome  have  formed  the  sub- 
ject of  some  of  the  most  persistent  and  searching  investigations  of 
recent  cytology.  Van  Beneden,  followed  by  Boveri  and  many  later 
workers,  regarded  the  centrosome  as  a  distinct  and  persistent  cell- 
organ,  which  Hke  the  nucleus  was  handed  on  by  division  from  one 
cell-generation  to  another.  Physiologically  it  was  regarded  as  being 
the  especial  organ  of  cell-division,  and  in  this  sense  as  the  *' dy- 
namic centre"  of  the  cell.     In  Boveri's  beautiful  development  of  this 

the  following  year,  in  dycyemids.  The  name  is  due  to  Boveri  ('88,  2,  p.  68).  Van  Beneden's 
and  Boveri's  independent  identification  of  centrosome  in  Ascaris  as  a  permanent  cell-organ 
('87)  was  quicklv  supported  by  numerous  observations  on  other  animals  and  on  plants.  In 
rapid  succession  the  centrosome  and  attraction-sphere  were  found  to  be  present  m  pig- 
ment-cells of  fishes  (Solger,  '89,  '90),  in  the  spermatocytes  of  Amphil)ia  (Hermann.  Vto),  hi 
the  leucocytes,  endothelial  cells,  connective  tissue-cells,  and  lung-epithelium  of  salamanders 
(Flemming,  '91),  in  various  plant-cells  (Guignard,  '91),  in  the  one-celled  diatoms  (Butschli. 
'91),  in  the  giant-cells  and  other  cells  of  bone-marrow  (Heidenhain,  Van  Ban.heke,  \  an  der 
Stricht,  '91),  in  the  flagellate  Noctiluca  (Ishikawa,  '91),  in  the  cells  of  marine  alg:v  (Stras- 
burger,  '92),  in  cartilage-cells  (Van  der  Stricht,  '92),  in  cells  of  cancerous  growths  j epitheli- 
oma, Lustig  and  Galeotti,  '92),  in  the  young  germ-cells  as  already  described,  in  gland-cells 
(Vom  Rath,  '95),  in  nerve-cells  (Lenhossek,  '95),  in  smooth  muscle-hbres  (Lenhossek,  99), 
and  in  embryonic  cells  of  manv  kinds  (Heidenhain,  '97)-  •'^'any  others  have  conhrmed 
and  extended  this  list.  Guignard's  identification  of  the  centrosomes  in  higher  plants  i. 
open  to  grave  doubt  (r/  p.  82).  Q-  P-  57- 


52  GEXERAL    SKETCH   OF   THE    CELL 

view  it  was  regarded  further  as  the  especial  fertiHzing  element  in  the 
spermatozoon,  which,  when  introduced  into  the  fgg,  endowed  the 
latter  with  the  power  of  division  and  development.  Van  Beneden's 
and  Boveri's  hypothesis,  highly  attractive  on  account  of  its  simplicity 
and  lucidity,  is  supported  by  many  facts,  and  undoubtedly  contains  an 
element  of  truth  ;  yet  recent  researches  have  cast  grave  doubt  upon 
its  generality,  and  necessitate  a  suspension  of  judgment  upon  the 
entire  matter.  Many  of  the  most  competent  recent  workers  on  the 
cytologv  of  higher  plants  have  been  unable  to  find  centrosomes, 
whether  in  the  resting-cells,  in  the  apparatus  of  cell-division,  or  dur- 
ing the  process  of  fertilization,  notwithstanding  the  fact  that  undoubted 
centrosomes  occur  in  some  of  the  lower  plants.  Among  zoologists, 
too,  an  increasing  number  of  recent  investigators,  armed  with  the 
best  technique,  have  maintained  the  total  disappearance  of  the  cen- 
trosome  at  the  close  of  cell-division  or  during  the  process  of  fertili- 
zation, agreeing  that  in  such  cases  the  centrosome  is  subsequently 
formed  dc  novo.  Experimental  researches,  also,  have  given  strong 
ground  for  the  conclusion  that  cells  placed  under  abnormal  chemical 
conditions  may  form  new  centrosomes  (p.  306).  If  these  strongly 
supported  results  be  well  founded.  Van  Beneden's  hypothesis  must 
be  abandoned  in  favour  of  the  view  that  the  centrosome  is  but  a  sub- 
ordinate part  of  the  general  apparatus  of  mitosis,  and  one  which  may 
be  entirelv  dispensed  with.  Thus  regarded,  the  centrosome  would 
lose  somewhat  of  the  significance  first  attributed  to  it,  though  still 
remaining  a  highly  interesting  object  for  further  research.^ 

F.     Other  Organs 

The  cell-substance  is  often  differentiated  into  other  more  or  less 
definite  structures,  sometimes  of  a  transitory  character,  sometimes 
showing  a  constancy  and  morphological  persistency  comj^arable  with 
that  of  the  nucleus  and  centrosome.  From  a  general  point  of  view 
the  most  interesting  of  these  are  the  bodies  known  as /A? jr//V/jr  ox  proto- 
plasts{Y\g.  6),  which,  like  the  nucleus  and  centrosome,  are  capable  of 
growth  and  division,  and  may  thus  be  handed  on  from  cell  to  cell. 
The  most  important  of  these  are  the  cJirouiatopJiores  or  cJironioplastids, 
which  are  especially  characteristic  of  plants,  though  they  occur  in 
some  animals  as  well.  These  are  definite  bodies,  varying  greatly 
in  form  and  size,  which  possess  the  power  of  growth  and  division,  and 
have  in  some  cases  been  traced  back  to  minute  colourless  plastids  or 

^  Cf.  pp.  Ill,  304.  Eisen  ('97)  asserts  that  in  the  blood  of  a  salamander,  Bairachoseps, 
the  attraction-sphere  (•'  archosome  ")  containing  the  centrosomes  may  separate  from  the 
remainder  of  the  cell  (nucleated  red  corpuscles)  to  form  an  independent  form  of  blood- 
corpuscle  or  "  plasmocyte,"  which  leads  an  active  life  in  the  blood. 


OTHER    ORGANS  53 

leucoplastids  in  the  embryonic  cells.  By  enlargement  and  differen- 
tiation these  give  rise  to  the  starch-builders  (amyloplastids),  to  the 
chlorophyll-bodies  (chloroplastids),  and  to  other  ])igment-bodies 
(chromoplastids),  all  of  which  may  retain  the  power  of  division.  The 
embryonic  leucoplastids  are  also  believed  to  multiply  by  division  and 
to  arise  by  the  division  of  plastids  in  the  parental  organism  ;  but  it 
remains  an  open  question  whether  this  is  their  only  mode  of  origin, 
and  the  same  is  true  of  the  more  highly  differentiated  forms  of  plas- 
tids to  which  they  may  give  rise. 

The  contractile  or  pulsating  vacuoles  that  occur  in  most  Protozoa 
and  in  the  swarm-spores  of  many  Algae  are  also  known  in  some 
cases  to  multiply  by  division  ;  and  the  same  is  true,  according  to  the 
researches  of  De  Vries,.  Went,  and  others,  of  the  non-pulsating  vacu- 
oles of  plant-cells.  These  vacuoles  have  been  shown  to  have,  in  many 
cases,  distinct  walls,  and  they  are  regarded  by  De  Vries  as  a  special 
form  of  plastid  ("tonoplasts  ")  analogous  to  the  chromatophores  and 
other  plastids.  It  is,  however,  probable  that  this  view  is  only  appli- 
cable to  certain  forms  of  vacuoles. 

The  plastids  possess  in  some  cases  a  high  degree  of  morphological 
independence,  and  may  even  live  for  a  time  after  removal  from 
the  remaining  cell-substance,  as  in  the  case  of  the  "yellow  cells"  of 
Radiolaria.  This  has  led  to  the  view,  advocated  by  Brandt  and  others, 
that  the  chlorophyll-bodies  found  in  the  cells  of  many  Protozoa  and 
a  few  Metazoa  {Hydra,  Spongilla,  some  planarians)  are  in  reality  dis- 
tinct Alg^  living  symbiotically  in  the  cell.  This  view  is  probably 
correct  in  some  cases,  e.g.  in  the  Radiolaria ;  but  it  may  be  doubted 
whether  it  is  of  general  appUcation.  In  the  plants  the  plastids  are. 
almost  certainly  to  be  regarded  as  differentiations  of  the  protoplasmic 

substance. 

The  existence  of  cell-organs  which  have  the  power  of  independent 
assimilation,  growth,  and  division  is  a  fact  of  great  theoretical  interest 
in  its  bearing  on  the  general  problem  of  cell-organization  ;  for  it  is 
one  of  the  main  reasons  that  have  led  De  Vries,  Wiesner,  and  many 
others  to  regard  the  entire  cell  as  made  up  of  elementary  self-propa- 
gating units. 

G.     The  Cell-membrane 

The  structure  and  origin  of  the  cell-wall  or  membrane  form  a 
subject  somewhat  apart  from  our  general  purpose,  since  the  wall 
belongs  to  the  passive  or  metaplasmic  products  of  protoplasm  rather 
than  to  the  living  cell  itself.  We  shall  therefore  treat  it  very  briefly. 
Broadly  speaking,  animal  cells  are  in  general  characterized  by  the 
slight  development  and  relative  unimportance  of  the  cell-walls,  while 


54  GEXERAL   SKETCH   OE   THE    CELL 

the  reverse  is  the  case  in  plants,  where  the  cell-walls  play  a  very 
important  role.  In  the  latter  the  wall  sometimes  attains  a  great 
thickness,  usually  displays  a  distinct  stratification,  and  often  has  a 
complex  sculj)ture.  Such  massive  walls  very  rarely  occur  in  the 
case  of  animal  tissues,  though  the  intercellular  matrix  of  cartilage 
and  bone  is  to  a  certain  extent  analogous  to  them,  and  the  thick  and 
often  highly  sculptured  envelopes  of  some  kinds  of  eggs  and  of 
various  Protozoa  may  be  placed  in  the  same  category. 

It  is  open  to  question  whether  any  cells  are  entirely  devoid  of  an 
enclosing  envelope;  for  even  in  such  ** naked"  cells  as  leucocytes, 
rhizopods,  or  membraneless  eggs,  the  boundary  of  the  cell  is  usually 
formed  by  a  more  resistant  layer  of  protoplasm  or  "  pellicle  "(Biitschli) 
which  may  be  so  marked  as  to  simulate  a  true  membrane,  as  is  the 
case,  for  example,  in  the  red  blood-corpuscles  (Ranvier,  Waldeyer, 
etc.).  Such  pellicles  probably  differ  from  true  membranes  only  in 
degree ;  but  it  is  still  an  open  question  both  in  animals  and  in  plants, 
how  far  true  membranes  arise  by  direct  transformation  of  the  periph- 
eral protoplasmic  layer  (the  "  Hautschicht "  of  botanists),  and  how 
far  as  a  secretion-product  of  the  protoplasm.  In  the  case  of  animal 
cells,  Leydig  long  since  proposed  ^  to  distinguish  between  "  cuticular  " 
membranes,  formed  as  secretions  and  usually  occurring  only  on  the 
free  surfaces  (as  in  epithelia),  from  *'  true  membranes  "  arising  by 
direct  transformation  of  the  peripheral  protoplasm.  Later  researches, 
including  those  of  Leydig  himself,  have  thrown  so  much  doubt  on 
this  distinction  that  most  later  writers  have  used  the  term  cuticular 
in  a  purely  topographical  sense  to  denote  membranes  formed  only 
on  one  (the  free)  side  of  the  ccll,^  leaving  open  the  question  of  origin. 
The  formation  and  growth  of  the  cell-wall  have  been  far  more  thor- 
oughly studied  in  plants  than  in  animals,  yet  even  here  opinion  is 
still  divided.  Most  recent  researches  tend  to  sustain  the  early  view 
of  Nageli  that  the  cell-wall  is  in  general  a  secretion-product,  though 
there  are  some  cases  in  which  a  direct  transformation  of  protoplasm 
into  membrane-stuff  seems  to  occur. ^  In  the  division  of  plant-cells 
the  daughter-cells  are  in  almost  all  cases  cut  apart  by  a  cell-plate 
which  arises  in  the  protoplasm  of  the  mother-cell  as  a  transverse 
series  of  thickenings  of  the  spindle-fibres  in  the  equatorial  region 
(Fig.  34).  This  fact,  long"  regarded  by  Strasburger  and  others  as 
a  proof  of  the  direct  origin  of  the  membrane  from  the  protoplasmic 
substance,  is  shown  by  Strasburger's  latest  work  ('98)  to  be  open 
to  a  quite  different  interpretation,  the  actual  wall  being  formed  by 
a  splitting  of  the  cell-plate  into  two  layers  between  which  the  wall 
appears  as  a  secretion-product.  Almost  all  observers  further  are 
ao:reed  that  the  formation  of  new  membranes  on  naked  masses  of 

1  Cf.  '85,  p.  12.  "^  C/.O.  Hertwig,  '93.  »  Cf.  Strasburger,  '98. 


POLARITY   OF   THE    CELL 


55 


protoplasm  produced  by  plasmolysis  are  likewise  secretion-products, 
and  that  the  secondary  thickening  of  plant-membranes  is  produced 
in  the  same  way.  These  facts,  together  with  the  scanty  available 
zoological  data,  indicate  that  the  formation  of  membranes  by  secre- 
tion is  the  more  usual  and  typical  process. ^ 

The  chemical  composition  of  the  membrane  or  intercellular  sub- 
stance varies  extremely.  In  plants  the  membrane  consists  of  a  basis 
of  cellidose,  a  carbohydrate  having  the  formula  CgHjoOg  ;  but  this  sub- 
stance is  very  frequently  impregnated  with  other  substances,  such 
as  silica,  Hgnin,  and  a  great  variety  of  others.  In  animals  the  inter- 
cellular substances  show  a  still  greater  diversity.  Many  of  them  are 
nitrogenous  bodies,  such  as  keratin,  chitin,  elastin,  gelatin,  and  the 
like ;  but  inorganic  deposits,  such  as  silica  and  carbonate  of  lime,  are 
common. 

H.     Polarity  of  the  Cell 

In  a  large  number  of  cases  the  cell  exhibits  a  definite  polarity,  its 
parts  being  symmetrically  grouped  with  reference  to  an  ideal  organic 
axis  passing  from  pole  to  pole.  No  definite  criterion  for  the  identi- 
fication of  the  cell-axis  has,  however,  yet  been  determined ;  for  the 
general  conception  of  cell-polarity  has  been  developed  in  two  differ- 
ent directions,  one  of  which  starts  from  purely  morphological  con- 
siderations, the  other  from  physiological,  and  a  parallelism  between 
them  has  not  thus  far  been  fully  made  out. 

On  the  one  hand.  Van  Beneden  ('83)  conceived  cell-polarity  as  a 
primary  morphological  attribute  of  the  cell,  the  organic  axis  being 
identified  as  a  line  drawn  through  the  centre  of  the  nucleus  and  the 
centrosome  (Fig.  22,  A).  With  this  view  Rabl's  theory  ('85)  of 
nuclear  polarity  harmonizes,  for  the  chromosome-loops  converge 
toward  the  centrosome,  and  the  nuclear  axis  coincides  with  the  cell- 
axis.  Moreover,  it  identifies  the  polarity  of  the  Qg^.  which  is  so 
important  a  factor  in  development,  with  that  of  the  tissue-cells;  tor 
the  egg-centrosome  almost  invariably  appears  at  or  near  one  pole  of 

the  ovum. 

Heidenhain  ('94,  '95)  has  recently  developed  this  conception  of 
polarity  in  a  very  elaborate  manner,  maintaining  that  all  the  struc- 
tures of  the  cell  have  a  definite  relation  to  the  primary  axis,  and  that 
this  relation  is  determined  by  conditions  of  tension  in  the  astral  rays 

1  Strasburger  ('97,  3,  '98)  believes  membrane-formation  in  general  to  be  especially  con- 
nected with  the  activity  of  the  "kinoplasm,"  or  tilar  plasm  of  which  he  considers  the  "  Haut- 
schicht,"  as  well  as  the  spindle-fibres,  to  be  largely  composed.  In  support  ol  this  may  be 
mentioned,  besides  the  mode  of  formation  of  the  partition-walls  in  the  division  of  plant- 
cells,  Harper's  ('97)  very  interesting  observations  on  the  formation  of  the  ascospores  m 
Erysiphe  (Fig.  IZ),  where  the  spore-membrane  appears  to  arise  directly  from  the  astral 
rays. 


56 


GENERAL   SKETCH   OE   THE    CELL 


focussed  at  the  centrosome.  On  this  basis  he  endeavours  to  explain 
the  position  and  movements  of  the  nucleus,  the  succession  of  division- 
planes,  and  many  related  phenomena.^ 

Hatschek  {^^^)  and  Rabl  ('89,  92),  on  the  other  hand,  have  ad- 
vanced a  quite  different  hypothesis  based  on  physiological  considera- 
tions. By  "cell-polarity"  these  authors  mean,  not  a  predetermined 
morphological  arrangement  of  parts  in  the  cell,  but  a  polar  differen- 
tiation of  the  cell-substance  arising  secondarily  through  adaptation  of 
the  cell  to  its  environment  in  the  tissues,  and  having  no  necessary 
relation  to  the  polarity  of  Van  Beneden  (Fig.  22,  B,  C).     This  is 


, 

\ 

r.. 

.  ^ 

• 

• 

•  • . 

•  0 

• 

• 

•  •• 

• 

• 

•  0 

9  •    • 

.  • 

•  • 

1 

A 

Van  Beneden. 


B  C 

Rabl,  Hatschek. 


Fig.  22.  —  Diagrams  of  cell-polarity. 

A.  Morphological  polarity  of  Van  Beneden.  Axis  passing  through  nucleus  and  centrosome. 
Chromatin-threads  converging  toward  the  centrosome.  B.C.  Physiological  polarity  of  Rabl  and 
Hatschek,  Zj'  in  a  gland-cell,  C'in  a  ciliated  cell. 

typically  shown  in  epithelium,  which,  as  Kolliker  and  Haeckel  long 
since  pointed  out,  is  to  be  regarded,  both  ontogenetically  and  phy- 
logenetically,  as  the  most  primitive  form  of  tissue.  The  free  and 
basal  ends  of  the  cells  here  differ  widely  in  relation  to  the  food- 
supply,  and  show  a  corresponding  structural  differentiation.  In  such 
cells  the  nucleus  usually  lies  nearer  the  basal  end,  toward  the  source 
of  food,  while  the  differentiated  products  of  cell-activity  are  formed 
either  at  the  free  end  (cuticular  structures,  cilia,  pigment,  zymogen- 
granules),  or  at  the  basal  end  (muscle-fibres,  nerve-fibres).  In  the 
non-epithelial  tissues  the  polarity  may  be  lost,  though  traces  of  it 
are  often  shown  as  a  survival  of  the  epithelial  arrangement  of  the 
embryonic  stages. 

1  Cf.  p.  105. 


POLARITY  OF   THE    CELL 


57 


But,  although  this  conception  of  polarity  has  an  entirely  different 
point  of  departure  from  Van  Beneden's,  it  leads,  in  some  cases  at 
least,  to  the  same  result ;  for  the  cell-axis,  as  thus  determined,  may 
coincide  with  the  morphological  axis  as  determined  by  the  position 
of  the  centrosome.  This  is  the  case,  for  example,  with  both  the 
spermatozoon  and  the  ovum  ;  for  the  morphological  axis  in  both  is 
also  the  physiological  axis  about  which  the  cytoplasmic  differentia- 
tions are  grouped.  Recent  researches  have  further  shown  that  the 
same  is  the  case  in  many  forms  of  epithelia,  where  the  centrosomes 
lie  in  the  outer  end  of  the  cell,  often  very  near  the  surface.^     (Fig-  '2-1) 


A 


B 


WM) 

km   ■ 

te^J-i 

'.'.i 

C  •  D 

Fig.  23.  — Centrosomes  in  epithelial  and  other  cells.     [A,  D,  ZiMMERMANN  ;  E,  Heidenhain 

and  COHN;  F,  Heidenhain.] 

A.  From  gastric  glands  of  man ;  dead  cell  at  the  left.  B.  Uterine  epithelium,  man.  C.  From 
human  duodenum  ;  goblet-cell,  with  centrosome  in  the  middle.  D.  Corneal  epithelium  of  monkev. 
E.  Epithelial  cells  from  mesoblast-somites,  embryo  duck.  F.  Red  blood-corpuscles  from  the  duck- 
embryo.     The  centrosomes  are  double  in  nearly  all  cases. 

and  the  recent  observations  of  Henneguy  ('98)  and  Lenhossek  (98,1) 
give  reason  to  believe  that  the  ** basal  bodies"  to  which  the  ciHa  of 
ciliated  epithelium  are  attached  may  be  the  centrosomes.-  These 
facts  are  of  very  high  significance;  for  the  position  of  the  centro- 
some, and  hence  the  direction  of  the  axis,  is  here  obviously  related 
to  the  cell-environment,  and  it  is  difficult  to  avoid  the  conclusion  that 
the  latter  must  be  the  determining  condition  to  which  the  intracellular 
relations  conform.  When  applied  to  the  germ-cells,  this  conclusion 
becomes  of  high  interest ;  for  the  polarity  of  the  Qgg  is  one  of  the 


1  Zimmermann,  '98;  Heidenhain  and  Cohn,  '97. 


2  cf.  p.  356. 


58  GEXERAL   SKETCH   OF   THE    CELL 

primary  conditions  of  development,  and  we  have  here,  as  I  beUeve, 
a  clue  to  its  determination.^ 


I.     Till:  Cell  in  Relation  to  the  Multicellular  Body 

In  analyzing  the  structure  and  functions  of  the  individual  cell  we 
are  accustomed,  as  a  matter  of  convenience,  to  regard  it  as  an  inde- 
pendent elementary  organism  or  organic  unit.  Actually,  however, 
it  is  such  an  organism  only  in  the  case  of  the  unicelkilar  j)lants  and 
animals  and  the  germ-cells  of  the  multicellular  forms.  When  we 
consider  the  tissue-cells  of  the  latter,  we  must  take  a  somewhat  dif- 
ferent view.  As  far  as  structure  and  origin  are  concerned  the  tissue- 
cell  is  unquestionably  of  the  same  morphological  value  as  the 
one-celled  plant  or  animal ;  and  i)i  tliis  sense  the  multicellular  body 
is  equivalent  to  a  colony  or  aggregate  of  one-celled  forms.  Physi- 
ologically, however,  the  tissue-cell  can  only  in  a  limited  sense  be 
regarded  as  an  independent  unit ;  for  its  autonomy  is  merged  in  a 
greater  or  less  degree  into  the  general  life  of  the  organism.  From 
this  point  of  view  the  tissue-cell  must  in  fact  be  treated  as  merely 
a  localized  area  of  activity,  provided  it  is  true  with  the  complete 
apparatus  of  cell-life,  and  even  capable  of  independent  action 
within   certain   limits,  yet   nevertheless  a  part   and   not  a   whole. 

There  is  at  present  no  biological  question  of  greater  moment  than 
the  means  by  which  the  individual  cell-activities  are  coordinated,  and 
the  organic  unity  of  the  body  maintained  ;  for  upon  this  question 
hangs  not  only  the  problem  of  the  transmission  of  acquired  charac- 
ters, and  the  nature  of  development,  but  our  conception  of  life  itself. 
Schwann,  the  father  of  the  cell-theory,  very  clearly  perceived  this ; 
and  after  an  admirably  lucid  discussion  of  the  facts  known  to  him 
(*39),  drew  the  conclusion  that  the  life  of  the  organism  is  essentially 
a  composite  ;  that  each  cell  has  its  independent  life  ;  and  that  "  the 
whole  organism  subsists  only  by  means  of  the  reciprocal  action  of  the 
single  elementary  parts."  ^  This  conclusion,  afterward  elaborated  by 
Virchow  and  Haeckel  to  the  theory  of  the  **  cell-state,"  took  a  very 
strong  hold  on  the  minds  of  biological  investigators,  and  is  even  now 
widely  accepted.  It  is,  however,  becoming  more  and  more  clearly 
apparent  that  this  conception  expresses  only  a  part  of  the  truth,  and 
that  Schwann  went  too  far  in  denying  the  influence  of  the  totality  of 
the  organism  upon  the  local  activities  of  the  cells.  It  would  of 
course  be  absurd  to  maintain  that  the  whole  can  consist  of  more  than 
the  sum  of  its  parts.     Yet,  as  far  as  growth  and  development  are  con- 

^  Cf.  pp.  384,  424.  We  should  remember  that  the  germ-cells  are  themselves  epithelial 
products.  2  Untersuchungen,  Trans.,  p.  181. 


THE    CELL    LN  RELATION   TO    THE   MULTICELLULAR   BODY         59 

cerned,  it  has  now  been  clearly  demonstrated  that  only  in  a  Hmited 
sense  can  the  cells  be  regarded  as  cooperating  units.  Thcv  are 
rather  local  centres  of  a  formative  power  pervading  the  growing 
mass  as  a  whole,^  and  the  physiological  autonomy  of  the  individual 
cell  falls  into  the  background.  It  is  true  that  the  cells  may  acquire 
a  high  degree  of  physiological  independence  in  the  later  stages  of 
embryological  development.  The  facts  to  be  discussed  in  the  eighth 
and  ninth  chapters  will,  however,  show  strong  reason  for  the  conclu- 
sion that  this  is  a  secondary  result  of  development,  through  which  the 
cells  become,  as  it  were,  emancipated  in  a  greater  or  less  degree 
from  the  general  control.  Broadly  viewed,  therefore,  the  life  of  the 
multicellular  organism  is  to  be  conceived  as  a  whole ;  and  the  appar- 
ently composite  character  which  it  may  exhibit  is  owing  to  a  second- 
ary distribution  of  its  energies  among  local  centres  of  action.  ^ 

In  this  light  the  structural  relations  of  tissue-cells  become  a  ques- 
tion of  great  interest ;  for  we  have  here  to  seek  the  means  by  which 
the  individual  cell  comes  into  relation  with  the  totality  of  the  organ- 
ism, and  by  which  the  general  equihbrium  of  the  body  is  maintained. 
It  must  be  confessed  that  the  results  of  microscopical  research  have 
not  thus  far  given  a  very  certain  answer  to  this  question.  Though 
the  tissue-cells  are  often  apparently  separated  from  one  another  by  a 
non-living  intercellular  substance,  which  may  appear  in  the  form  of 
solid  walls,  it  is  by  no  means  certain  that  their  organic  continuity  is 
thus  actually  severed.  Many  cases  are  known  in  which  division  of 
the  nucleus  is  not  followed  by  division  of  the  cell-body,  so  that  multi- 
nuclear  cells  or  syncytia  are  thus  formed,  consisting  of  a  continuous 
mass  of  protoplasm  through  which  the  nuclei  are  scattered.  Heitz- 
mann  long  since  contended  (  '73),  though  on  insufficient  evidence,  that 
division  is  incomplete  in  nearly  all  forms  of  tissue,  and  that  even  when 
cell-walls  are  formed  they  are  traversed  by  strands  of  protoplasm  by 
means  of  which  the  cell-bodies  remain  in  organic  continuity.  The 
whole  body  was  thus  conceived  by  him  as  a  syncytium,  the  cells 
being  no  more  than  nodal  points  in  a  general  reticulum,  and  the  body 
forming  a  continuous  protoplasmic  mass. 

This  interesting  view,  long  received  with  scepticism,  has  been  to  a 
considerable  extent  sustained  by  later  researches,  and  though  it  still 
x^m-d^x^-^siibjiidice,  has  been  definitely  accepted  in  its  entirety  by  some 
recent  workers.  The  existence  of  protoplasmic  cell-bridges  between 
the  sieve-tubes  of  plants  has  long  been  known  ;  and  Tangl's  dis- 
covery, in  1879,  of  similar  connections  between  the  endosperm-cells 
was  followed  by  the  demonstration  by  Gardiner,  Kienitz-Gerloff,  A. 
Meyer,  and  many  others,  that  in  nearly  all  plant-tissues  the  cell-walls 

1  (7:  Chapters  VIII..  IX. 

2  for  a  fuller  discussion  see  pp.  388  and  413. 


60  GENERAL   SKETCH   OF   THE    CELL 

are  traversed  by  delicate  intercellular  bridges.  Similar  bridges  have 
been  conclusively  demonstrated  by  Ranvier,  J^izzozero,  Rctzius,  Flem- 
ming,  Pfitzner,  and  many  later  observers  in  nearly  all  forms  of  epithe- 
lium ( Fig.  I ) ;  and  they  are  asserted  to  occur  in  the  smooth  muscle-fibres, 
in  cartilage-cells  and  connective  tissue-cells,  and  in  some  nerve- 
cells.  Dendy  ('88),  Paladino  (  '90),  and  Retzius  ('89)  have  endeav- 
oured to  show,  further,  that  the  follicle-cells  of  the  ovary  are 
connected  by  protoplasmic  bridges  not  only  with  one  another,  but  also 
with  tJic  oviiDi ;  and  similar  protoplasmic  bridges  between  germ-cells 
and  somatic  cells  have  been  also  demonstrated  in  a  number  of  plants, 
e.g.  by  Goroschankin  (  '83)  and  Ikeno  (  '98)  in  the  cycads  and  by  A. 
Meyer  ('96)  in  ]\)hox.  On  the  strength  of  these  observations  some 
recent  writers  have  not  hesitated  to  accept  the  probability  of  Heitz- 
mann's  original  conception,  A.  Meyer,  for  example,  expressing  the 
opinion  that  both  the  plant  and  the  animal  individual  are  continuous 
masses  of  protoplasm,  in  which  the  cytoplasmic  substance  forms  a 
morphological  unit,  whether  in  the  form  of  a  single  cell,  a  multi- 
nucleated cell,  or  a  system  of  cells. ^  Captivating  as  this  hypothesis 
is,  its  full  acceptance  at  present  would  certainly  be  premature  ;  and 
as  far  as  adult  animal  tissues  are  concerned,  it  still  remains  unde- 
termined how  far  the  cells  are  in  direct  protoplasmic  continuity.  It 
is  obvious  that  no  such  continuity  exists  in  the  case  of  the  corpuscles 
of  blood  and  lymph  and  the  wandering  leucocytes  and  pigment-cells. 
In  case  of  the  nervous  system,  which  from  an  a  priori  point  of  view 
would  seem  to  be  above  all  others  that  in  which  protoplasmic  con- 
tinuity is  to  be  expected,  its  occurrence  and  significance  are  still  a 
subject  of  debate.  When,  however,  we  turn  to  the  embryonic  stages 
we  find  strong  reason  for  the  belief  that  a  material  continuity  between 
cells  here  exists.  This  is  certainly  the  case  in  the  early  stages  of 
many  arthropods,  where  the  whole  embryo  is  at  first  an  unmistakable 
syncytium  ;  and  Adam  Sedgwick  has  endeavoured  to  show  that  in 
Pcripatns  and  even  in  the  vertebrates  the  entire  embryonic  body,  up 
to  a  late  stage,  is  a  continuous  syncytium.  I  have  pointed  out  (  '93) 
that  even  in  a  total  cleavage,  such  as  that  of  AnipJiioxus  or  the  echi- 
noderms,  the  results  of  experiment  on  the  early  stages  of  cleavage 
are  difficult  to  explain,  save  under  the  assumption  that  there  must 
be  a  structural  continuity  from  cell  to  cell  that  is  broken  by  mechan- 
ical displacement  of  the  blastomeres.  This  conclusion  is  supported 
by  the  recent  work  of  Hammar  (  '96,  '97),  whose  observations  on 
sea-urchin  eggs  I  can  in  the  main  confirm. 

Among    the    most    interesting  observations    in    this    direction   are 
those  of  Mrs.  Andrews  ('97),^  who  asserts  that  during  the  cleavage 

1  '96,  p.  212.      Cf.  also  the  views  of  Hanstein,   Strasburger.    Russow,   and   others    there 
cited.  ^  Cf.  also  E.  A.  Andrews,  '98,  I,  '98,  2. 


THE    CELL   IN  RELATION   TO    THE  MULTICELLULAR  BODY        6 1 

of  the  echinoderm-egg  the  blastomeres  **  spin  "  dehcate  protoplasmic 
filaments,  by  which  direct  protoplasmic  continuity  is  established 
between  them  subsequent  to  each  division.  These  observations,  if 
correct,  are  of  high  importance  ;  for  if  protoplasmic  connections  may 
be  broken  and  re-formed  at  will,  as  it  were,  the  adverse  evidence 
of  the  blood-corpuscles  and  wandering  cells  loses  much  of  its  weight. 
Meyer  ('96)  adduces  evidence  that  in  Volvox  the  cell-bridges  are 
formed  anew  after  division ;  and  Flemming  has  also  shown  that 
when  leucocytes  creep  about  among  epithehal  cells  they  rupture  the 
protoplasmic  bridges,  which  are  then  formed  anew  behind  them.^ 

We  are  still  almost  wholly  ignorant  of  the  precise  physiological 
meaning  of  the  cell-bridges ;  but  the  facts  indicate  that  they  are  not 
merely  channels  of  nutrition,  as  some  authors  have  maintained,  but 
paths  of  subtler  physiological  impulse.  Beside  the  facts  determined 
by  the  isolation  of  blastomeres,  referred  to  above,  may  be  placed 
Townsend's  recent  remarkable  experiments  on  plants,  described  at 
page  346.  If  correct,  these  experiments  give  clear  evidence  of  the 
transference  of  physiological  influences  from  cell  to  cell  by  means  of 
protoplasmic  bridges,  showing  that  the  nucleus  of  one  cell  may  thus 
control  the  membrane-forming  activity  in  an  enucleated  fragment 
of  another  cell.  The  field  of  research  opened  up  by  these  and 
related  researches  seems  one  of  the  most  promising  in  view;  but 
until  it  has  been  more  fully  explored,  judgment  should  be  reserved 
regarding  the  whole  question  of  the  occurrence,  origin,  and  physio- 
logical meaning  of  the  protoplasmic  cell-bridges. 


LITERATURE.     I  •'- 

Altmann,  R.--Die  Elementarorganismen  und  ihre  Beziehungen  zu  den  Zellen,  2d 

ed.     Leipzig,  1894. 
TAnnee  Biologique.  — /^^7;v>,  1895-96.     (Full  Reviews  and  Literature-lists.) 
Bohm  and  Davidoff .  —  Lehrbuch  der  Histologic  des  Menschen.     Wiesbaden,  1895. 
Boveri,  Th.—  (See  Lists  IV.,  V.) 
Biitschli,  0.  —  Untersuchungen  liber  mikroskopische  Schaume  und  das  Protoplasma. 

Zt'/^z^s/;^  (Engelmann),  1892. 
Id.  —  Untersuchungen  liber  Struktur.     Leipzig,  1898. 
Carnoy,  J.  B.  —  La  Biologie  Cellulaire.     /./Wvr.  1884. 
Engelmann,  T.  W.  — Zur  Anatomic  und  Pliysiologie  der  Flimmerzellen:  Arch.  ges. 

Phvs.,  XXIII.     1880.  ,  .^    , 

Erlanger,  R.  v.     Neuere   Ansichten   liber   die   Struktur  des   Protoplasmas :    ZooL 

CentralbUUl-^j9'     1896. 
Fischer,  A.    Fixierung,  Farbung  und  Bau  des  Protoplasmas.    Jemu  1899. 
Flemming,  W.     Zellsubstanz,  Kern  und  Zellteikmg.     Leipzig,  1882. 
Id.     ZeW^:  MerkelundBonHet^sErgebnisse,\.-\'n.     1891-97-     (Admirable  reviews 

and  literature-lists.) 

1  '95,  pp.  lo-i  I ;    '97,  p.  261 .  '  See  also  Introductory  list,  p.  14. 


62  GENERAL   SKETCH  OF   THE    CELL 

Heidenhain,  M.  —  Uber  Kern  und  Protoplasma :  FestscJir.  z.  ^o-Jii/ir.  Doctorjub.  7'Oh 

7'.  KolUkcr.     Leipzig^  i^93- 
Klein,  E.  —  Observations  on  the  Structure  of  Cells  and  Nuclei  :    (J/^d^'i-  Journ.  Mic. 

.SV/..  Will.      1878. 
Kolliker,  A. —  Handbucii  der  Gewebelehre,  6th  ed.     Leipzig,  1889. 
Leydig,  Fr.  —  Zelle  und  Gewebe.     Boiui.  1885. 
Schafer,  E.  A.  —  General  Anatomy  or  Histology;    in  Quaifi's  Anatof/ty,  I.,  2,  loth 

ed.     London,  1891. 
Schiefferdecker  &  Kossel.  —  Die  Gewebe  des  Menschlichen  Korpers.    Braunschweig, 

I  Sc)  I . 
Schwarz,  Fr.  —  Die   morphologische  und   chemische  Zusammcnsetzung  des  Proto- 

plashias.     Jyres/au,  1887. 
Strasburger,  E.  —  Zcllbildung  und  Zellteilung.  3d  ed.     1880. 
Id.  —  Das  Botanische  Practicum.  3d  ed.    Jena.  1897. 
Strasburger,  Noll,  Schenck,  and  Schimper.  —  Lehrbuch  der  Botanik,  3d  ed.    Jenay 

1897. 
Strieker,  S.  —  Handbuch  der  Lehre  von  den  Geweben.     Leipzig,  1871. 
Thoma,  R.  —  Text-book  of  General  Pathology  and  Pathological  Anatomy:  trans,  by 

Alex.  Bruce.     London,  1896. 
Van  Beneden,  E.  —  (See  Lists  II..  IV.) 
De  Vries,  H.  —  Intracellulare  Pangenesis,    /ena,  1889. 
Waldeyer,  W.  —  Die  neueren  Ansichten  liber  den  Bau  und  das  Wesen  der  Zelle : 

Deu'.sch.  Med.  IVoc/ienschr.,  Oct.,  Nov.,  1895. 
Wiesner,  J.  —  Die    Elementarstruktur   u.    das  Wachstum    der   lebenden    Substanz : 

U'ien,  Holder.     1892. 
Wilson,  E.  B.  —  The  Structure  of  Protoplasm:  Journ.  Morph.,  XV.  Suppl. ;    also 

Wood's  U oil  Biol.  Lectures,  1899. 
Zimmermann,  A.  —  Beitrage   zur  Morphologic  und  Physiologic  der  Pflanzenzelle. 

Tubingen,  1893. 
Id.  —  Die  Morphologic  und  Physiologic  des  Pflanzlichen  Zellkernes.    Jena,  1896. 


CHAPTER    II 

CELL-DIVISION 

"  Wo  eine  Zelle  entsteht,  da  muss  eine  Zelle  vorausgegangen  sein,  ebenso  wie  das  Thier 
nur  aus  deni  Thiere,  die  Pflanze  nur  aus  der  Pflanze  entstehen  kann.  Auf  diese  Weise  ist 
wenngleich  es  einzelne  Punkte  im  Korper  gibt,  wo  der  strenge  Nachweis  noch  nicht  gelie- 
fert  ist,  doch  das  Princip  gesichert,  dass  in  der  ganzen  Reihe  alles  Lebendigen,  dies  mogen 
nun  ganze  Pflanzen  oder  thierische  Organismen  oder  integrirende  Theile  derselben  sein,  ein 
ewiges  Gesetz  der  coiithiuir lichen  EntzvicklunghQ.%i&\i\.r  ViKCHow.^ 

The  law  of  genetic  cellular  continuity,  first  clearly  stated  by  Vir- 
chow  in  the  above  words,  has  now  become  one  of  the  primary  data 
of  biology,  and  the  advance  of  research  is  ever  adding  weight  to  the 
conclusion  that  the  cell  has  no  other  mode  of  origin  than  by  division 
of  a  preexisting  cell.  In  the  multicellular  organism  all  the  tissue- 
cells  arise  by  continued  division  from  the  original  germ-cell,  and 
this  in  its  turn  arises  by  the  division  of  a  cell  preexisting  in  the 
parent-body.  By  cell-division,  accordingly,  the  hereditary  substance 
is  split  off  from  the  parent-body ;  and  by  cell-division,  again,  this 
substance  is  handed  on  by  the  fertilized  egg-cell  or  oosperm  to  every 
part  of  the  body  arising  from  it.^  Cell-division  is,  therefore,  one  of 
the  central  facts  of  development  and  inheritance. 

The  first  two  decades  after  Schleiden  and  Schwann  ('40-'6o)  were 
occupied  with  researches,  on  the  part  both  of  botanists  and  of  zool- 
ogists, which  finally  demonstrated  the  universality  of  this  process 
and  showed  the  authors  of  the  cell-theory  to  have  been  in  error  in 
asserting  the  independent  origin  of  cells  out  of  a  formative  blastema.'"^ 
The  mechanism  of  cell-division  was  not  precisely  investigated  until 
long  afterward,  but  the  researches  of  Remak  ('41),  Kolliker  (44), 
and  others  showed  that  an  essential  part  of  the  process  is  a  division 
of  both  the  nucleus  and  the  cell-body.  In  1855  {I.e.,  pp.  174,  175),  and 
again  in  1858,  Remak  gave  as  the'  general  result  of  his  researches 
the  following  synopsis  or  scheme  of  cell-division.  Cell-division,  he 
asserted,  proceeds  from  the  centre  toward  the  periphery.  It  begins 
with  the  division  of  the  nucleolus,  is  continued  by  simple  constriction 
and  division  of  the  nucleus,  and  is  completed  by  division  of  the  cell- 

1  Cellularpathologie,  p.  25,  1858.  2  Cf.  Introduction,  p.  10. 

3  For  a  full  historical  account  of  this  period,  see  Remak,  Untersttchungen  iiher  die  Ent- 
wickhmg  derWirbelthiere,  1855,  PP-  ^ 64-1 80.  Also  Tyson  on  the  Cell-doctrine  and  Sachs's 
Geschichte  der  Botajiik. 

63 


64 


CELL-DIVISION 


body  and  membrane  (Fig.  24).  For  many  years  this  account  was 
accepted,  and  no  essential  advance  beyond  Rcmak's  scheme  was 
made  for  nearly  twenty  years.  A  number  of  isolated  observations 
were,  however,  from  time  to  time  made,  even  at  a  very  early  period, 
which  seemed  to  show  that  cell-division  was  by  no  means  so  simple 
an  operation  as  Remak  believed.  In  some  cases  the  nucleus  seemed 
to  disappear  entirely  before  cell-division  (the  germinal  vesicle  of  the 
ovum,  according  to  Reichert,  Von  Ikier,  Robin,  etc.);  in  others  to 
become  lobed  or  star-shaped,  as  described  by  Virchow  and  by  Remak 
himself  (Fig.  24,/).  It  was  not  until  1873  that  the  way  was  opened 
for  a  better  understanding  of  the  matter.  \\\  this  year  the  discoveries 
of  Anton  Schneider,  quickly  followed  by  others  in  the  same  direction 
by  Biitschli,  Fol,  Strasburger,  Van  Beneden,  Flemming,  and  Hertwig, 
showed  cell-division  to  be  a  far  more  elaborate  process  than  had  been 

supposed,  and  to  involve  a  com- 
plicated transformation  of  the 
nucleus  to  which  Schleicher 
('78)  afterward  gave  the  name 
of  karyokincsis.  It  soon  ap- 
peared, however,  that  this  mode 
of  division  was  not  of  universal 
occurrence ;  and  that  cell-divi- 
sion is  of  two  widely  different 
types,  which  Van  Beneden  {'j^^ 
distinguished  as  fnii^Di 01  ta t io n , 
T?;„    .       T^-     ♦  ^-  •  •        f  n^-,^  .oUc  ;^     Corresponding     nearly     to    the 

Fig.    24. —  Direct    division   of   blood-cells   in  J^  ^  J    ^ 

the   cmbrvo  chick,  illustrating   Remak's   scheme.      simple       proCCSS      described      by 

[Remak.]  Remak,  and  division,  involving 

a-e.    Successive    stasres    of    division;     f.   cell      ,,  t       i.    j 

dividing  by  mitosis.  '   -^  the   morc   Complicated    process 

of  karyokincsis.  Three  years 
later  Flemming  ('79)  proposed  to  substitute  for  these  the  terms  direct 
and  iJidircct  division,  which  are  still  used.  Still  later  ('82)  the  same 
author  suggested  the  terms  mitosis  (indirect  or  karyokinetic  division) 
and  auiitosis  (direct  or  akinetic  division),  which  have  rapidly  made 
their  w^ay  into  general  use,  though  the  earlier  terms  are  often  em- 
ployed. ' 

Modern  research  has  demonstrated  the  fact  that  amitosis  or  direct 
division,  regarded  by  Remak  and  his  immediate  followers  as  of  uni- 
versal occurrence,  is  in  reality  a  rare  and  exceptional  process ;  and 
there  is  reason  to  believe,  furthermore,  that  it  is  especially  char- 
acteristic of  highly  specialized  cells  incapable  of  long-continued 
multiplication  or  such  as  are  in  the  early  stages  of  degeneration,  for 
instance,  in  glandular  epithelia  and  in  the  cells  of  transitory  embry- 
onic envelopes,  where  it  is  of    frequent  occurrence.      Whether   this 


OUTLINE    OF  INDIRECT  DIVISION  65 

view  be  well  founded  or  not,  it  is  certain  that  in  all  the  hifjjher  and  in 
many  of  the  lower  forms  of  Ufe,  indirect  division  or  mitosis  is  the 
typical  mode  of  cell-division.  It  is  by  mitotic  division  that  the  germ- 
cells  arise  and  are  prepared  for  their  union  during  the  process  of 
maturation,  and  by  the  same  process  the  oosperm  segments  and  gives 
rise  to  the  tissue-cells.  It  occurs  not  only  in  the  highest  forms  of 
plants  and  animals,  but  also  in  such  simple  forms  as  the  rhizopods, 
flagellates,  and  diatoms.  We  may,  therefore,  justly  regard  it  as  the 
most  general  expression  of  the  "eternal  law  of  continuous  develop- 
ment" on  which  Virchow  insisted. 


A.    Outline  of  Indirect  Division  or  Mitosis  (Karyokixesis) 

In  the  present  state  of  knowledge  it  is  somewhat  difficult  to  give  a 
connected  general  account  of  mitosis,  owing  to  the  uncertainty  that 
hangs  over  the  nature  and  functions  of  the  centrosome.  For  the  pur- 
pose of  the  following  preliminary  outline,  we  shall  take  as  a  type 
mitosis  in  which  a  distinct  and  persistent  centrosome  is  present,  as 
has  been  most  clearly  determined  in  the  maturation  and  cleavage  of 
various  animal  eggs,  and  in  the  division  of  the  testis-cells.  In  such 
cases  the  process  involves  three  parallel  series  of  changes,  which  affect 
the  nucleus,  the  centrosome,  and  the  cytoplasm  of  the  cell-body 
respectively.  For  descriptive  purposes  it  may  conveniently  be  divided 
into  a  series  of  successive  stages  or  phases,  which,  however,  graduate 
into  one  another  and  are  separated  by  no  well-defined  limits.  These 
are:  (i)  The  Pi'opJiases,  or  preparatory  changes;  (2)  the  Mctaphasc, 
which  involves  the  most  essential  step  in  the  division  of  the  nucleus  ; 
(3)  the  Anaphases,  in  which  the  nuclear  material  is  distributed  ;  (4)  the 
Telophases,  in  which  the  entire  cell  divides  and  the  daughter-cells  are 
formed. 

I.  Prophases.  — {a)  The  Nticletis.  As  the  cell  prepares  for  division, 
the  most  conspicuous  fact  is  a  transformation  of  the  nuclear  substance, 
involving  both  physical  and  chemical  changes.  The  chromatin-sub- 
stance  rapidly  increases  in  staining-power,  loses  its  net-like  arrange- 
ment, and  finally  gives  rise  to  a  definite  number  of  separate  intensely 
staining  bodies,  usually  rod-shaped,  known  as  cliroiuosoines.  As  a  rule 
this  process,  exemplified  by  the  dividing  cells  of  the  salamander-epi- 
dermis (Fig.  i)  or  those  of  plant-meristem  (Fig.  2),  takes  place  as  fol- 
lows. The  chromatin  resolves  itself  little  by  little  into  a  more  or  less 
convoluted  thread,  known  as  the  5/r/;/(Knauel)or  spireme,  and  its  sub- 
stance stains  far  more  intensely  than  that  of  the  reticulum  (Fig.  25). 
The  spireme-thread  is  at  first  fine  and  closely  convoluted,  forming  the 
close  spireme."     Later  the  thread  thickens  and  shortens  and  the 


n 


66 


CELL-DIVISION 


convolution  becomes  more  open  ('*  open  spireme").     In  some  cases 
there  is  but  a  single  continuous  thread ;  in  others,  the  thread  is  from 


E 


Fig.  25.  —  Diagrams  showing  the  prophases  of  mitosis. 

A.  Resting  cell  with  reticular  nucleus  and  true  nucleolus;  at  c  the  attraction-sphere  containing 
two  centrosomes.  B.  Early  prophase ;  the  chromatin  forming  a  continuous  spireme,  nucleolus  still 
present :  abo%-e,  the  amphiaster  {a).  C.  D.  Two  different  types  of  later  prophases.  C.  Disappear- 
ance of  the  primary  spindle,  divergence  of  the  centrosomes  to  opposite  poles  of  the  nucleus  (exam- 
ples, some  plant-cells,  cleavage-stages  of  many  eggs).  D.  Persistence  of  the  primary  spindle  (to 
form  in  some  cases  the  "  central  spindle"),  fading  of  the  nuclear  membrane,  ingrowth  of  the  astral 
rays,  segmentation  of  the  spireme-thread  to  form  the  chromosomes  (examples,  epidermal  cells  of 
salamander,  formation  of  the  polar  bodies).  E.  Later  prophase  of  type  C\  fading  of  the  nuclear 
membrane  at  the  poles,  formation  of  a  new  spindle  inside  the  nucleus;  precocious  splitting  of  the 
chromosomes  (the  latter  not  characteristic  of  this  tvpe  alone).  E.  The  mitotic  figure  established; 
e.f.  the  equatorial  plate  of  chromosomes.     {€/.  Figs.  21,  27,  32,  etc.) 


OUTLINE    OF  INDIRECT  DIVISION  6/ 

its  first  appearance  divided  into  a  number  of  separate  pieces  or  seg- 
ments, forming  a  segmented  spireme.  In  either  case  it  ultimately 
breaks  transversely  to  form  the  chromosomes,  which  in  most  cases  have 
the  form  of  rods,  straight  or  curved,  though  they  are  sometimes  spher- 
ical or  ovoidal,  and  in  certain  cases  may  be  joined  together  in  the 
form  of  rings.  The  staining-povver  of  the  chromatin  is  now  at  a  maxi- 
mum. As  a  rule  "the  nuclear  membrane  meanwhile  fades  away  and 
finally  disappears,  though  there  are  some  cases  in  which  it  persists 
more  or  less  completely  through  all  the  phases  of  division.  The 
chromosomes  now  lie  naked  in  the  cell,  and  the  ground-substance 
of  the  nucleus  becomes  continuous  with  the  surrounding  cytoplasm 

(Fig.  25,  A£,/^V 

The  remarkable  fact  has  now  been  established  with  high  probability 
that  eveiy  species  of  plant  or  animal  has  a  fixed  and  characteristic  niDH- 
ber  of  cJiromosomes,  zvhicJi  regularly  recurs  in  the  division  of  all  of  its 
cells  ;  and  in  all  forms  arising  by  sexual  reproduction  the  number  is 
even.  Thus,  in  some  of  the  sharks  the  number  is  36 ;  in  certain  gas- 
teropods  it  is  32  ;  in  the  mouse,  the  salamander,  the  trout,  the  lily,  24  ; 
in  the  worm  Sagitta,  18  ;  in  the  ox,  guinea-pig,  and  in  man  -  the  num- 
ber is  said  to  be  16,  and  the  same  number  is  characteristic  of  the  onion. 
In  the  grasshopper  it  is  12;  in  the  hepatic  Pallavicinia  and  some  of 
the  nematodes,  8  ;  and  in  Ascaris,  another  thread-worm,  4  or  2.  In  the 
crustacean  Artemia  it  is  168.^  Under  certain  conditions,  it  is  true, 
the  number  of  chromosomes  may  be  less  than  the  normal  in  a  given 
species;  but  these  variations  are  only  apparent  exceptions  (p.  %'j). 
The  even  number  of  chromosomes  is  a  most  interesting  fact,  which,  as 
will  appear  hereafter  (p.  205 ),  is  due  to  the  derivation  of  one-half  the 
number  from  each  of  the  parents. 

The  nucleoli  differ  in  their  behaviour  in  different  cases.  Net-knots, 
or  chromatin-nucleoli,  contribute  to  the  formation  of  the  chromosomes; 
and  in  cases  such  as  Spirogyra  {ViQ.\m\Q.x,  "^6,  and  Moll,  '93 )  or  .-^r//- 
nosphcerium  (R.  Hertwig,  '99),  where  the  whole  of  the  chromatin  is  at 
one  period  concentrated  into  a  single  mass,  the  whole  chromatic  figure 
thus  appears  to  arise  from  a  "nucleolus."  True  nucleoli  or  plasmo- 
somes  sooner  or  later  disappear  ;  and  the  greater  number  of  observers 
agree  that  they  do  not  take  part  in  the  chromosome-formation.  In  a 
considerable  number  of  forms  {e.g.  during  the  formation  of  the  j^olar 

1  The  spireme-formation  is  by  no  means  an  invariable  occurrence  in  mit»)sis.  In  a  consid- 
erable number  of  cases  the  chromatin-network  resolves  itself  ilirectly  into  tlie  chromosomes, 
the  chromatic  substance  becoming  concentrated  in  separate  masses  which  never  form  a  con- 
tinuous thread.     Such  cases  are  connected  by  various  gradations  with  the  "  segmented  spi- 


reme." 


2  Flemmino  believes  the  number  in  man  to  be  considerably  greater  than  16. 
^  For  a  more  complete  list  see  p.  206. 


6  8  CELL-Di  I  vs/oy 

bodies  in  various  eggs)  the  nucleolus  is  cast  out  into  the  cytoplasm  as 
the  spindle  forms,  to  persist  as  a  "  metanucleus  "  for  some  time  before 
its  final  disappearance  (Fig.  104).  More  commonly  the  nucleolus 
fades  away  /;/  s/t//,  sometimes  breaking  into  fragments  meanwhile, 
while  the  chromosomes  and  spindle  are  forming.  The  fate  of  the 
material  is  in  this  case  only  conjectural.  An  interesting  view  is  that 
of  Strasburger  ('95,  '97),  who  suggests  that  the  true  nucleoli  are  to  be 
regarded  as  storehouses  of  "  kinoplasmic  "  material,  which  is  either 
directly  used  in  the  formation  of  the  spindle,  or,  upon  being  cast  out 
of  the  nucleus,  adds  to  the  cytnj-)lasmic  store  of  "  kinoplasm  "  avail- 
able for  future  mitosis. 

{d)  The  AinpJiiastcr.  Meanwhile,  more  or  less  nearly  parallel  with 
these  changes  in  the  chromatin,  a  complicated  structure  known  as  the 
avipJiiastcr  {Yo\,  '77)  makes  its  appearance  in  the  position  formerly 
occupied  by  the  nucleus  (Fig.  2^,B-F).  This  structure  consists  of 
a  fibrous  spindle-shaped  bodv,  the  spindle,  at  either  pole  of  which  is 
a  star  or  aster  formed  of  rays  or  astral  fibres  radiating  into  the  sur- 
rounding cytoplasm,  the  whole  strongly  suggesting  the  arrangement 
of  iron  filings  in  the  field  of  a  horseshoe  magnet.  The  centre  of  each 
aster  is  occupied  by  a  minute  body,  known  as  the  centrosoine  (Boveri, 
'ZZ\  which  may  be  surrounded  by  a  spherical  mass  known  as  the 
^rw/rt^j/Z/^/r  (Strasburger,  '93).  As  the  amphiaster  forms,  the  chro- 
mosomes group  themselves  in  a  plane  passing  through  the  equator  of 
the  spindle,  and  thus  form  what  is  known  as  the  equatorial  plate. 

The  amphiaster  arises  under  the  influence  of  the  centrosome  of  the 
resting  cell,  which  divides  into  two  similar  halves,  an  aster  being 
developed  around  each  while  a  spindle  stretches  between  them  (Figs. 
25,  27).  In  most  cases  this  process  begins  outside  the  nucleus,  but 
the  subsequent  phenomena  vary  considerably  in  different  forms.  In 
some  forms  (tissue-cells  of  the  salamander) the  amj^hiaster  at  first  lies 
tangentially  outside  the  nucleus,  and  as  the  nuclear  membrane  fades 
away,  some  of  the  astral  rays  grow  into  the  nucleus  from  the  side, 
become  attached  to  the  chromosomes,  and  finally  pull  them  into  posi- 
tion around  the  equator  of  the  spindle,  which  is  here  called  the  ecu- 
tral  spiudle  (Figs.  25,  D,  F ;  27).  In  other  cases  the  original  spindle 
disappears,  and  the  two  asters  pass  to  o]:>])osite  poles  of  the  nucleus 
(some  plant  mitoses  and  in  many  animal-cells).  A  spindle  is  now 
formed  from  ravs  that  grow  into  the  nucleus  from  each  aster,  the 
nuclear  membrane  fading  away  at  the  poles,  though  in  some  cases  it 
may  be  pushed  in  by  the  spindle-fibres  for  some  distance  before  its 
disappearance  (Figs.  25,  32).  In  this  case  there  is  apparently  ho 
central  spindle.  In  a  few  exceptional  cases,  finally,  the  amphiaster 
may  arise  inside  the  nucleus  (p.  304). 

The   entire    structure,    resulting  from    the    foregoing    changes,  is 


OUTLINE    OF  I.XDIRECT  DIVISION 


69 


known  as  the  karyokiiictic  or  mitotic  figure.  It  may  be  described  as 
consisting  of  two  distinct  parts;  namely,  i,  the  chromatic  figure, 
formed  by  the  deeply  staining  chromosomes  ;  and,  2,  the  achromatic 
figure,  consisting  of  the  spindle  and  asters  which,  in  general,  stain 
but  slightly.     The  fibrous  substance  of  the  achromatic  figure  is  gener- 


Fig.  26.  —  Diagrams  of  the  later  phases  of  mitosis. 

G.  Metaphase;  splitting  of  the  chromosomes  {e.p.^.  ».  The  cast-off  nucleolus.  //.  Ana- 
phase ;  the  daughter-chromosomes  diverging,  between  them  the  interzonal-fibres  (/./.).  or  central 
spindle;  centrosomes  already  doubled  in  anticipation  of  the  ensuing -division.  /.  Late  anaphase 
or  telophase,  showing  division  of  the  cell-body,  mid-body  at  the  equator  of  the  spindle  and  bcgm- 
ning  reconstruction  of  the  daughter-nuclei.     J.   Division  completed. 

ally  known  as  archoplasm  (Boveri,  '88),  but  this  term  is  not  applied 
to  the  centrosome  within  the  aster. 

2.  Metaphase.  — TYiO,  prophases  of  mitosis  are,  on  the  whole,  pre- 
paratory in  character.  The  metaphase,  which  follows,  forms  the 
initial  phase  of  actual  division.  Each  chromosome  splits  lengthwise 
into  two  exactly  similar  halves,  which  afterward  diverge  to  opposite 
poles  of  the  spindle,  and  here  each  group  of  daughter-chromosomes 


70 


CELL-DIVISIOX 


finally  gives  rise   to   a  daiii^^hter-nucleus  (Fig.   26).      In   some   cases 
the    splitting  of    the  chromosomes  cannot    be  seen  until  they  have 
grouped  themselves  in  the  equatt)rial  plane  of  the  spindle  ;  and  it  is 
only  in  this  case  that  the  term  "  metaphase  "  can  be  applied  to  the 
mitotic  figure  as  a  whole.      In  a  large  number  of  cases,  however,  the 
splitting  may  take  place  at  an  earlier  period  in  the  spireme-stage,  or 
even,  in  a  few  cases,  in  the  reticulum  of  the  mother-nucleus  (Figs. 
54.   55).     Such  variations  do  not,  however,  affect  the  essential  fact 
that  the  clnvDiatic  nctivork  is  converted  into  a  thread^  which,  ivhether 
continuous    or   discontinuous,   splits  tJiroughojit   its   entire  lengtJi  into 
two  exactlj  equivalent  halves.       The   splitting  of   the  chromosomes, 
discovered  by  Flemming  in  1880,  is  the  most  significant  and  funda- 
mental operation' of  cell-division;  for  by  it,  as  Roux  first  pointed  out 
{^^}i\  the  entire  substance  of  the  chromatic  network  is  precisely  halved, 
and  the  daughter-nuclei  receive  precisely  equivalent  portions  oj  chro- 
matin from  the  mother-nucleus.     It  is  very  important  to  observe  that 
the  nuclear  division  always  shows  this  exact  quality,  whether  division 
of  the  cell-body  be  equal  or  unequal.     The  minute  polar  body,  for 
example  (p.  238),  receives  exactly  the  same  amount  of  chromatin  as 
the  Q^^g,  though  the  latter  is  of  gigantic  size  as  compared  with  the 
former.     On  the  other  hand,  the  size  of  the  asters  varies  with  that 
of  the  daughter-cells  (Figs.  58,  175),  though  not  in  strict  ratio.     The 
fact  is  one  of  great  significance  for  the  general  theory  of    mitosis, 
as  will  appear  beyond. 

3.  Anaphases.  —  After  splitting  of  the  chromosomes,  the  daughter- 
chromosomes,  arranged  in  two  corresponding  groups,^  diverge  to  oppo- 
site poles  of  the  spindle,  where  they  become  closely  crowded  in  a  mass 
near  the  centre  of  the  aster.  As  they  diverge,  the  two  groups  of 
daughter-chromosomes  are  connected  by  a  bundle  of  achromatic 
fibres,  stretching  across  the  interval  between  them,  and  known  as  the 
interzonal  fibres  or  connecting  fibres?  In  some  cases  these  differ  in  a 
marked  degree  from  the  other  spindle-fibres ;  and  they  are  believed 
by  many  observers  to  have  an  entirely  different  origin  and  function. 
A  view  now  widely  held  is  that  of  Hermann,  who  regards  these  fibres 
as  belonging  to  a  central  spindle,  surrounded  by  a  peripheral  layer 
of  mantle-fibres  to  which  the  chromosomes  are  attached,  and  only 
exposed  to  view  as  the  chromosomes  separate.^  Almost  invariably 
in  the  division  of  plant-cells  and  often  in  that  of  animal  cells  these 

1  It  was  this  fact  that  led  Flemming  to  employ  the  word  mitosis  (fiiros,  a  thread). 

2  This  stage  is  termed  by  Flemming  the  dyasier,  a  term  which  should,  however,  be  aban- 
doned in  order  to  avoid  confusion  with  the  earlier  word  amphiafter.  The  latter  convenient 
and  appropriate  term  clearly  has  priority. 

3  Verlnndutigsfasern  of  German  authors  ;  filaments  reunissants  of  Van  Beneden. 

*  Cf.  p.  105. 


OUTLINE    OF  INDIRECT  DIVISION 


71 


fibres  show  during  this  period  a  series  of  deeply  staining  thickenings 
in  the  equatorial  plane  forming  the  cell-plate  or  mid-body.  In  plant- 
mitoses  this  is  a  very  conspicuous  structure  ( Fig.  34).  In  animal  cells 
the  mid-body  is  usually  less  developed  and  sometimes  rudimentary, 
being  represented  by  only  a  few  granules  or  even  a  single  one 
(Fig.  29).     Its  later  history  is  described  below. 

4.  Telophases. —  In  the  final  phases  of  mitosis,  the  entire  cell 
divides  in  two  in  a  plane  passing  through  the  equator  of  the  spindle, 
each  of  the  daughter-cells  receiving  a  group  of  chromosomes,  half  of 
the  spindle,  and  one  of  the  asters  with  its  centrosome.  Meanwhile, 
a  daughter-nucleus  is  reconstructed  in  each  cell  from  the  group  of 
chromosomes  it  contains.  The  nature  of  this  process- differs  greatly 
in  different  kinds  of  cells.  Sometimes,  as  in  the  epithelial  cells  of 
Amphibia,  especially  studied  by  Flemming  and  Rabl,  and  in  many 
plant-cells,  the  daughter-chromosomes  become  thickened,  contorted, 
and  closely  crowded  to  form  a  daugJiter-sph'enie ,  closely  similar  to  that 
of  the  mother-nucleus  (Fig.  29);  this  becomes  surrounded  by  a  mem- 
brane, the  threads  give  forth  branches,  and  -thus  produce  a  reticular 
nucleus.  A  somewhat  similar  set  of  changes  takes  place  in  the  seg- 
menting eggs  of  Ascaris  (Van  Beneden,  Boveri).  In  other  cases,  as 
in  many  segmenting  ova,  each  chromosome  gives  rise  to  a  hollow 
vesicle,  after  which  the  vesicles  fuse  together  to  produce  a  single 
nucleus  (Fig.  52).  When  first  formed,  the  daughter-nuclei  are  of 
equal  size.  If,  however,  division  of  the  cell-body  has  been  unequal, 
the  nuclei  become,  in  the  end,  correspondingly  unequal  —  a  fact 
which,  as  Conklin  and  others  have  pointed  out,  proves  that  the  size 
of  the  nucleus  is  controlled  by  that  of  the  cytoplasmic  mass  in  which 
it  lies. 

The  fate  of  the  achromatic  structures  varies  considerably,  and  has 
been  accurately  determined  in  only  a  few  cases.  As  a  rule,  the 
spindle-fibres  disappear  more  or  less  completely,  but  a  portion  of 
their  substance  sometimes  persists  in  a  modified  form  i^e.g.  the 
Nebenkern,  p.  163).  In  dividing  plant-cells,  the  cell-plate  finally 
extends  across  the  entire  cell  and  splits  into  two  layers,  between 
which  appears  the  membrane  by  which  the  daughter-cells  are  cut 
apart.i  A  nearly  similar  process  occurs  in  a  few  animal  cells,-  but 
as  a  rule  the  cell-plate  is  here  greatly  reduced  and  forms  no  mem- 
brane, the  cell  dividing  by  constriction  through  the  equatorial  plane. 
Even  in  this  case,  however,  the  division-plane  is  often  indicated 
before  division  takes  place  by  a  peculiar  modification  of  the  cyto- 
plasm in  the  equatorial  plane  outside  the  spindle  (Fig.  30)-  This 
region  is  sometimes  called  the  cytoplasmic  plate,  in  contradistinction 
to  the  spindle-plate,  or  mid-body  proper.  In  the  proi)hases  and  meta- 
1  Cf.  Strasburger,  '98.  2  Cf.  Hoffmann,  '98. 


7^ 


CELL-DIVISION 


phases  the  astral  rays  often  cross  one  another  in  the  equatorial  region 
outside  the  spindle.  During  the  anaphases,  however,  this  crossing 
disappears,  the  rays  from  the  two  asters  now  meeting  at  an  angle 
along  the  cytoplasmic  plate  (Fig.  31).  Constriction  and  division  of 
the  cell  then  occur.^ 

The  aster  may  in  some  cases  entirely  disappear,  together  with  the 
centrosome  (as  occurs  in  the  mature  dgg).  In  a  large  number  of 
cases,  however,  the  centrosome  persists,  lying  either  outside  or  more 
rarely  inside  the  nucleus  and  dividing  into  two  at  a  very  early  period. 
This'division  is  clearly  a  precocious  preparation  for  the  ensuing  divi- 
sion of  the  daughter-cell,  and  it  is  a  remarkable  fact  that  it  occurs  as 
a  rule  during  the  early  anaphase,  before  the  mother-cell  itself  has 
divided.  There  are  apparently,  however,  some  cases  in  which  the 
centrosome  remains  undivided  during  the  resting  stage  and  only 
divides  as  the  process  of  mitosis  begins. 

Like  the  centrosome,  the  aster  or  its  central  portion  may  persist  in 
a  more  or  less  modified  form  throughout  the  resting  state  of  the  cell, 
forming  a  structure  generally  known  as  the  attraction-sphere.  This 
body  often  shows  a  true  astral  structure  with  radiating  fibres  (Figs. 
8,  49);  but  it  is  sometimes  reduced  to  a  regular  spherical  mass  which 
may  represent  only  a  portion  of  the  original  aster  (Fig.  7). 


B.     Origin  of  the  Mitotic  Figure 

The  nature  and  source  of  the  material  from  which  the  mitotic 
figure  arises  form  a  problem  that  has  been  almost  continuously  under 
discussion  since  the  first  discovery  of  mitosis,  and  is  even  now  but 
partially  solved.  The  discussion  relates,  however,  almost  solely  to 
the  achromatic  figure  (centrosome,  spindle,  and  asters) ;  for  every  one 
is  agreed  that  the  chromatic  figure  (chromosomes)  is  directly  derived 
from  the  chromatin-network,  as  described  above,  so  that  there  is  no 
breach  in  the  continuity  of  the  chromatin  from  one  cell-generation  to 
another.  With  the  achromatic  figure  the  case  is  widely  different. 
The  material  of  the  spindle  and  asters  must  be  derived  from  the 
nucleus,  from  the  cytoplasm,  or  from  both  ;  and  most  of  the  earlier 
research  was  devoted  to  an  endeavour  to  decide  between  these 
possibilities.  The  earhest  observers  {'71-7S)  supposed  the  achro- 
matic figure  to  disappear  entirely  at  the  close  of  cell-division,  and 
most  of  them  (Butschli,  Strasburger,  Van  Beneden,  '75)  believed 
it  to  be  re-formed  at  each  succeeding  division  out  of  the  nuclear 
substance.  The  entire  mitotic  figure  was  thus  conceived  as  a 
metamorphosed  nucleus.     Later  researches  ('75- 85)  gave  contradic- 

i  See  p.  318.      Cf.  Kostanecki,  '97,  and  Hoffmann,  '98. 


ORIGIN  OF   THE  MITOTIC  FIGURE 


73 


tory  and  apparently  irreconcilable  results.  Fol  ('79)  derived  the 
spindle  from  the  nuclear  material,  the  asters  from  the  cytoplasm. 
Strasburger  ('80)  asserted   that  the   entire    achromatic  figure   arose 


A 


c 


%-!#  »^ 


D 


Fig.  27.  — The  prophases  of  mitosis  (heterotypical  form)  in  primary  spermatocytes  of 
Salama)idra.     [Meves.] 

A.  Early  segmented  spireme ;  two  centrosomes  outside  the  nucleus  in  the  remains  of  the 
attraction-sphere.  B.  Longitudinal  splitting  of  the  spireme,  appearance  of  the  astral  rays,  disin- 
tegration of  the  sphere.  C.  Early  amphiastcr  and  central  spindle.  D.  Chromosomes  in  the  form 
of  rings,  nuclear  membrane  disappeared,  amphiaster  enlarging,  mantle-fibres  developing. 

from  the  cytoplasm,  and  to  that  view,  in  a  modified  form,  he  still 
adheres.  Flemming  ('82),  on  the  whole,  inclined  to  the  opinion 
that  the  achromatic  figure  arose  inside  the  nucleus,  yet  expressed  the 


y^  CELL-DIVISION 

opinion  that  the  question  of  nuclear  or  cytoplasmic  origin  was  one  of 
minor  importance.  A  long  series  of  later  researches  on  both  plants 
and  animals  has  fully  sustained  this  opinion,  showing  that  the  origin 
of  the  achromatic  figure  does  in  fact  differ  in  different  cases.  Thus 
in  Infusoria  the  entire  mitotic  figure  is  of  intranuclear  origin  (there 
are,  however,  no  asters);  in  echinoderm  eggs  the  spindle  is  of  nuclear, 
the  asters  of  cytoplasmic,  origin ;  in  the  testis-cells  and  some  tissue- 
cells  of  the  salamander,  a  complete  amphiaster  is  first  formed  in  the 
cytoplasm,  but  to  this  are  afterward  added  elements  probably  derived 
from  the  linin-network;  while  in  higher  plants  there  is  some  reason 
to  believe  that  the  entire  achromatic  figure  may  be  of  cytoplasmic 
origin.  Such  differences  need  not  surprise  us  when  we  reflect  that 
the  achromatic  part  of  the  nucleus  (linin-network,  etc.)  is  probably  of 
the  same  general  nature  as  the  cytoplasm. ^ 

Many  observers  have  maintained  that  the  material  of  the  astral 
rays  and  spindle-fibres  is  directly  derived  from  the  substance  of  the 
protoplasmic  meshwork,  whether  nuclear,  cytoplasmic,  or  both  ;  but 
its  precise  origin  has  long  been  a  subject  of  debate.  This  question, 
critically  considered  in  Chapter  VI.,  will  be  here  only  briefly  sketched. 
By  Klein  {;7%\  Van  Beneden  ('83),  Carnoy  ('84,  '85),  and  a  large  num- 
ber of  later  observers,  the  achromatic  fibres,  both  of  spindles  and  of 
asters,  are  regarded  as  identical  with  those  of  a  preexisting  reticulum 
which  have  merely  assumed  a  radiating  arrangement  about  the  cen- 
trosome.  The  amphiaster  has,  therefore,  no  independent  existence, 
but  is  merely  an  image,  as  it  were,  somewhat  like  the  bipolar  figure 
arising  when  iron  filings  are  strewn  in  the  field  of  a  horseshoe  magnet. 
Boveri,  on  the  other  hand,  who  has  a  small  but  increasing  following, 
maintains  that  the  amphiastral  fibres  are  not  identical  with  those  of 
the  preexisting  meshwork,  but  a  new  formation  which,  as  it  were, 
"crystallizes  anew  "  out  of  the  general  protoplasmic  substance.  The 
amphiaster  is  therefore  a  new  and  independent  structure,  arising  in, 
or  indirectly  from,  the  preexisting  material,  but  not  by  a  direct  mor- 
phological transformation  of  that  material.  This  view,  which  has 
been  advocated  by  Druner  ('94),  Braus  ('95),  Meves  ('97,  4,  '98), 
and  with  which  my  own  later  observations  ('99)  also  agree,  is  more 
fully  discussed  at  page  318. 

In  1887  an  important  forward  step  was  taken  through  the  inde- 
pendent discovery  by  Van  Beneden  and  Boveri  that  in  the  ^gg  of 
Ascaris  the  centrosome  does  not  disappear  at  the  close  of  mitosis,  but 
remains  as  a  distinct  cell-organ  lying  beside  the  nucleus  in  the  cyto- 

1  In  the  case  of  echinoderm  eggs,  I  have  found  reason  ('95,  2)  for  the  conclusion  that  the 
spindle- fibres  are  derived  not  merely  from  the  linin-substance,  but  also  from  the  chromatm. 
Despite  some  adverse  criticism,  I  have  found  no  reason  to  change  my  opinion  on  this  point. 
The  possible  significance  of  such  a  derivation  is  indicated  elsewhere  (p.  302). 


ORIGIN   OF   THE   MITOTIC  FIGURE 


75 


plasm.  These  investigators  agreed  that  the  amphiaster  is  formed 
under  the  influence  of  the  centrosome,  which  by  its  division  creates 
two  new  "centres  of  attraction"  about  which  the  astral  systems  arise, 
and  which  form  the  foci  of  the  entire  dividing  system.  In  them  are 
centred  the  fibrillae  of  the  astral  system,  toward  them  the  daughter- 


F  0 

Fig.  28.  -  Metaphase  and  anaphases  of  mitosis  in  cells  (spermatocytes)  of  the  salamander. 
[Druner.] 

E    Metaphase.     Tlie  continuous  central  spindle-fibres  pass  from  pole  to  pole  of  the  spmdle 
Outside  them  the  thm  layer  of  contractile  mantle-fibres  attached  to  the  d.vKlcd  ^^^'-:^'^^''^^'^} 
which  only  two  are  shown!    Centrosomes  and  asters.     F.  Transverse  section  ^ -- ^  \   ^^  "^  !?^ J 
figure  showing  the  ring  of  chromosomes  surrounding  the  central  spmdle.  the  cut  ^^'^^/^^  '  rL'''!"^/ 
a;peanng  as ''dots.     G.  Anaphase;  divergence  of  the  ^»-^g'^^--f-°"--"\^.^- ^j^J^^^  "S„f^,^^^ 
tral  spin5leas  the  interzonal  fibres;  contractile  fibres  ^vn^^^'^-\^or..^-'  \ '^Xj^^^^^^^ 
shown.     H.  Later  anaphase  (dyaster  of  Flemming)  ;  the  central  spmdle    uly  exposed  to  Mev. 
mantle-fibres  attached  to  the  chromosomes.     Immediately  aftenvard  the  cell  d.vdes  (see  P  .g.  ^) . 

chromosomes  proceed,  and  within  their  respective  spheres  of  influ- 
ence are  formed  the  resulting  daughter-cells.  Both  Van  Beneden  and 
Boveri  fully  recognized  the  importance  of  then"  discovery.  "We  are 
justified,"  said  Van  Beneden,  ^  in  regarding  the  attraction-sphere  with 
its  central  corpuscle  as  forming  a  permanent  organ,  not  only  of  the 
early  blastomeres,  but  of  all  cells,  and  as  constituting  a  cell-organ  equal 


76 


CELL-DIVISION 


in  rank  to  the  nucleus  itself ;  and  we  may  conclude  that  every  central 
corpuscle  is  derived  from  a  preexisting  corpuscle,  every  attraction- 
sphere  from  a  preexisting  sphere,  and  that  division  of  the  sphere 
precedes  that  of  the  cell-nucleus."  ^  Boveri  expressed  himself  in 
similar  terms  regarding  the  centrosome  in  the  same  year  {"^y,  2, 
p.  153),  and  the  same  general  result  was  reached  by  Vejdovsky 
nearlv  at  the  same  time,-  though  it  was  less  clearly  formulated  than 
bv  either  Boveri  or  Van  Beneden. 

All  these  observers  agreed,  therefore,  that  the  achromatic  figure 
arose  outside  the  nucleus,  in  the  cytoplasm ;  that  the  primary  impulse 
to  cell-division  was  given,  not  by  the  nucleus,  but  by  the  centrosome, 
and  that  a  new  cell-organ   had  been  discovered  whose  special  office 


Fig.  29. —  Final  phases  (telephases)  of  mitosis  in  salamander  cells.     [Flemming.] 

/.  Epithelial  cell  from  the  lung;  chromosomes  at  the  poles  of  the  spindle,  the  cell-body  divid- 
ing; granules  of  the  "mid-body"  or  '/Auischcnkorpcr  ■M  the  equator  of  the  disappearing  spindle. 
y.  Connective  tissue-ceil  (lung)  immediately  after  division;  daughter-nuclei  reforming,  the  cen- 
trosome just  outside  of  each ;  mid-body  a  single  granule  in  the  middle  of  the  remains  of  the 
spindle.  • 


was  to  preside  over  cell-division.  **The  centrosome  is  an  indepen- 
dent permanent  cell-organ,  which,  exactly  like  the  chromatic  elements, 
is  transmitted  by  division  to  the  daughter-cells.  77ic  cciitrosojue  rep- 
resents the  dynamic  cejitre  of  cell T  "^ 

That  the  centrosome  does  in  many  cases,  especially  in  embryonic 
cells,  behave  in  the  manner  stated  by  Van  Beneden  and  Boveri  seems 
at  present  to  admit  of  no  doubt ;  and  it  has  been  shown  to  occur  in 


1  '87,  p.  279. 


2  '88,  pp.  151,  etc. 


3  Boveri,  '87,  2,  p.  153. 


ORIGIN  OF   THE  MITOTIC  FIGURE  yy 

many  kinds  of  adult  tissue-cells  during  their  resting  state  ;  for  example 
in  pigment-cells,  leucocytes,  connective  tissue-cells,  epithelial  and 
endothelial  cells,  in  certain  gland-cells  and  nerve-cells,  in  the  cells 
of  some  plant-tissues,  and  in  some  of  the  unicellular  plants  and  ani- 
mals, such  as  the  diatoms  and  flagellates  and  rhizopods.  On  the  other 
hand.  Van  Beneden's  conception  of  the  attraction-sphere  has  proved 
untenable ;  for  this  structure  has  been  clearly  shown  in  some  cases 
to  disintegrate  and  disappear  at  the  close  or  the  beginning  of  mitosis^ 

(Fig-  27). 

Whether  the  centrosome  theory  can  be  maintained  is  still  in  doubt ; 

but  evidence  against  it  has  of  late  rapidly  accumulated. 

In  the  first  place,  it  has  been  shown  that  the  primary  impulse  to 
cell-division  cannot  be  given  by  fission  of  the  centrosome,  for  there  are 
several  accurately  determined  cases  in  which  the  chromatin-elements 
divide  independently  of  the  centrosome,  and  it  is  now  generally  agreed 
that  the  division  of  chromatin  and  centrosome  are  two  parallel  events, 
the  nexus  between  which  still  remains  undetermined.^ 

Secondly,  an  increasing  number  of  observers  assert  the  total  disap- 
pearance of  the  centrosome  at  the  close  of  mitosis  ;  while  some  very 
convincing  observations  have  been  made  favouring  the  view  that  cen- 
trosomes  may  be  formed  de  novo  without  connection  with  preexisting 
ones  (pp.  213,  305). 

Thirdly,  a  large  number  of  recent  observers  (including  Strasburger 
and  many  of  his  pupils)  of  mitosis  in  the  flowering  plants  and 
pteridophytes  agree  that  in  these  forms  no  centrosome  exists  at  any 
stage  of  mitosis,  the  centre  of  the  aster  being  occupied  by  a  vague 
reticular  mass,  and  the  entire  achromatic  figure  arising  by  the 
gradual  grouping  of  fibrous  cytoplasmic  elements  (kinoplasm  or 
filar  plasm)  about  the  nuclear  elements.^  If  we  can  assume  the  cor- 
rectness of  these  observations,  the  centrosome-theory  must  be  greatly 
modified,  and  the  origin  of  the  amphiaster  becomes  a  far  more  com- 
plex problem  than  it  appeared  under  the  hypothesis  of  Van  Ik-neden 
and  Boveri.  That  such  is  indeed  the  case  is  indicated  by  nothing 
more  strongly  than  by  Boveri's  own  remarkable  recent  experiments 
on  cell-division  (referred  to  at  page  108). 


C.     Details  of  Mitosis 

Comparative  study  has  shown  that  almost  every  detail  of  the  pro- 
cesses described  above  is  subject  to  variation  in  different  forms  of  cells. 
Before  considering  some  of  these  modifications  it  may  be  well  to  pomt 
out  what  we  are   at   present  justified  in   regarding   as   its   essential 

1  Cf.  p.  323.  2  cf.  p.  108.  «  Cf.  p.  82. 


78 


CELL-DIVISION 


features.  These  are  :  (i)  The  formation  of  the  chromatic  and  achro- 
matic figures;  (2)  the  longitudinal  splitting  of  the  chromosomes  or 
spireme-thread ;  (3)  the  transportal  of  the  chromatin-halves  to  the 
respective  daughter-cells.  Each  of  these  three  events  is  endlessly 
varied  in  detail ;  yet  the  essential  phenomena  are  everywhere  the  same, 
with  one  important  exception  relating  to  the  division  of  the  chromo- 
somes that  occurs  in  the  maturation  of  certain  eggs  and  spermatozoa.^ 
It  maybe  stated  further  that  the  study  of  mitosis  in  some  of  the  lower 
forms  (Protozoa)  gives  reason  to  believe  that  the  asters  are  of  second- 
ary importance  as  compared  with  the  spindle,  and  that  the  formation 
of  spireme  and  chromosomes  is  but  tributary  to  the  division  of  the 
smaller  chromatin-masses  of  which  they  are  made  up. 

I.     J'aricties  of  the  Mitotic  Figure 

(a)  TJic  Achromatic  Figure.  The  phenomena  involved  in  the  his- 
tory of  the  achromatic  figure  are  in  general  most  clearly  displayed 
in  embryonic  or  rapidly  dividing  cells,  especially  in  egg-cells  (Figs. 
31,  60),  where  the  asters  attain  an  enormous  development,  and  the 
centrosomes  are  especially  distinct.  In  adult  tissue-cells  the  asters 
are  relatively  small  and  difficult  of  demonstration,  the  spindle  large 
and  distinct ;  and  this  is  particularly  striking  in  the  cells  of  higher 
plants  where  the  asters  are  but  imperfectly  developed.  Plant-mitoses 
are  characterized  by  the  prominence  of  the  cell-plate  (Fig.  34),  which 
is  rudimentary  or  often  wanting  in  animals,  a  fact  correlated  no 
doubt  with  the  greater  development  of  the  cell-membrane  in  plants. 
With  this  again  is  correlated  the  fact  that  division  of  the  cell-body  in 
animal  cells  generally  takes  place  by  constriction  in  the  equatorial 
plane  of  the  spindle ;  while  in  plant-cells  the  cell  is  usually  cut  in 
two  by  a  cell-wall  developed  in  the  substance  of  the  protoplasm  and 
derived  in  large  part  from  the  cell-plate. 

In  animal  cells  we  may  distinguish  two  general  types  in  the  forma- 
tion of  the  amphiaster,  which  are,  however,  connected  by  interme- 
diate gradations.  In  the  first  of  these,  typically  illustrated  by  the 
division  of  epithelial  and  testis-cells  in  the  salamander  (Flemming, 
Hermann,  Drijner,  Meves),  a  complete  amphiaster  is  first  formed  in 
the  cytoplasm  outside  the  nucleus,  while  the  nuclear  membrane  is 
still  intact.  As  the  latter  fades  away  and  the  chromosomes  appear, 
some  of  the  astral  rays  grow  into  the  nuclear  space  and  become 
attached  to  the  chromosomes,  which  finally  arrange  themselves  in  a 
ring  about  the  original  spindle  (Figs.  27,  28).  In  the  completed 
amphiaster,  therefore,  we  may  distinguish  the  original  central  spi7idle 
(Hermann,  '91)  from  the  surrounding  mantle-fibres,  the  latter  being 

1  Cf.  Chapter  V. 


DETAILS   OF  MITOSIS 


79 


attached  to  the  chromosomes,  and  being,  according  to  Hermann,  the 
principal  agents  by  which  the  daughter-chromosomes  are  dragged 
apart.  The  mantle-fibres  thus  form  two  hollow  cones  or  half-spin- 
dles, separated  at  their  bases  by  the  chromosomes  and  completely 
surrounding  the  continuous  fibres  of  the  central  spindle,  which  come 
into  view  as  the  ''interzonal  fibres"  during  the  anaphases  (Fig.  28). 
There  is  still  considerable  uncertainty  regarding  the  origin  and 
relation  of  these  two  sets  of  fibres.  It  is  now  generally  agreed  with 
Van  Beneden  that  the  mantle-fibres  are  essentially  a  part  of  the 
asters,  i.e.  are  simply  those  astral  rays  that  come  into  connection 
with  the  chromosomes  — 
wholly  cytoplasmic  in  ori- 
gin (Herma.nn,  Driiner, 
MacFarland),  or  in  part 
cytoplasmic,  in  part  dif- 
ferentiated from  the  linin- 
network  (Flemming, 
Meves).  Driiner  ('95), 
Braus  ('95)  (salamander), 
and  MacFarland  (yPleicro- 
phyllidia,  '97)  believe  the 
central  spindle  to  arise 
secondarily  through  the 
union  of  two  opposing 
groups  of  astral  rays  in 
the  area  between  the 
centrosomes.  On  the 
other  hand,  Hermann 
('91),  Flemming  ('91), 
Heidenhain  ('94),  Kos- 
tanecki  ('97),  Van  der 
Stricht  ('98),  and  others 
believe  the  central  spindle 
to  exist  from  the  first  in 
the  form  of  fibres  stretching  between  the  diverging  centrosomes ;  and 
Heidenhain  believes  them  to  be  developed  from  a  special  substance, 
forming  a  ''primary  centrodesmus,"  which  persists  in  the  resting  cell, 
and  in  which  the  centrosomes  are  embedded.^  MacFarland's  observa- 
tions on  gasteropod-eggs  ('97)  indicate  that  even  nearly  related  torms 
may  differ  in  the  origin  of  the  central  spindle,  since  in  Plcurophyllidia 
it  is  of  secondary  origin,  as  described  above,  while  in  Diaiilula  it  is  a 
primary  structure  developed  from  what  he  describes  as  the  "  centro- 
some,"  but  which,  as  shown  at  page  314,  is  probably  to  be  regarded  as 

ir/p.  315- 


Fig.  30.  —  Mid-body  in  embryornc  cells  oiUinax.     [HOFF- 
MANN.] 

Earlier  stage  above,  showing  thickenings  along  the  line 
of  cleavage.     Later  stage,  below,  showing  spindle-plate  and 

cytoplasmic  plate. 


8o 


CELL-DIVISIOy 


an  attraction-sphere  surrounding;   the   centrosomes,   and    is    perhaps 
comparable   to   Heidenhain's   "  centrodesmus." 

In  the  second  type,  ilhistrated  in  the  cleavaf]^e  of  echinoderm, 
annehd,  niolhiscan,  and  some  other  egjrs,  a  central  s])indle  may  be 
formed, — sometimes  already  during  the  anaphases  of  the  preceding 
mitosis  (Figs.  99,  155),  —  but  afterward  disappears,  the  asters  moving 


Fig.  31.  —  The  middle  phases  ot  mitosis  in  the  first  cleavage  of  the  Ascaris-^gg.     [BOVERI.] 

y-f.  Closing  prophase,  the  equatorial  plate  forming,  B.  Metaphase;  equatorial  plate  estab- 
lished and  the  chromosomes  split;  b.  the  equatorial  plate,  viewed  en  face,  showing  the  four  chro- 
mosomes, C.  Early  anaphase;  divergence  of  the  daughter-chromosomes  (polar  body  at  one 
side),     D.  Later  anaphase;  p.b.  second  polar  body, 

(For  preceding  stages  see  Fig.  90;  for  later  stages  Fig.  145.) 

to  opposite  poles  of  the  nucleus.  Between  these  two  poles  a  new 
spindle  is  then  formed  in  the  nuclear  area,  while  astral  rays  grow 
out  into  the  cytoplasm.  There  is  strong  evidence  that  in  this  case 
the  entire  spindle  may  arise  inside  the  nucleus,  i.e.  from  the  sub- 
stance of  the  linin-network,  as  occurs,  for  example,  in  the  eggs  of 
echinoderms  (Fig,  25,  E),  and  in  the  testis-cells  of  arthropods.  In 
other  cases,  however,  a  part  at  least  of  the  spindle  is  of  cytoplasmic 


DETAILS   OF  MITOSIS 


8l 


origin,  since  the  ends  of  the  spindle  begin  to  form  before  dissokition 
of  the  nuclear  membrane,  and  the  latter  is  pushed  inwards  in  folds 
by  the  ingrowing  fibres  (Figs.  25,  C,  99).!  In  some  cases,  however, 
it  seems  certain  that  the  nuclear  membrane  fades  away  before  com- 
pletion of  the  spindle  (first  maturation-division  of  TJialasscma,  CJice^- 
toptcriis),  and  it  is  probable  that  the  middle  region  of  the  spindle  is 
here  formed  from  the  Hnin-network.  In  most,  if  not  all,  mitoses  of 
the  second  type  the  chromosomes  do  not  form  a  ring  about  the 
equator  of  the  spindle,  but  extend  in  a  flat  plate  completely  through 


u-'---«- 


/  / 


%l 


\ 


N. 


D 


Fig.  32.  —  Mitosis  in  Sfypocaulon.     [SWINGLE.] 

A.  Early  prophase  witli  single  aster  and  centrosonie.  B.  Initial  formation  of  intraniK  car 
spindle.  C.  Divergence  of  the  daughter-centrosomes.  D.  Early  anaphase ;  nuclear  nieniiji..ne 
still  intact. 


its  substance.  Here,  therefore,  it  is  impossible  to  speak  of  a  "  cen- 
tral spindle."  It  is  nevertheless  probable  that  the  spindle-fibres  are 
of  two  kinds,  viz.  continuous  fibres,  which  form  the  interzonal  fibres 
seen  during  the  anaphases,  and  half-spindle  fibres,  extending  only 
from  the  poles  to  the  chromosomes.  It  is  possible  that  these  two 
kinds  of  fibres,  though   having  the  same  origin,  respectively  corre- 

1  Cf.  Platner  ('86)   on  Avion  and  Lepidoptera,  Watase   ('91)  on  Loligo,  Braus  ('95)  on 
Triton,  and  Griffin  ('96,  '99)  on   Thalassema.     Erlanger  ('97,  5)  endeavours  to  show  that  in 
the  mitosis  of  embryonic  cells  in  the  cephalopods  (^Scpia'),  where  the  inpushing  of  the  mem- 
brane was  previously  shown  by  Watase,  the  entire  spindle  arises  from  the  nucleus. 
G 


82  CELL-DIVISION 

spond  in  function  to  those  of  the  central  spindle  and  to  the  mantle- 
fibres.  It  seems  probable  that  the  difference  between  the  two  types 
of  spindle-formation  may  be  due  to,  or  is  correlated  with,  the  fact 
that  the  nuclear  transformation  takes  place  relatively  earlier  in  the 
"first  type.  When  the  nucleus  lags  behind  the  spindle-formation  the 
centrosomes  take  up  their  position  prematurely,  as  it  w^ere,  the  cen- 
tral spindle  disappearing  to  make  w^ay  for  the  nucleus. 

It  is  in  the  mitosis  of  plant-cells  that  the  most  remarkable  type  of 
achromatic  figure  has  been  observed.  In  some  of  the  lower  forms 
(Alga?)  mitosis  has  been  clearly  shown  to  conform  nearly  to  the 
process  observed  in  animal  cells,  the  amphiaster  being  provided  with 
very  large  asters  and  distinct  centrosomes,  and  its  genesis  corre- 
sponding broadly  with  the  second  type  described  above  (Figs.  32,  33), 
though  with  some  interesting  modifications  of  detail.^  Swingle  ('97) 
describes  in  Stytopocauloii  a  process  closely  similar  to  that  seen  in 
many  animal  cells,  the  minute  but  very  distinct  centrosomes  being 
surrounded  by  quite  typical  cytoplasmic  asters,  passing  to  opposite 
poles  of  the  nucleus,  and  a  spindle  then  developing  between  them 
out  of  the  achromatic  nuclear  substance  (Fig.  32).  In  the  flowering 
plants  and  pteridophytes,  on  the  other  hand,  mitosis  seems  to  be  of  a 
quite  different  type,  apparently  taking  place  in  tJie  entire  absence  of 
centrosomes.  Guignard  ('91,  i,  '92,  2)  clearly  described  and  figured 
typical  centrosomes  and  attraction-spheres  both  in  the  ordinary 
mitosis  (Fig.  34)  and  in  the  fertilization  of  the  higher  plants,  giving 
an  account  of  their  behaviour  nearly  agreeing  with  the  views  then 
prevaiUng  among  zoologists.  Although  these  accounts  have  been 
supported  by  some  other  workers,^  and  have  recently  been  in  part 
reiterated  by  Guignard  himself  ('98,  i),  they  have  not  been  sustained 
by  some  of  the  best  and  most  careful  later  observers,  who  describe  a 
mode  of  spindle-formation  differing  radically  from  that  seen  in  thal- 
lophytes  and  in  animals  generally.-^  According  to  these  observations, 
begun  by  Farmer  and  Belajcff,  and  strongly  sustained  by  the  care- 
ful studies  of  Osterhout,  Mottier,  Nemec,  and  others,  the  achromatic 
figure  is  almost  wholly  of  cytoplasmic  origin,  arising  from  a  fibrillar 
material  ("  kinoplasm  "  or  "  filar  plasm,"  of  Strasburger),  which  at  the 
beginning  of  mitosis  forms  a  net-like  mass  surrounding  the  nucleus, 
from  which  fibrilloe  radiate  out  into  the  cytoplasm.  As  the  nuclear 
membrane  fades,  these  fibrillae,  continually  increasing,  invade  the 
nuclear  area,  gather  themselves  into  bundles,  converging  to  a  number 

1  See  especially  Swingle  ('97)  on  Sphacelariacea,  Strasburger  ('97)  on  Fucus,  Mottier 
('98)  on  Dictyota  ;  cf.  also  Harper  ('97)  on  Erysiphe  and  Peziza. 

2  Cf.  Schaffner  ('98),  Fulmer  ('98). 

^  See  Osterhout  ('97)  on  Eqidsettim,  Mottier  ('97,  I,  '97,  2)  on  Lilium,  Lawson  ('98)  on 
Cobcca,  Nemec  ('99)  on  Allium,  Debski  ('97,  '99)  on  Chara ;  also  Belajeff  ('94)  and 
Farmer  ('95). 


DETAILS   OF  MITOSIS 


83 


B 


of  centres  (without  centrosomes),  and  thus  give  rise  to  an  irregular 
multipolar  figure  (Figs.  36,  133).  This  figure  finally  resolves  itself 
into  a  definite  bipolar  spindle  which  is  devoid  of  centrosomes,  and 
in  the  earlier  stages  also  of  asters,  though  in  the  later  phases  some- 
what irregular  asters  are  formed.  On  the  basis  of  these  observations 
Mottier^  proposes  to  distinguish  provisionally  two  well-defined  types 
of  mitosis  in  plants  which  he  designates  as  the  ''thallophyte  "  and  the 
"cormophyte"  types.  The  latter  seems  wholly  irreconcilable  with 
the  process  observed  in  animal-cells ;  for  the  whole  course  of  spindle- 
formation  seems  diametrically  opposed  in  the  two  cases,  and  should 
the  cormophyte-type  be  established  it  would,  to  say  the  least,  greatly 
restrict  the  application  of  the  centrosome-theory  of  Van  Beneden  and 
Boveri.  Only  future  re- 
search can  definitely  de- 
termine the  question. 
There  can  be  no  doubt 
that  the  descriptions  of 
Guignard  and  his  follow- 
ers do  not  rest  upon  pure 
imagination  ;  for  it  is  easy 
to  observe  at  the  spindle- 
poles  in  some  prepara- 
tions {e.g.  sections  of  root- 
tips  of  AlliuDi,  Liliuin, 
etc.)  deeply  staining- 
bodies  such  as  these 
authors  describe.  These 
''centrosomes"  seem, 
however,  to  be  of  quite 
inconstant  occurrence  ; 
and  the  careful  studies  of 
Osterhout,  Mottier,  and  Nemec  seem  to  give  good  ground  for  the 
conclusion  that  they  have  no  such  significance  as  the  centrosomes  of 
lower  plants  or  of  animals.  It  should  nevertheless  be  borne  in  mind 
that  true  centrosomes  (*'  blepharoplasts  ")  have  been  demonstrated  in 
the  spermatogenic  divisions  of  some  of  the  vascular  cryptogams,  and 
that  analogous  bodies  occur  in  the  corresponding  divisions  of  the 
cycads  (p.  175).  We  should  therefore  still  hold  open  the  possibility 
that  centrosomes  may  occur  in  the  vegetative  mitoses  of  the  higher 
plants,  their  apparent  absence  being  possibly  due  to  lack  of  staining- 
capacity  or  similar  conditions  rendering  their  demonstration  difificult.- 


^  ■■■.   -     

Fig.  33.  — Mitosis  in  ascus-nuclei  of  a  fungus,  Erysiphe. 
[Harper.]  . 

A.  Resting  nucleus  with  disc-shaped  centrosome  [c^. 
B.  Early  prophase  with  aster.  C.  Later  prophase;  amphi- 
aster;  intranuclear  spindle  forming.  D.  Spindle  estab- 
lished. E.  Daughter-nucleus  after  division ;  spore-mem- 
brane developing  from  astral  rays. 


1  ' 


97,  2,  p.  183.  . 

2  Mention  may  here  be  made  of  the  barrel-shaped  truncated  spindles  described  m  some 
of  the  plants.     In  Basidiobolus,  Fairchild  ('97)  finds  spindles  of  this  type,  having  no  asters 


84 


CELL-DIVISIOX 


A  no  less  remarkable  mode  of  spindle-formation,  which  is  in  a  cer- 
tain way  intermediate  between  the  cormophyte-type  and  the  usual 
animal  type  is  described  by  Mead  ('97,  '98,  i)  in  the  first  maturation- 
division  of  CJuctoptcnis.  Here  the  completed  am]:)hiaster  is  of  quite 
typical  form,  and  the  centrosomes  persist  for  the  followin^^  mitosis; 
yet  Mead  is  convinced  that  the  amphiaster  is  synthetically  formed  by 
the  union  of  two  separate  asters  and  centrosomes  (Fig.  150)  which 


B 

Fig-  34-  —  Division  of  pollen-mother-cells  in  the  lily  as  described  by  Gl'IGNARD. 
A.  An.ii)hase  of  the  first  division,  showing  the  twelve  daughter-chromosomes  on  eacli  side,  the 
interzonal  fibres  stretching  between  them,  and  the  centrosomes,  already  double,  at  the  spindle- 
poles.  D.  Later  stage,  showing  the  cell-plate  at  the  equator  of  the  spindle  and  the  daughter- 
spiremes  (dispireme-stage  of  Flemming).  C.  Division  completed;  double  centrosomes  in  the 
resting  cell.  D.  Ensuing  division  in  progress;  the  upper  cell  at  the  close  of  the  proj^hases,  the 
chromosomes  and  centrosomes  still  undivided;  lower  cell  in  the  late  anaphase,  cell-plate  not  yet 
formed. 


have  no  genetic  connection,  arising  independently  dc  novo  in  the 
cytoplasm.^  Improbable  as  such  a  conclusion  may  seem  on  a  priori 
grounds,  it  is  supported  by  very  strong  evidence,^  and,  taken  together 

and  nearly  parallel  fibres,  each  of  which  terminates  in  a  deeply  staining  granule.  Nearly 
similar  spindles  have  been  described  by  Strasburger  ('So)  in  Spirogyra,  and  in  the  embryo- 
sac  o{  Monoiropa.  It  is  not  impossible  that  such  spindles  may  represent  a  type  intermediate 
between  the  "cormoptyte  "  and  "  thallophyte  "  types  of  Mottier. 

1  C/:  p.  306. 

2  I  have  had  the  privilege  of  examining  some  of  Mead's  beautiful  preparations. 


DETAILS   OF  MITOSIS 


85 


with  the  facts  described  in  plants,  it  indicates  that  the  forces  involved 
in  sjDindle-formation  are  far  more  complex  than  Van  Ik'neden's  and 
Boveri's  hypothesis  would  lead  one  to  suppose.^ 

The  centrosome  and  centrosphere  appear  to  present  great  varia- 
tions that  have  not  yet  been  thoroughly  cleared  up  and  will  be  more 
critically  discussed  beyond. ^  They  are  known  to  underg(j  extensive 
changes  in  the  cycle  of  cell-division  and  to  vary  greatly  in  different 
forms  (Fig.  152).  In  some  cases  the  aster  contains  at  its  centre 
nothing  more  than  a  minute  deeply  staining  granule,  which  doubtless 


A 


^^g-  36-  —  Division  of  spore-mother-cells  in  Eqiiisetum,  sliowing  spindle-formation.     [OsTERHOfT.] 

A.  Early  prophase, "  kinoplasmic  "  fibrillae  in  the  cytoplasm.  B.  Multipolar  fibrillar  figure  invad- 
ing the  nuclear  area,  after  disappearance  of  the  nuclear  membrane.  C.  Multipolar  spindle. 
D.   Quadripolar  spindle  which  finally  condenses  into  a  bipolar  one. 


represents  the  centrosome  alone.  In  other  cases  the  granule  is  sur- 
rounded by  a  larger  body,  which  in  turn  lies  within  the  centrosphere 
or  attraction-sphere.  In  still  other  cases  the  centre  of  the  aster  is 
occupied  by  a  large  reticular  mass,  within  which  no  smaller  body  can 
be  distinguished  {e.g.  in  pigment-cells);  this  mass  is  sometimes  called 
the  centrosome,  sometimes  the  centrosphere.  Sometimes,  again,  the 
spindle-fibres   are   not   focus.sed    at  a   single   point,  and   the   spindle 

^  See  p.  276  for  the  peculiar  spindles,  devoid  of  asters,  observed  during  the  maturation  of 
the  egg  in  certain  forms.  Cf.  also  Morgan's  experiments  on  the  artificial  production  of  asters 
and  centrosomes,  p.  307.  -  See  p.  304. 


35  CELL-DIVISIOX 

appears  truncated  at  the  ends,  its  fibres  terminating  in  a  transverse 
row  of  granules  (maturation-spindles  of  Ascaris,  and  some  plant-cells). 
It  is  not  entirely  certain,  however,  that  such  spindles  observed  in 
]:)reparations  represent  the  normal  structure  during  life. 

b.     Tlic    CJinwiatic    Fii^mr. — The     variations     ot     the     chromatic 
figure   must   for  the   most   part   be   considered   in   the   more   special 
parts  of   this   work.      There    seems   to  be  no   doubt    that   a    single 
continuous    spireme-thread    may    be    formed   {rf.    p.    113).    ^^'^it   it    is 
equally  certain    that    the    thread    may    appear   from    the    beginning 
in    a    number   of    distinct    segments,    i.e.    as    a    segmented    spireme, 
and    there   are    some    cases    in    which    no    distinct    spireme    can   be 
seen,  the  reticulum  resolving  itself  directly   into  the   chromosomes. 
The    chromosomes,    when    fully    formed,    vary    greatly    in    appear- 
ance.      In   many  of   the   tissues   of   adult   plants   and   animals  they 
are   rod-shaped   and   are   often   bent  in  the   middle   like   a   V  (Figs. 
28,  131  ).       Thev  often   have   this   form,  too,  in  embryonic   cells,  as 
in    the    segmentation-stages    of    the    Qgg    in    Ascaris   (Fig.    31)  and 
other  forms.     The  rods  may,  however,  be  short  and   straight  (seg- 
menting eggs  of  echinoderms,  etc),  and  may  be  reduced  to  spheres, 
as  in  the  maturation-stages  of  the  germ-cells.     In  the  equatorial  plate 
the  V-shaped  chromosomes  are  placed  with  the  apex  of  the  V  turned 
toward    the    spindle  (Fig.    28),   while    the   straight  rods    are    placed 
with  one  end   toward   the   spindle.       In   either   case    the    daughter- 
chromosomes    first   begin    to    move    apart   at   the   point  nearest  the 
spindle,  the  separation  proceeding  thence  toward  the    free   portion. 
The  V-shaped  chromosomes,  opening  apart  from  the  apex,  thus  give 
rise  in  the  early  anaphase  to  <>-shaped  figures;  while  rod-shaped 
chromosomes  often  produce  A-  ^'I'ld  i-shaped  figures  (the  stem  of  the 
1  being  double).     The  latter,  opening  farther  apart,  form  straight 
rods  twice  the  length  of  the  original  chromosome  (since  each  consists 
of  two  daughter-chromosomes  joined  at  one  end).     This  rod  finally 
breaks  across  the  middle,  thus  giving  the  deceptive  appearance  of  a 
transverse  instead  of  a  longitudinal  division  (Fig.   52).      The    <>- 
shaped  figures    referred  to  above    are   nearly   related  to  those   that 
occur  in  the  so-called  Jictcrotypical  mitosis.     Under  this  ^name   Flem- 
ming  ('87)  first  described  a  peculiar  modification  of  the  division  of  the 
chromosomes  that  has  since  been  shown  to  be  of  very  great  impor- 
tance in  the  early  history  of  the  germ-cells,  though  it  is  not  confined 
to  them.     In  this  form  the  chromosomes  split  at  an  early  period,  but 
the  halves  remain  united  by  their  ends.     Each  double  chromosome 
then  opens  out  to  form  a  closed  ring  (Fig.  37),  which  by  its  mode  of 
origin  is  shown  to  represent  two  daughter-chromosomes,  each  forming 
half  of  the  ring,  united  by  their  ends.     The  ring  finally  breaks  in  two 
to  form  two  U-shaped  chromosomes  which  diverge  to  opposite  poles 


DETAILS   OF  MITOSIS 


s? 


of  the  spindle  as  usual.  As  will  be  shown  in  Chapter  V.,the  divisions 
by  which  the  germ-cells  are  matured  are  in  many  cases  of  this  type ; 
but  the  primary  rings  here  in  many  cases  represent  not  two  but  four 
chromosomes,  into  which  they  afterward  break  up. 


Fig-  37-  ~  Heterotypical  mitosis  in  spermatocytes  of  the  salamander.  [Flemminc] 
A.  Prophase,  chromosomes  in  the  form  of  scattered  rings,  each  of  which  represents  lui. 
daughter-chromosomes  joined  end  to  end.  B.  The  rings  ranged  about  the  equator  of  the  spindle 
and  dividing ;  the  swellings  indicate  the  ends  of  the  chromosomes.  C.  The  same  viewed  from  the 
spindle-pole.  D..  Diagram  (Hermann)  showing  the  central  spindle,  asters,  and  centrosomes,and 
the  contractile  mantle-fibres  attached  to  the  rings  (one  of  the  latter  dividing). 


2.    Bivalent  and  Plurivalent  Chroniosomes 

The  last  paragraph  leads  to  the  consideration  of  certain  varia- 
tions in  the  number  of  the  chromosomes.  Boveri  discovered  that  the 
species  Ascaris  mcgaloccpJiala  comprises  two  varieties  which  differ  in 
no  visible  respect  save  in  the  number  of  chromosomes,  the  germ-nuclei 
of  one  form  ("  variety  bivalens  "  of  Hertwig)  having  two  chromosomes, 


g3  CELL-DIVISIOX 

while  in  the  other  form  ("  variety  iinivalens  ")  there  is  but  one.  Braiier 
discovered  a  similar  fact  in  the  phyllopod  Artcmia,  the  number  of 
somatic  chromosomes  being   i68  in  some  individuals,  in  others  only 

84  (p.  281). 

It  will  appear  hereafter  that  in  some  cases  the  primordial  germ- 
cells  show  only  half  the  usual  number  of  chromosomes,  and  in 
Cyclops  the  same  is  true,  according  to  Hacker,  of  all  the  cells  of 
the  early  cleavage-stages. 

In  all  cases  where  the  number  of  chromosomes  is  apparently 
reduced  ("pseudo-reduction"  of  Riickert)  it  is  highly  probable  that 
each  chromatin-rod  represents  not  one  but  two  or  more  chromosomes 
united  together,  and  Hacker  has  accordingly  proposed  the  terms 
bivalent  and  plnrivaloit  for  such  chromatin-rods.^  •  The  truth 
of  this  view,  which  originated  with  Vom  Rath,  is,  I  think,  conclusively 
shown  by  the  case  of  Artcniia  described  at  page  281,  and  by  many  facts 
in  the  maturation  of  the  germ-cells  hereafter  considered.  In  Ascaris 
we  may  regard  the  chromosomes  of  Hertwig's  "variety  univalens  " 
as  really  bivalent  or  double,  i.e.  equivalent  to  two  such  chromosomes 
as  appear  in  "  variety  bivalens."  These  latter,  however,  are  probably 
in  their  turn  plurivalent,  i.e.  represent  a  number  of  units  of  a  lower 
order  united  together;  for,  as  described  at  page  148,  each  of  these 
normally  breaks  up  in  the  somatic  cells  into  a  large  number  of  shorter 
chromosomes  closely  similar  to  those  of  the  related  species  Ascaris 
liunbricoides,  where  the  normal  number  is  24. 

Hacker  has  called  attention  to  the  striking  fact  that  plurivalent 
mitosis  is  very  often  of  the  heterotypical  form,  as  is  very  common 
in  the  maturation-mitoses  of  many  animals  (Chapter  V.),  and  often 
occurs  in  the  early  cleavages  of  Ascaris ;  but  it  is  doubtful  whether 
this  is  a  universal  rule. 

3.    Mitosis  in  the  Unicellular  Plants  and  Animals 

The  process  of  mitosis  in  the  one-celled  plants  and  animals  has  a 
peculiar  interest,  for  it  is  here  that  we  must  look  for  indications  of 
its  historical  origin.  But  although  traces  of  mitotic  division  were 
seen  in  the  Infusoria  by  Balbiani  ('58-61),  Stein  ('59),  and  others 
long  before  it  was  known  in  the  higher  forms,  it  has  only  recently 
received  adequate  attention  and  is  still  imperfectly  understood. 

Mitotic  division  has  now  been  observed  in  many  of  the  main  divi- 
sions of  Protozoa  and  unicellular  plants ;  but  in  the  present  state  of 

1  The  words  bivalent  and  uui-'alcni  have  been  used  in  precisely  the  opposite  sense 
by  Hertwig  in  the  case  of  Ascaris.  the  former  term  being  applied  to  that  variety  having  two 
chromosomes  in  the  germ-cells,  the  latter  to  the  variety  with  one.  These  terms  certainly 
have  priority,  but  were  applied  only  to  a  specific  case.  Hacker's  use  of  the  words,  which  is 
strictly  in  accordance  with  their  etymology,  is  too  valuable  for  general  descriptive  purposes  to 
be  rejected. 


DETAILS   OF  MITOSIS 


89 


the  subject  it  must  be  left  an  open  question  whether  it  occurs  in  all. 
In  some  of  the  gregarines  and  Heliozoa,  the  process  is  of  nearly  or 
quite  the  same  type  as  in  the  Metazoa.  From  such  mitoses,  how- 
ever, various  gradations  may  be  traced  toward  a  much  simpler  pro- 
cess, such  as  occurs  in  Aniceba  and  the  lower  flagellates ;  and  it  is  not 
improbable  that  we  have  here  representatives  of  more  primitive  con- 
ditions. Among  the  more  interesting  of  these  modifications  may  be 
mentioned  :  — 

I.    Even  in  forms  that  nearly  approach  the  mitosis  of  higher  types 


B 


D 


Fig.  38.  —  Mitotic  division  in  Infusoria,     [k.  Hkr  rwiG.] 

A-C.  jSIacronucleus  of  Spirochona,  showing  pole-plates.  D-H.  Successive  stagres  in  the 
division  of  the  micronucleus  of  Paravicecium.  D.  The  earliest  stage,  showing  reticulum.  G.  Fol- 
lowing stage  ("  sickle-form  ")  with  nucleolus.  E.  Chromosomes  and  pole-plates.  /•:  Late  ana- 
phase.    H.  Final  phase. 


the  nuclear  membrane  may  persist  more  or  less  completely  through 
every  stage  {Noctiliica,  EuglypJia,  ActinospJicerium). 

2.  Asters  maybe  present  (Heliozoa,  gregarines)  or  wanting  (In- 
fusoria, Radiolaria). 

3.  In  one  series  of  forms  the  centrosome  or  sphere  is  represented 
by  a  persistent  intranuclear  body  (nucleolo-centrosome)  of  consider- 
able size,  which  divides  to  form  a  kind  of  central  spindle  { Huo^/ina 
Aniocbuy  Infusoria.''). 

4.  In   a   second   series   the   centrosome  or  sphere  is  a  persistent 


90 


CELL-DIVISION 


cxtranuclear  body,  as   in  most  Metazoa  {Heliozoa,  Noctilucay  Para- 
moeba ). 

5.  In  a  few  forms  havin<;  a  scattered  nucleus  the  chromatin-gran- 
ules  are  only  collected  about  the  apparently  j)ersistent  sphere  or 
centrosome  at  the  time  of  its  division,  and  afterward  scatter  throuirh 
the  cell,  leaving  the  sphere  lying  in  the  general  cell-substance 
(  Tctrainitiis). 

6.  The  arrangement  of  the  chromatin-granules  to  form  chromo- 
somes appears  to  be  of  a  secondary  importance  as  compared  with 


A  BCD 

Fig-  39-  —  Mitosis  in  the  rhizopod,  Euglyplia.     [SCHEWIAKOFF.] 
In  this  form  the  body  is  surrounded  by  a  firm  shell  which  prevents  direct  constriction  of  the 
cell-body.     The  latter  therefore  divides  by  a  process  of  budding  from  the  opening  of  the  shell 
(the  initial  phase  shown  at  .-/)  ;  the  nucleus  meanwhile  divides,  and  one  of  the  daughter-nuclei 
aftenvard  wanders  out  into  the  bud. 

A.  Early  prophase;  nucleus  near  lower  end  containing  a  nucleolus  and  numerous  chromo- 
somes. D.  lu]u.-itorial  plate  and  spindle  formed  insirle  the  nucleus;  pole-bodies  or  pole-plates 
(i.e.  attraction-spheres  or  centrosomes)  at  the  spindle-poles.  C.  Metaphase.  D.  Late  ana- 
phase, spindle  dividing;  after  division  of  the  spindle  the  outer  nucleus  wanders  out  into  the  bud. 

higher  forms,  and  the  essential  feature  in  nuclear  division  a])pears  to 
be  the  fission  of  the  individual  crranules. 

We  may  first  consider  especially  the  achromatic  figure.  The  basis 
of  our  knowledge  in  this  field  was  laid  by  Richard  Hertwig  through  his 
studies  on  an  infusorian,  SpirocJioiia  {  ^yy),  and  a  rhizo})od,  Actiiio- 
spJicenum{'?i/\).  In  both  these  forms  a  typical  spindle  and  equatorial 
plate  are  formed  inside  tJie  nuclear  membrane  by  a  direct  transfor- 
mation of  the  nuclear  substance.     In   SpirocJiona  (Fig.  38,  A-C)  a 


DETAILS   OF  MITOSIS 


91 


hemispherical  ''end-plate"  or  *' pole-plate  "  is  situated  at  either  pole 
of  the  spindle,  and  Hertwig's  observations  indicated,  though  they 
did  not  prove,  that  these  plates  arose  by  the  division  of  a  large 
"nucleolus."  Nearly  similar  pole-plates  were  somewhat  described  by 
Schewiakoff  ('88)  in  Eiiglypha  (Fig.  39),  and  it  seems  clear  that  they 
are  the  analogues  of  the  centrosomes  or  attraction-spheres  in  higher 
forms.  In  Euglcna,  as  shown  by  Keuten,  the  pole-plates,  or  their 
analoo-ues,  certainly  arise  by  division  of  a  distinct  and  persistent  intra- 
nuclear body  ("nucleolus"  or  "  nucleolo-centrosome  ")  which  elon- 


Fig.  40.  —  Mitosis  in  the  flagellate,  Eugleua.     [KEUTEN.] 
A    Preparing  for  division ;  the  nucleus  contains  a  "  nucleolus  "  or  nucleolo-centrosome  sur- 
rounded Ty  a  g-up  of  chromosomes.     B.  Division  of  the  ;•  nucleolus     to  form  an  mtranuclear 
spindle.     C.  Later  stage.    D.  The  nuclear  division  completed. 

gates  to  form  a  kind  of  central  spindle  around  which  the  chromatin 
elements  are  grouped  (Fig.  40);  and  Schaudinn  ("95)  described  a 
similar  process  in  Am(^ba.  Richard  Hertwig's  latest  work_  on 
Infusoria  ('95)  indicates  that  a  similar  process  occurs  m  the  micro- 
nuclei  of  Paramcccinm,  which  at  first  contain  a  large  "  nucleolus 
and  afterward  a  conspicuous  pole-plate  at  either  end  ot  the  spindle 
(Fio-  :;8  D-H\  TJie  origin  of  the  pole-plates  was  not,  however, 
po^tively  determined.  A  corresponding  dividing  body  is  foupd  m 
Ccratiinn  (Lauterborn,  '95),  and  as  in  the  Infusoria  the  entire 
nucleus    transforms    itself    into    a    fibrillar   spindle-like    body. 


92  CELL-DIVISION 

Still  simpler  conditions  are  found  in  some  of  the  flagellates.^  In 
Chilonionas  the  sphere  may  still  be  regarded  as  intranuclear,  since  it 
lies  in  the  middle  of  an  irregular  mass  of  chromatin-granules,  though 
the  latter  are  apparently  not  enclosed  by  a  membrane.  Nuclear 
division  is  here  accomplished  by  fission  of  the  sphere  and  the  aggre- 
gation of  the  chromatin-granules  around  the  two  products.  In 
Tctramitus,  finally  (Fig.  i6),  the  nucleus  is  represented  by  chromatin- 
granules  that  are  scattered  irregularly  through  the  cell  and  only  at 
the  time  of  division  collect  about  the  dividing  sphere. 


t^ 


CvOl,  ,  <^&< 


« 
r 


Vc. 


*-*'<■* 


B  C 


—A  ♦I''-  , 


O 


^i:^^'  --^ 


F 


Fig.  41.  —  Mitosis  in  the  Heliozoa.     [SCHAUDINN.] 
A.  Sp/ifrrastruiN  :  vegetative  cell  showing  nucleus,  "central  gmnule"  (centrosonic),  and  axial 
rays.     li-G.  Acanthocystis.     B-D.  Prophases  of  mitosis.      E.  Budding   to    form    swarm-spores. 
F.  Swarm-spores,  devoid  of  centrosomes.     G.  Swarm-spores  preparing  for  division  ;  intranuclear 
origin  of  centrosome. 

In  a  second  series  of  forms,  represented  by  Noctiliica  (Ishikawa, 
'94,  98),  (Calkins,  '98,  2),  raranurba  (Schaudinn,  '96,  i),  Actinophrys 
and  Acanthoi-ystis  (Schaudinn,  '96,  2),  and  the  diatoms  (Lauterborn, 
'96),  the  sphere  lies  outside  the  nucleus  in  the  cytoplasm  and  the 
mitosis  is  closely  similar  to  that  observed  in  most  Metazoa.  This  is 
most  striking  in  the  Heliozoa,  where  the  centrosome  persists  through 
the  vegetative  condition  of  the  cell  as  the  *•  central  granule,"  to  which 
the  axial  filaments  of  the  pseudopodia  converge.  Schaudinn  ('96,  2) 
shows  that  by  the  division  of  this  body  a  typical  extranuclear  amphi- 
aster  and  central  spindle  are  formed  (Fig.  41),  while  the  chromatin 

^  Calkins,  '98,  I,  '98,  2. 


DETAILS    OF  MITOSIS 


93 


passes  through  a  spireme-stage,  breaks  into  very  short  rod-shaped 
chromosomes  which  spHt  lengthwise  and  arrange  themselves  in  the 
equator  of  the  spindle,  while  the  nuclear  membrane  fades  away. 
Noctiluca  (Fig.  42),  as  shown  by  Ishikawa  and  Calkins,  agrees  with 
this  in  the  main  points;  but  the  nuclear  membrane  does  not  at  any 
period  wholly  disappear,  and  a  distinct  centrosome  is  found  at  the 
centre  of  the  sphere.     The  latter  body,  which  is  very  large,  gives 


••;.-.r-.vA-r.:.r->; .,      -■«■"'.•.>•.. 
TS\:/.;;.\';::...  ■.  .  ..  ■■;  ^*!?Xi; 


^^V}r;:-\:.-:, 


D 

Fig.  42.  —  Mitosis  in  Noctiluca.  [Calkins.] 
A.  Prophase;  division  of  the  sphere  to  form  the  central  spindle;  chromosomes  convertjing  to 
the  nuclear  pole.  B.  Late  anapliase.  in  horizontal  section,  showing  centrosomes;  the  crntral 
spindle  has  sunk  into  the  nucleus;  nuclear  membrane  still  intact  except  at  the  polos.  C.  \-A\\y 
anaphase;  mantle-fibres  connected  with  the  diverging  chromosomes.  D.  Kinal  anaphase  (which 
is  also  the  initial  prophase  of  the  succeeding  division  of  spore-forming  mitosis)  ;  doubling  of  cen- 
trosome and  splitting  of  chromosomes. 


rise  by  a  division  to  a  fibrillated  central  spindle,  about  which  the 
nucleus  wraps  itself  while  mantle-fibres  are  developed  from  the 
sphere-substance  and  become  attached  to  the  chromosomes,  the  nu- 
clear membrane  fading  away  along  the  surface  of  contact  with  the 
central  spindle  (Calkins).     Broadly  speaking,  the  facts  are  similar  in 


94  CELL-DIVISION 

the  diatoms  {Surirclla,  t.  Lauterborn),  where  the  central  spindle, 
arisinf(  by  a  peculiar  ]:)rocess  from  an  extranuclear  centrosome, 
(sphere  ?)  sinks  into  the  nucleus  in  a  manner  strongly  suggesting  that 
observed  in  Xoctiluca. 

In  the  interesting  form  raiamaini,  as  described  by  Schaudinn 
('96,  I ),  the  sphere  (*'  Nebenkorper  "),  which  is  nearly  as  large  as  the 
nucleus,  divides  to  form  a  central  spindle,  about  the  equator  of  which 
the  chromatin-elements  become  arranged  in  a  ring  (Fig.  43);  but  no 
centrosome  has  yet  been  demonstrated  in  the  sj^here.  J\\rai)uvba 
appears  to  differ  from  Rugloia  mainly  in  the  fact  that  at  the  close  of 
dixision  the  sphere  is  in  the  former  left  outside  the  daughter-nucleus 
and  in  the  latter  enclosed  within  it.^  The  connecting  link  is  perfectly 
given  by  Tctramitus,  where  no  morphological  nucleus  is  formed,  and 
the  sphere  lies  in  the  general  cell-substance  (p.  92);  and  we  could 
have  no  clearer  demonstration  that  the  extra-  or  intranuclear  position 
of  sphere  or  centrosome  is  of  quite  secondary  importance.  As 
regards  the  formation  of  the  spheres  (pole-plates)  ActinospJiceriuin 
(Figs.  44,  45)  seems  to  show  a  simpler  condition  than  any  of  the 
above  forms,  since  no  permanent  sphere  exists,  and  Brauer  ('94)  and 
R.  riertwig  ('98)  agree  that  the  pole-plates  are  formed  by  a  gradual 
accumulation  of  the  achromatic  substance  of  the  nucleus  at  opposite 
poles. 

A  distinct  centrosome  (centriole  t)  in  the  interior  of  the  sphere  has 
thus  far  only  been  observed  in  a  few  forms  {Noctiluca,  ActitiospJice- 
riu7ii),  and  neither  its  origin  nor  its  relation  to  the  sphere  has  yet 
been  sufficiently  cleared  up.  Both  Ishikawa('94)  and  Calkins  ('98,  2) 
somewhat  doubtfully  concluded  that  in  Xoctiluca  the  centrosomes 
arise  within  the  nucleus,  migrating  thence  out  into  the  extranuclear 
sphere.  With  this  agree  R.  Hertwig's  latest  studies  on  Actiiiosphce- 
riiiDi  ('98),  the  spindle-poles  being  first  formed  from  the  pole-plates 
(themselves  of  nuclear  origin),  and  the  centrosomes  then  passing  into 
them  from  the  nucleus.  HertwMg  reaches  the  further  remarkable 
conclusion  that  the  centrosomes  arise  as  portions  of  the  cJiromatin- 
iictivork  extruded  at  the  nuclear  poles  (Fig.  45),  first  forming  a 
spongy  irregular  mass,  but  afterward  condensing  into  a  deeply 
staining  pair  of  granules  which  pass  to  the  respective  poles  of  the 
spindle.  It  is  a  remarkable  fact  that  these  centrosomes  are  only 
found  in  the  two  maturation-divisions,  and  are  absent  from  the  ordi- 
nary vegetative  mitoses  where  the  spindle-poles  arc  formed  by  two 
cytoplasmic  masses  derived,  as  Hertwig  believes,  from  the  intra- 
nuclear plates.  Schaudinn  ('96,  3)  likewise  describes  and  clearly 
figures  an  intranuclear  origin  of  the  centrosome  in  buds  of  AcantJio- 
cystis  (Fig.  41),  which  are  derived  by  direct  division  of  the  mother- 

1  Cf.  Calkins,  '98,  i,  p.  388. 


DETAILS   OF  MITOSIS 


95 


nucleus  with  no  trace  of  a  centrosome.  In  this  same  form,  as 
described  above,  the  ordinary  vegetative  mitoses  are  quite  of  the 
metazoan  type,  with  a  persistent  extranuclcar  centrosome. 

The  history  of  the  chromatin  in  the  mitosis  of  unicellular  forms 
shows  some  interesting  modifications.  In  a  considerable  number  of 
forms  a  more  or  less  clearly  marked  spireme-stage  precedes  the  forma- 
tion of  chromosomes  (diatoms,  Infusoria,  dinoflagellates,  Iuio/yp/i(i)\ 
in  others,  long  chromosomes  are  formed  without  a  distinct  spireme- 
stage  (N'octihtca).  It  has  been  clearly  demonstrated  that  in  some 
cases  these  chromosomes  split  lengthwise,  as  in   Metazoa  {A^octilnca, 


V 


«c^;> 


X, 


-O 


Fig.  43.  —  Mitosis  in  Parafnccba.     [SCJIAUDINN.] 
At  the  left,  amoeboid  phase,  showing  nucleus  and  "  Nebenkorper."     At  the  right,  four  stages 
of  division  in  the  swarm-spores. 


diatoms,  Actinophrys,  probably  in  EuglypJia) ;  but  in  some  cases  they 
are  stated  to  divide  transversely  in  the  middle  (Infusoria  according 
to  Hertwig,  Ccratiuin  according  to  Lauterborn).  These  chromosomes 
appear  always  to  arise,  as  in  Metazoa,  through  the  linear  arrangement 
of  chromatin-granules  {^Noctihica,  Actinospluvriuni,  Euglciia\  wliich 
themselves  in  many  cases  arise  by  the  preliminary  fragmentation  of 
one  or  more  large  chromatin-masses  {^c.g.  in  Noctiluca  or  ActiuosplicB- 
rijun).  In  other  forms  no  such  linear  aggregates  are  formed,  and 
direct  fission  of  the  chromatin-granules  appears  to  take  place  without 
the  formation  of  bodies  morphologically  comparable  with  the  chromo- 
somes of  such  forms  as  Noctiluca.  This  is  apparently  the  case  in 
Tetrajnitus,    and    Achromatium,   other    forms    having    a    distributed 


96 


CELL-DIVISION 


nucleus,!  ^nd  in  such  forms  as  CJiilomouas  and  Tmchclomonas,  where 
the  <^ranules  are  permanently  aggret^ated  about  a  central  body.  Too 
little  is  known  of  the  facts  to  justify  a  very  positive  statement;  but 
on  the  whole  they  point  toward  the  conclusion  that  in  the  simplest 


qqOs, 


o    «.«' .'."• ,12 


o 


'ooo» 


,"oOoo 


.oo'">% 


».."""%„ 


^ 


% 


\ 


"imd 


I 


^'oirv^Baio^' 


oOQOooo 


Fig.  44.  —  Mitosis  in  the  rhizoped  Actinospluvrium.     [Braukr.] 

A.  Nucleus  and  surrounding  structures  in  the  early  prophase;  above  and  below  the  reticular 
nucleus  lie  the  semilunar  "  pole-plates."  and  outside  these  the  cytoplasmic  masses  in  which  the 
asters  afterward  develop.  B.  Later  stage  of  the  nucleus.  D.  Mitotic  figure  in  the  metaphase, 
showing  equatorial  plate,  intra-nuclear  spindle,  and  pole-plates  {p.p.).  C.  Equatorial  plate, 
viewed  ^////r*",  consisting  of  double  chromatin-granules.  E.  Early  anaphase.  E.G.  Later  ana- 
phases. H.  F^inal  anaphase.  /.  Telophase;  daughter-nucleus  forming,  chromatin  in  loop-shaped 
threads;  outside  the  nuclear  membrane  the  centrosome,  already  divided,  and  the  aster.  J.  1-ater 
stage;  the  daughter-nucleus  established;  divergence  of  the  centrosomes.  Beyond  this  point  the 
centrosomes  have  not  been  followed. 


types  of  mitosis  no  true  chromosome-formation  occurs,  thus  sustaininp^ 
Brauer's  conclusion  that  the  es.sential  fact  in  the  history  of  the  chro- 
matin in  mitosis  is  the  fission  of  the  individual  granules.^ 

1  The  fission  of  the  individual  granules  is  carefully  described  and  figured  by  Schewiakoff 
in  Achromatimn. 
■     '-^  For  speculations  on  the  historical  origin  of  the  centrosome,  etc.,  see  p.  315. 


DETAILS   OF  MITOSIS 


97 


4.    Pathological  Mitoses 

Under  certain  circumstances  the  delicate  mechanism  of  cell-division 
may  become  deranged,  and  so  give  rise  to  various  forms  of  patho- 
logical mitoses.  Such  a  miscarriage  may  be  artificially  produced,  as 
Hertwig,  Galeotti,  and  others  have  shown,  by  treating  the  dividing 
cells  with  poisons  and  other  chemical  substances  (quinine,  chloral, 
nicotine,  potassic  iodide,  etc.).     Pathological  mitoses  may,  however, 


x: 


\ 


B 


^^^F 


C  D 

Fig.  45.  —  Mitosis  in  Actinosphcerium.  [R.  Hertwig.] 
A.  Encysted  form,  with  resting  nucleus;  chromatin  aggregated  into  large  nucleolus-like  body. 
B.  prophase  of  division  of  the  encysted  form,  showing  chromosome-like  bodies  formed  of  granules, 
and  spindle  without  centrosomes.  C.  Earlier  prophase  of  the  first  maturation  division,  showing 
extrusion  of  chromatic  substance  to  form  the  centrosome.  D.  Later  stage,  showing  centrosome 
and  aster. 

occur  without  discoverable  external  cause ;  and  it  is  a  very  interesting 
fact,  as  Klebs,  Hansemann,  and  Galeotti  have  especially  pointed  out, 
that  they  are  of  frequent  occurrence  in  abnormal  growths  such  as 
cancers  and  tumours. 

The  abnormal  forms  of  mitoses  are  arranged  by  Hansemann  in  two 
general  groups,  as  follows:  (i)  asymmetrical  mitoses,  in  which  the 
chromosomes  are  unequally  distributed  to  the  daughter-cells,  and  (2) 
multipolar  mitoses,  in  which  the  number  of  centrosomes  is  more  than 

H 


98 


CELL-DIVISIOX 


two,  and  more  than  one  spindle  is  formed.  Under  the  first  group  are 
inckided  not  only  the  cases  of  unequal  distribution  of  the  daughter- 
chromosomes,  but  also  those  in  which  chromosomes  fail  to  be  drawn 
into  the  equatorial  })late  and  hence  are  lost  in  the  cytoplasm. 

Klebs  first  pointed  out  the  occurrence  of  asymmetrical  mitoses  in 
carcinoma-cells,  where  they  have  been  carefully  studied  by  Hanse- 
mann  and  Galeotti.  The  inequality  is  here  often  extremely  marked, 
so  that  one  of  the  daughter-cells  may  receive  more  than  twice  as 
much  chromatin  as  the  other  (Fig.  46).     Hansemann,  whose  conclu- 


E  F 

Fig.  46.  —  Pathological  mitoses  in  human  cancer-cells.     [Galeotti.] 

A.  Asymmetrical  mitosis  with  unequal  centrosomes.  B.  Later  stage,  showing  unequal  distri- 
bution of  the  chromosomes.  C.  Quadripolar  mitosis.  D.  Tripolar  mitosis.  E.  Later  stage. 
F.  Trinucleate  cell  resulting. 

sions  are  accepted  by  Galeotti,  believes  that  this  asymmetry  of  mito- 
sis gives  an  explanation  of  the  familiar  fact  that  in  cancer-cells  many 
of  the  nuclei  are  especially  rich  in  chromatin  (hyperchromatic  cells), 
while  others  are  abnormally  poor  (hypochromatic  cells).  Lustig  and 
Gale(^tti  ('93)  showed  that  the  unequal  distribution  of  chromatin  is 
correlated  with  and  probably  caused  by  a  corresponding  inequality  in 
the  centrosomes  which  causes  an  asymmetrical  development  of  the 
amphiaster.  A  very  interesting  discovery  made  by  Galeotti  ('93)  is 
that  asymmetrical  mitoses,  exactly  like  those  seen  in  carcinoma,  may 
be  artificially  produced  in  the  epithelial  cells  of  salamanders  (Fig.  47) 
by  treatment  with  dilute  solutions  of  various  drugs  (antipyrin,  cocaine, 
quinine). 


DETAILS   OF  MITOSIS 


99 


Normal  multipolar  mitoses,  though  rare,  sometimes  occur,  as  in  the 
division  of  the  pollen-mother-cells  and  the  endosperm-cells  of  flower- 
ing plants  (Strasburger) ;  but  such  mitotic  figures  arise  through  the 
union  of  two  or  more  bipolar  amphiasters  in  a  syncytium  and  are 
due  to  a  rapid  succession  of  the  nuclear  divisions  unaccompanied  by 
fission  of  the  cell-substance.  These  are  not  to  be  confounded  with 
pathological  mitoses  arising  by  premature  or  abnormal  division  of  the 
centrosome.  If  one  centrosome  divide,  while  the  other  does  not, 
triasters  are  produced,  from  which  may  arise  three  cells  or  a  tri- 
nucleated  cell.  If  both  centrosomes  divide,  tetrasters  or  polyasters 
are  formed.  Here  again  the  same  result  has  been  artificially  attained 
by  chemical  stimulus  {cf.  Schottlander,  '^S).     Multipolar  mitoses  are 


^  B 

Fig.  47.  —  Pathological  mitoses  in  epidermal  cells  of  salamander  caused  by  poisons. 
[Galeotti.] 

A.  Asymmetrical  mitosis  after  treatment  with  0.05  %  antipyrin  solution.  B.  Tripolar  mitosis 
after  treatment  with  0.5%  potassic  iodide  solution. 

also  common  in  regenerating  tissues  after  irritative  stimulus  (Strobe); 
but  it  is  uncertain  whether  such  mitoses  lead  to  the  formation  of 
normal  tissue.^ 

The  frequency  of  abnormal  mitoses  in  pathological  growths  is  a 
most  suggestive  fact,  but  it  is  still  wholly  undetermined  whether  the 
abnormal  mode  of  cell-division  is  the  cause  of  the  disea.se  or  the 
reverse.  The  latter  seems  the  more  probable  alternative,  since  nor- 
mal mitosis  is  certainly  the  rule  in  abnormal  growths  ;  and  Galeotti's 
experiments  suggest  that  the  pathological  mitoses  in  such  growths 
may  be  caused  by  the  presence  of  deleterious  chemical  products  in 
the  diseased  tissue,  and  perhaps  point  the  way  to  their  medical 
treatment. 

1  The  remarkable  polyasters  formed  in  polyspermia  fertilization  of  the  egg  are  de- 
scribed at  page  198. 


lOO 


CELL-DIVISIOX 


D.     The  Mfxhamsm  of  Mitosis 

We  now  pass  to  a  consideration  of  the  forces  at  work  iii   mitotic 
division,   which    leads   us   into  one   of   the   most   debatable    helds  of 

cytological  inquiry. 


I.   Function  of  the  Aviphi- 
astcr 


CZ- 


in.  z. 


v-ac 


a.c 


All  observers  agree  that 
the  amphiaster  is  in  some 
manner  an  expression  of 
the  forces  by  which  cell- 
division  is  caused,  and 
many  accept,  in  one  form 
or  another,  the  first  view 
clearly  stated  by  Fol,^  that 
the  asters  represent  in 
some  manner  centres  of 
attractive  forces  focussed 
in  the  centrosome  or  dv- 
namic  centre  of  the  cell. 
Regarding  the  nature  of 
these  forces,  there  is,  how- 
ever, so  wide  a  divergence 
of  opinion  as  to  compel  the 
admission  that  we  have 
thus  far  accomplished  little 
more  than  to  clear  the 
ground  for  a  ])recise  in- 
vestigation of  the  subject ; 

Fig.  48. —  Slightly  schematic  figures  of  dividing  eggs  ^  .  '        . 

o{  Ascaris,  illustrating  Van  Bencdcn's  theory  of  mitosis,      and   the   mccnanism  Ot    mi- 

[Van  Beneden  and  juLiN.]  tosis  Still  lies  before  us  as 

A.  Early  anaphase;    each  chromosome  has  divided  qj-j^.  ^f  ^\^^^  moSt  fascinating 
into  two.      B.  Later  anaphase  during  divergence  of  the  ^  . 

daughter-chromosomes,      a.c.  Antipodal   cone   of  astral  problems  01  CytOlOgy. 
rays;    f.s.  cortical  zone  of  the  attraction-sphere ;  /.    inter-  (^(ji\      'J'Jw     TllCOiy     of     Fl- 

zonal    fibres   stretching  between    the    daughter-chromo-  u,.;i]  ^,.  r  ...f,- ,  -tUi t^,  HTUo 

somes;    m.z.   medullary  zone   of  the   attraction-sphere;  ^^ '^^^^'  Coutl Octlllty . —^\x<. 

p.c.  principal   cone,  forming  one-half  of  the  contractile  vicVV    that      haS     taken     the 

spindle  (the  action  of  these  fibres  is  reenforced  by  that  of  cf-rnno'est     hold     On     rCCCnt 
tlie  antipodal  cone)  ;  s.e.c.  subequatorial  circle,  to  which  t>         ^  _ 

the  astral  rays  are  attached.  research   IS   the    hypothesis 

of  fibrillar  contractility. 
First  suggested  by  Klein  in  1878,  this  hypothesis  was  independ- 
ently   put    forward   by    Van    Beneden    in    1883,   and   fully   outlined 


1 ' 


73.  P-  473- 


THE  MECHANISM   OF  MITOSIS  10 1 

by  him  four  years  later  in  the  following  words:  *' In  our  opinion 
all  the  internal  movements  that  accompany  cell-division  have  their 
immediate  cause  in  the  contractility  of  the  protoplasmic  fibrilla:^  and 
their  arrangement  in  a  kind  of  radial  muscular  system,  composed  of 
antagonizing  groups"  {i.e.  the  asters  with  their  rays).  "  In  this  sys- 
tem the  central  corpuscle  (centrosome)  plays  the  part  of  an  organ  of 
insertion.  It  is  the  first  of  all  the  various  organs  of  the  cells  to  divide, 
and  its  division  leads  to  the  grouping  of  the  contractile  elements  in 
two  systems,  each  having  its  own  centre.  The  presence  of  these  two 
systems  brings  about  cell-division,  and  actively  determines  the  paths 
of  the  secondary  chromatic  asters"  {i.e.  the  daughter-groups  of  chro- 
mosomes) ''  in  opposite  directions.  An  important  part  of  the  phe- 
nomena of  (karyo-)  kinesis  has  its  efficient  cause,  not  in  the  nucleus, 
but  in  the  protoplasmic  body  of  the  cell."  ^  This  beautiful  hypothesis 
was  based  on  very  convincing  evidence  derived  from  the  study  of  the 
Ascaris  eofo:,  and  it  was  here  that  Van  Beneden  first  demonstrated  the 
fact,  already  suspected  by  Flemming,  that  the  daughter-chromosomes 
move  apart  to  the  poles  of  the  spindle  and  give  rise  to  the  two 
respective  daughter-nuclei.^ 

Van  Beneden's  general  hypothesis  was  accepted  in  the  following 
year  by  Boveri  ('88,  2),  who  contributed  many  important  additional 
facts  in  its  support,  though  neither  his  observations  nor  those  of  later 
investigators  have  sustained  Van  Beneden's  account  of  the  grouping 
of  the  astral  rays.  Boveri  showed  in  the  clearest  manner  that,  during 
the  fertilization  of  Ascaris,  the  astral  rays  become  attached  to  the 
chromosomes  of  the  germ-nuclei ;  that  each  comes  into  connection 
with  rays  from  both  the  asters  ;  that  the  chromosomes,  at  first  irregu- 
larly scattered  in  the  ^gg,  are  drawn  into  a  position  of  equilibrium  in 
the  equator  of  the  spindle  by  the  shortening  of  these  rays  (Figs.  90. 
147);  and  that  tJie  rays  thicke^i  as  tJiey  sJiorten.  He  showed  that  as 
the  chromosome  splits,  each  half  is  connected  only  with  rays  (spindle- 
fibres)  from  the  aster  on  its  own  side ;  and  he  followed,  step  by  step. 
the  shortening  and  thickening  of  these  rays  as  the  daughter-chromo- 
somes diverge.     In  all  these  operations  the  behaviour  of  the  rays  is 

1  '87,  p.  280. 

2  '83,  p.  544.  Van  Beneden  describes  the  astral  rays,  both  in  Ascaris  and  in  tunicates,  as 
differentiated  into  several  groups.  One  set,  forming  the  "jirincipal  cone."  are  attached  to 
the  chromosomes  and  form  one-half  of  the  spindle,  and,  l)y  the  contractions  of  these  lilires, 
the  chromosomes  are  passively  dragged  apart.  An  opposite  group,  forming  the  "  antipodal 
cone,"  extend  from  the  centrosome  to  the  cell-periphery,  the  base  of  the  cone  forming  the 
"polar  circle."  These  rays,  opposing  the  action  of  the  principal  cones,  not  only  hold  the 
centrosomes  in  place,  but,  by  their  contractions,  drag  them  apart,  and  thus  cause  an  actual 
divergence  of  the  centres.     The  remaining  astral  rays  are  attached  to  the  cell-periphery  and 

are  limited  by  a  subequatorial  circle  (Fig.  48).  Later  observations  indicate,  however,  that 
this  arrangement  of  the  astral  rays  is  not  of  general  occurrence,  and  that  the  rays  often  do 
not  reach  the  periphery,  but  lose  themselves  in  the  general  reticulum. 


I02 


CELL-DIVISIOy 


precisely  like  that  of  muscle-fibres ;  and  it  is  difficult  to  study  Bovcri's 
beautiful  figures  and  clear  descriptions  without  sharing  his  conviction 
that  "of  the  contractility  of  the  tibrilke  there  can  be  no  doubt."  ^ 

Very  convincing  evidence  in  the  same  direction  is  afforded  by 
pigment-cells  and  leucocytes  or  wandering  cells,  in  both  of  which 
there  is  a  very  large  permanent  aster  (attraction-sphere)  even  in  the 
resting  cell.  The  structure  of  the  aster  in  the  leucocyte,  where  it 
was  first  discovered  by  Flemming  in  1891,  has  been  studied  very 
carefully  by  Heidenhain  in  the  salamander.  The  astral  rays  here 
extend  throughout  nearly  the  whole  cell  (Fig.  49),  and  are  believed 

_.  _  B 


Fig.  49.  —  Leucocytes  or  wandering  cells  of  the  salamander.     [Heidenhain.] 

A.  Cell  with  a  single  nucleus  containing  a  very  coarse  network  of  chromatin  and  two  nucleoli 
(plasmosomes)  ;  s.  permanent  aster,  its  centre  occupied  by  a  double  centrosome  surrounded  by 
an  attraction-sphere.  i9.  Similar  cell,  with  double  nucleus;  the  smaller  dark  masses  in  the  latter 
are  oxychromatin-granules  (linin),  the  larger  masses  are  basichromatin  (chromatin  proper). 


by  Heidenhain  to  represent  the  contractile  elements  by  means  of 
which  the  cell  changes  its  form  and  creeps  about.  A  similar  con- 
clusion was  reached  by  Solger  ('91)  and  Zimmermann  ('93,  2)  in  the 
case  of  pigment-cells  (chromatophores)  in  fishes.  These  cells  have, 
in  an  extraordinary  degree,  the  power  of  changing  their  form  and  of 
actively  creeping  about.  Solger  and  Zimmermann  have  shown  that 
the  pigment-cell  contains  an  enormous  aster,  whose  rays  extend  in 
every  direction  through  the  pigment-mass,  and  it  is  almost  impos- 
sible to  doubt  that  the  aster  is  a  contractile  apparatus,  like  a  radial 
muscular  system,  by  means  of  which  the  active  changes  of  form  are 
produced  (Fig.  50).  This  interpretation  of  the  aster  receives  addi- 
tional  support  through   Schaudinn's  ('96,  3)  highly   interesting   dis- 

1  '88,  2,  p.  99. 


THE  MECHANISM  OF  MITOSIS 


10% 


covery  that  the  ''central  granule  "  of  the  Heliozoa  is  to  be  identified 
with  the  centrosome  and  plays  the  same  role  in  mitosis  (Fig.  41). 
In  these  animals  the  axial  filaments  of  the  radiating  pseudopodia  con- 
verge to  the  central  granule  during  the  vegetative  state  of  the  cell, 
thus  forming  a  permanent  aster  which  Schaudinn's  observations  prove 
to  be  directly  comparable  to  that  of  a  leucocyte  or  of  a  mitotic  figure. 
There  is  in  this  case  no  doubt  of  the  contractility  of  the  rays,  and  a 


B 


.' 


W/:i/^ 


-;-^  - 


Fig.  50.  —  Pigment-cells  and  asters  from  the  epidermis  of  fishes.     [ZiMMERMAN'N.] 

A.  Entire  pigment-cell,  from  Bleimius.  The  central  clear  space  is  the  central  mass  of  the  aster 
from  which  radiate  the  pigment-granules;  two  nuclei  below.  D.  Nucleus  (//)  and  aster  after  ex- 
traction of  the  pigment,  showing  reticulated  central  mass.  C.  Two  nuclei  and  aster  with  rod- 
shaped  central  mass,  from  Sargus. 

strong,  if  indirect,  argument  is  thus  given  in  favour  of  contractility  in 
other  forms  of  asters.^  The  contraction-hypothesis  is  beautifully 
illustrated  by  means  of  a  simple  and  easily  constructed  model,  devised 
by  Heidenhain  ('94,  '96),  which  closely  simulates  some  of  the  phenom- 
ena of  mitosis.  In  its  simplest  form  the  model  consists  of  a  circle, 
marked  on  a  flat  surface,  to  the  periphery  of  which  are  attached  at  equal 


1  For   an   interesting    discussion    and    develoj  ment    of    the    contraction-hypothesis   see 
Watase,  '94. 


104 


CELL-DIVISION 


Fig.  51.  —  Heidenhain's  model  of  mitosis  (mainly  from 
Heidenhain). 

A.  Dotted  lines  show  position  of  the  rays  upon  sever- 
ing connection  between  the  small  rings.  B.  Position  upon 
insertion  of  "  nuclcu=."  C.  D.  Models  with  fiexiblc  hinged 
hoops,   showing  division. 


intervals  a  series  of  rub- 
ber bands    (astral    rays). 
At  the  other  ends  these 
bands   are   attached  to  a 
pair  of  small  rings  (cen- 
trosomes)     fastened     to- 
gether.      In  the    position 
of  equilibrium,  when  the 
rays     are     stretched     at 
equal    tension,    the    rays 
form  a  symmetrical  aster 
with  the    centrosomes  at 
the  centre   of    the   circle 
(Fig.  51,  A).     If  the  con- 
nection between  the  cen- 
trosomes be  severed,  they 
are   immediately  dragged 
apart  to  a  new  position  of 
equilibrium  with  the  rays 
grouped  in  two  asters,  as 
in  the  actual  cell  (dotted 
lines  in  Fig.  51,  A).      If 
a    round    pasteboard  box 
of  suitable  size  (nucleus) 
be  inserted  between  two 
of   the    ravs,   it    assumes 
an  eccentric  position,  the 
cell-axis  being  formed  by 
a  line  passing  through  its 
centre    and    that    of    the 
\)?i\x    of    small  rings    {cf. 
the  epithelial  cell,  p.  57), 
and  upon  division   of  the 
aster  it  takes  up  a  position 
between  the    two    asters. 
In  a  second  form   of  the 
models      the      circle      is 
formed  of  two  half  rings 
of    flexible    steel,    joined 
by     hinges ;      the     diver- 
gence of  the  small  rings 
is   here   accompanied    by 
an  elongation  and  partial 
constriction  of  the  model 


THE   MECHANISM   OF  MITOSIS  IO5 

in  the  equatorial  plane ;  and  if,  finally,  the  hinge-connection  be  re- 
moved, each  half  of  the  ring  closes  to  form  a  complete  ring.^ 

Heidenhain  has  fully  worked  out  a  theory  of  mitosis  based  upon 
the  analogy  of  these  pretty  models.  The  astral  rays  of  the  cell 
(*' organic  radii")  are  assumed  to  be  in  like  manner  of  equal  length 
and  in  a  state  of  equal  tonic  contraction  or  tension,  the  centrosome 
forming  the  common  insertion-point  of  the  rays,  and  equilibrium  of 
the  system  being  maintained  by  turgor  of  the  cell.  Upon  disappear- 
ance of  the  nuclear  membrane  and  division  of  this  insertion-point,  the 
tension  of  the  rays  causes  divergence  of  the  centrosomes  and  forma- 
tion of  the  spindle  between  them,  and  by  further  contraction  of  the 
rays  both  the  divergence  of  the  daughter-chromosomes  and  the  division 
of  the  cell-body  are  caused.  A  new  condition  of  equilibrium  is  thus 
established  in  each  daughter-cell  until  again  disturbed  by  division  of 
the  centrosome.2  In  some  cases  (leucocytes)  the  organic  radii  are 
visible  at  all  periods.  More  commonly  they  are  lost  to  view  by 
breaking  up  into  the  cell-reticulum,  without,  however,  losing  their 
essential  relations. 

No  one  who  witnesses  the  operation  of  Heidenhain's  models  can 
fail  to  be  impressed  with  its  striking  simulation  of  actual  cell-division. 
Closer  study  of  the  facts  shows,  however,  that  the  contraction-hypothe- 
sis must  be  considerably  restricted,  as  has  been  done  by  the  successive 
modifications  of  Hermann  ('91),  Drijner('95),  and  others.  Hermann, 
to  whom  the  identiiication  of  the  central  spindle  is  due,  pointed  out 
that  there  is  no  evidence  of  contractility  in  the  central  spindle-fibres, 
which  elongate  instead  of  shorten  during  mitosis ;  and  he  concluded 
that  these  fibres  are  non-contractile  supporting  elements,  which  form 
a  basis  on  which  the  movements  of  the  chromosomes  take  place.  The 
ina7itle-fibres  are  the  only  contractile  elements  in  the  spindle,  and  it 
is  by  them  that  the  chromosomes  are  brought  into  position  about  the 
central  spindle  and  the  daughter-chromosomes  are  dragged  apart.^ 
Driiner  ('95)  still  further  restricts  the  hypothesis,  maintaining  that  the 
progressive  divergence  of  the  spindle-poles  is  caused  not  by  contrac- 
tion of  the  astral  rays  (''polar  fibres"),  as  assumed  by  Heidenhain 
(following  Van  Beneden  and  Boveri),  but  by  an  active  growth  or 
elongation  of  the  central  spindle,  which  goes  on  throughout  the 
whole  period  from  the  earliest  prophases  until  the  close  of  the  ana- 
phases.    This  view  is  supported  by  the  fact  that  the  central  spindle- 

1  In  a  modification  of  the  apparatus  devised  by  Rhumbler  ('97),  the  same  effect  is  pro- 
duced without  the  hinges. 

2  (7:  p.  57.  p^or  critique  of  this  hypothesis,  see  Fick  ('97),  Rhumbler  ('96,  '97),  and 
Meves  ('97,  4). 

3  Belajeff  ('94)  and  Strasburger  ('95)  have  accepted  a  similar  view  as  applied  to  mitosis 
in  plant-cells. 


I06  CELL-DIVISIOX 

fibres  are  always  contorted  during  the  metaphases,  as  if  pushing 
against  a  resistance  ;  and  it  harmonizes  with  the  facts  observed  in 
the  mitoses  of  infusorian  nuclei,  where  no  asters  are  present.  This 
view  has  been  accepted,  with  slight  modifications,  by  Flemming, 
Boveri,  Meves,  Kostanecki,  and  also  by  Heidenhain.  A  nearly 
decisive  argument  in  its  favour  is  given  by  such  cases  as  the  polar 
bodies,  or  the  mitosis  of  salamander  spermatocytes  as  described  by 
Meves  ('96,  '97,  3),  where  the  spindle-poles  are  pushed  out  to  the 
periphery  of  the  cell,  the  polar  astral  rays  meanwhile  nearly  or  quite 
disappearing  (Fig.  130).  This  not  only  strongly  indicates  the  push 
of  the  central  spindle,  but  also  shows  that  the  assumption  of  a  pull 
by  the  polar  rays  is  superfluous.  But  beyond  this  both  Driincr  and 
Meves  have  brought  arguments  against  contractility  in  the  other 
astral  rays,  endeavouring  to  show^  that  these,  like  the  spindle-fibres, 
are  actively  elongating  elements,  and  that  (Meves,  '97,  3)  the  actual 
grouping  of  the  rays  during  the  anaphases  is  such  as  to  suggest  that 
even  the  division  of  the  cell-body  may  be  thus  caused.  A  pushing 
function  of  the  astral  rays  is  also  indicated  by  infolding  of  the  nuclear 
membrane  caused  by  the  development  of  the  aster  as  described  by 
Platner,  Watase,  Braus,  Griffin,  and  others.^  The  contraction-hypothe- 
sis is  thus  restricted  by  Driiner  and  Meves  to  the  mantle-fibres 
alone,  though  many  others,  among  them  Flemming  and  Kostanecki, 
still  accept  the  contractility  of  the  astral  rays. 

{b)  OtJicr  Facts  and  TJicorics.  —  Even  in  the  restricted  form  indi- 
cated above  the  contraction-hypothesis  encounters  serious  difficulties, 
one  of  which  is  the  fact  urged  by  me  in  an  earlier  paper  ('95),  and 
subsequently  by  Richard  Hertwig  ('98),  that  in  the  eggs  of  echino- 
derms  and  many  other  dividing  cells  the  daughter-chromosome 
plates,  extending  through  the  whole  substance  of  the  spindle, 
wander  to  the  extreme  ends  of  the  spindle  —  a  process  which 
demands  a  contraction  of  the  fibres  almost  to  the  vanishing  point, 
while  in  point  of  fact  not  even  a  shortening  and  thickening  of 
the  fibres  can  be  seen  (Fig.  52).  Moreover,  in  these  cases,  no 
distinction  can  be  seen  between  central  spindle-fibres  and  mantle- 
fibres,  and  we  can  only  save  the  contraction-hypothesis  by  the 
improbable  assumption  that  fibres  indistinguishably  mingled,  and 
having  the  same  mode  of  origin,  structure,  and  staining-reaction,  have 
exactly  opposite  functions.  The  inadequacy  of  the  general  theory 
is    sufficiently    apparent    from    the  fact  that  in  amitosis  cells  many 

1  QC  p.  68.  It  should  be  pointed  out  that  the  originator  of  the  pushing  theory  was 
Watase  ('93),  who  ingeniously  developed  an  hypothesis  exactly  the  opposite  of  Van  Bene- 
den's,  assuming  both  astral  rays  and  spindle-fibres  to  be  actively  elongating  fibres,  dove-tailing 
in  the  spindle-region,  and  pushing  the  chromosomes  apart.  This  hypothesis  is,  I  believe,  in- 
consistent with  the  phenomena  observed  in  multiple  asters  and  elsewhere,  yet  it  probably 
contains  a  nucleus  of  truth  that  forms  the  basis  of  Driiner's  conception  of  the  central  spindle. 


THE  MECHANISM   OF  MITOSIS 


107 


divide  without  any  amphiaster  whatever.  In  Infusoria  mitosis  seems 
to  occur  in  the  entire  absence  of  asters,  althou^^h  the  cells  divide  by 
constriction,  and  the  analogy  with  Heidenhain's  model  entirely  fails. 


^vX      V 


■^> 


x 


\ 


/ 


/ 


-/  ^ 


o 


c 
oo 


/V'-'^ 


/'^ 


X 


"lr\:\\  Z)'  IW 


Mil''". ''      '^ 


1 1  \^^-      /    \ 


E 

Fig.  52.  —  The  later  stages  of  mitosis  in  the  G.gg  of  the  sea-urchin  Toxopneustcs  {.4-D,  X  1000; 
£-F,X5oo). 

A.  Metaphase;  daughter-chronaosomes  drawing  apart  but  still  united  at  one  end.  B.  Daugh- 
ter-chromosomes separating.  C.  Late  anaphase ;  daughter-chromosomes  lying  near  the  spindle- 
poles.  D.  Final  anaphase;  daughter  chromosomes  converted  into  vesicles.  E.  Immediately 
after  division,  the  asters  undivided ;  the  spindle  has  disappeared.  F.  Resting  2-cell  stage,  the 
asters  divided  into  tvvo  in  anticipation  of  the  next  division. 

In  Figs.  A  and  /?  the  centrosome  consists  of  a  mass  of  intensely  staining  granules,  which  in 
Cand  D  elongates  at  right  angles  to  the  spindle-axis.  In  /-'the  centrosome  appears  as  a  single 
or  double  granule,  which  in  later  stages  gives  rise  to  a  pluricorpuscular  centrum  like  that  in  .-/. 
The  connection  between  D  and  F  is  not  definitely  determined. 

In  Eiiglypha,  according  to  Schewiakoff  (Fig.  39),  division  of  the  cell- 
body  appears  to  take  place  quite  independently  of  the  mitotic  figure. 
Again,  a  considerable  number  of  cases  are  now  known  in  which  dur- 
ing the  fertilization  of  the  ^gg  a  large  amphiaster  is  formed,  with 


1 08  CELL-DI J  'I SI  ON 

astral  rays  sometimes  extending  throughout  almost  the  entire  Qgg^ 
only  to  disappear  or  become  greatly  reduced  without  the  occurrence 
of  division,  the  ensuing  cleavage  being  effected  by  a  new  am})hiaster 
or  by  the  recrudescence  of  the  old.^  For  these  and  other  reasons  we 
must  admit  the  probability  that  contractility  of  the  astral  fibrillx',  if 
it  exists,  is  but  the  expression  or  consequence  of  a  more  deeply  lying 
phenomena  of  more  general  significance.  The  subtlety  of  the  prob- 
lem is  strikingly  shown  by  Boveri's  remarkable  observations  on 
abnormal  sea-urchin  eggs  ('96),  which  show  (i)  that  the  periodic 
division  of  the  centrosome  and  formation  of  the  amphiaster  may  take 
place  independently  of  the  nucleus;  (2)  that  the  spindle,  as  well  as 
the  asters,  is  concerned  in  division  of  the  cell-body  ;  and  (3)  that  an 
amphiaster  without  chromosomes  is  unable  to  effect  normal  division 
of  the  cell-body.  The  first  and  third  of  these  facts  are  shown  by  eggs 
in  which  during  the  first  cleavage  all  of  the  chromatin  passes  to  one 
pole  of  the  spindle,  so  that  one  of  the  resulting  halves  of  the  tgg 
receives  no  nucleus,  but  only  a  centrosome  and  aster.  In  this  half 
perfect  amphiasters  are  formed  simultaneously  with  each  cleavage  in 
the  other  half,  yit  no  division  of  the  protoplasmic  mass  occurs?  The 
second  fact  is  shown  in  polyspermic  eggs,  in  which  multipolar  astral 
svstems  are  formed  by  union  of  the  several  sperm-asters  (Figs.  53,  loi ). 
In  such  eggs  cleavages  only  occur  between  asters  that  are  joined  by  a 
spindle.  Normal  cleavage  of  the  cell-body  thus  requires  the  complete 
apparatus  of  mitosis,  and  even  though  the  fibres  be  contractile  they 
cannot  fully  operate  in  the  absence  of  chromatin. 

We  may  now  turn  to  theories  based  on  the  hypothesis,  first  sug- 
gested by  Fol  in  1873,  that  the  astral  foci  {i.e.  centrosomes)  represent 
dynamic  centres  of  attractive  or  other  forces.  It  should  be  noted  that 
this  hy])othesis  involves  two  distinct  questions,  one  relating  to  the 
origin  of  the  amphiaster,  the  other  to  its  mode  of  action  ;  and  we  have 
seen  that  some  of  the  foremost  advocates  of  the  contraction-hypothesis, 
including  Van  Beneden  and  Boveri,  have  held  the  centrosomes  to  be 
attractive  centres.  Apart  from  the  movements  of  the  chromosomes, 
the  most  obvious  indication  that  the  centrosomes  are  dynamic  centres 
is  the  extraordinary  resemblance  of  the  amphiaster  to  the  lines  of  force 
in  a  magnetic  field  as  shown  by  the  arrangement  of  iron-filings  about 
the  poles  of  a  horseshoe  magnet  —  a  resemblance  pointed  out  by  Fol 
himself,   and  urged  by  many  later  writers,'^  especially  Ziegler  ('95) 

ir/:p.  213. 

'  This  result  is  opposed  to  Boveri's  earlier  work  on  Ascaris  (p.  355),  and  is  modified  by 
Ziegler  ('98),  who  observed  in  a  single  case  that  an  irregular  cleavage  occurred  in  the 
enucleated  half  after  two  or  three  divisions  of  the  centrosome.  On  the  other  hand,  it  is  sup- 
ported by  Morgan's  convincing  experiments  on  the  eggs  oi  Arbacia  (p.  308). 

2  Cf.  the  interesting  photographic  figures  of  Ziegler  ('95).  A  still  closer  simulacrum  of 
the  amphiaster  is  produced  by  fine  crystals  of  sulphate  of  quinine  (a  semiconductor)  sus- 


THE  MECHANISM   OF  MITOSIS 


109 


and  Gallardo  ('96,  '97).  It  is  impossible  to  regard  this  analogy  as 
exact  ;  first,  because  it  is  inconsistent  with  the  occurrence  of  tripolar 
astral  figures  ;  second,  as  Meves  has  recently  urged  ^  the  course  of  the 
astral  fibres  does  not  really  coincide  with  the  lines  of  force,  the  most 
important  deviation  being  the  crossing  of  the  rays  opposite  the  equa- 
torial region  of  the  spindle,  which  is  impossible  in  the  magnetic  or 
electric  field.  We  must,  however,  remember  that  the  amphiastcr  is 
formed  in  a  viscid  medium,  that  it  may  perform  various  movements, 
and  that  its  fibres  probably  possess  the  power  of  active  growth.     The 


C 


B 


D 


F 


Fig-  53-  —  Division  of  dispermic  eggs  in  sea-urchin  eggs,  schematic.     [Boveri.] 
A.  C.  E.     Eggs  before  division,  showing  various  connections  of  the  asters.     B.  D.  F.     Result- 
ing division  in  the  three  respective  cases,  showing  cleavage  only  between  centres  connected  by  a 

spindle. 

physical  or  chemical  effect  of  the  centres,  through  which  the  amphias- 
tcr primarily  arises,  may  thus  be  variously  disturbed  (^r  modified  in 
later  stages,  and  the  crossing  of  the  rays  is  therefore  not  necessarily 
fatal  to  the  assumption  of  dynamic  centres.  Biitschli  ('92,  '98)  has, 
moreover,  recently  shown  that  a  close  sijuii/acntin  of  the  amphiastcr, 
showing  a  distinct  crossing  of  the  rays,  may  be  produced  in  an  arti- 
ficial alveolar  structure  (coagulated  gelatine)  by  tractive  forces  ccn- 

pended  in  spirits  of  turpentine  (a  poor  conductor)  between  two  electric  poles.  This  experi- 
ment, devised  by  Faraday,  has  recently  been  applied  by  Gallardo  ('96,  '97)  to  an  analysis 
of  the  mitotic  figure.  ^  '96,  p.  371. 


I  lo  CELL-DIVISION 

tring  in  two  adjacent  points.  This  result  is  obtained  by  warming  and 
then  cooling  a  film  of  thick  gelatine-solution,  filled  with  air-bubbles, 
and  then  coagulating  the  mass  in  chromic  acid.  Such  a  film  shows  a 
fine  alveolar  structure,  which  assumes  a  radial  arrangement  about  the 
air-bubbles,  owing  to  the  traction  exerted  on  the  surrounding  structure 
by  shrinkage  of  the  bubbles  on  cooling.  The  amphiastral  simjilacra 
are  produced  about  two  adjacent  bubbles,  —  a  "  spindle  "  being  formed 
between  them,  and  the  "  astral  rays  "  sometimes  showing  a  crossing 
like  that  seen  in  the  actual  amphiaster  (Biitschli  is  himself  unable  to 
explain  fully  how  the  crossing  arises).  The  protoplasmic  asters  are 
maintained  by  Biitschli  to  be,  in  like  manner,  no  more  than  a  radial 
configuration  of  the  alveolar  cell-substance  caused  by  centripetal 
diffusion-currents  toward  the  astral  centres.^  The  most  interesting 
part  of  this  view  is  the  assumption  that  these  currents  are  caused  by 
specific  chemical  cJianges  taking  place  in  the  centrosome  which  causes 
an  absorption  of  liquid  from  the  surrounding  region.  **  The  astral 
bodies  are  structures  which,  under  certain  circumstances,  function  in 
a  measure  as  centres  from  which  emanate  chemical  actions  upon  pro- 
toplasm and  nucleus  ;  and  the  astral  phenomena  which  appear  about 
the  centrosomes  are  only  a  result  incidental  to  this  action  of  the  central 
bodies  upon  the  plasma."  ^  Through  centripetal  currents  thus  caused 
arise  the  asters,  and  they  may  even  account,  in  a  measure,  for  the  move- 
ments of  the  chromosomes.^  This  latter  part  of  BiitschU's  conception 
is,  I  believe,  quite  inadequate ;  but  the  hypothesis  of  definite  chemical 
activity  in  the  centrosome  is  a  highly  important  one,  which  is  sustained 
by  the  staining-reactions  of  the  centrosome  and  by  its  definite  morpho- 
logical changes  during  the  cycle  of  cell-division. 

More  or  less  similar  chemical  hypotheses  have  been  suggested  by 
several  other  writers.^  Of  these  perhaps  the  most  interesting  is 
Strasburger's  suggestion,^  that  the  movements  of  the  chromosomes 
may  be  of  a  chemotactic  character,  which  I  suspect  may  prove  to  have 
been  one  of  the  most  fruitful  contributions  to  the  subject.  Beside  this 
may  be  placed  Carnoy's  still  earlier  hypothesis  ('85),  that  the  asters 
are  formed  under  the  influence  of  specific  ferments  emanating  from 
the  poles  of  the  nucleus.  Mathews  ('99,  2)  has  recently  pointed  out 
that  there  is  a  considerable  analogy  between  the  formation  of  the 
astral  rays  and  that  of  fibrin-fibrils  under  the  infiuence  of  fibrin-fer- 
ment, adding  the  suggestion  that  the  centrosome  may  actually  contain 

1  Carnoy  ('S5)  and  Plainer  ('86)  had  previously  held  a  similar  view,  suggesting  that  not 
only  the  spindle-formation,  but  also  the  movements  of  the  chromosomes,  might  be  explained 
as  the  result  of  protoplasmic  currents. 

2 '92.  I,  p.  538. 

8 '92,  2,  p.  160;    '92,3,  p.  10. 

*  Cf.  the  first  edition  of  this  work,  p.  77,  also  Ziegler  ('95).  ®'93»  2. 


THE   MECHANISM   OF  MITOSIS  I  i  j 

fibrin-ferment.  Attention  may  be  called  here  to  the  fact,  now  definitely 
determined  by  experiment,^  that  cell-division  may  be  incited  by  chemi- 
cal stimulus.  In  most  of  the  cases  thus  far  experimentally  examined 
the  divisions  so  caused  are  pathological  in  character,  but  in  others 
they  are  quite  normal,  as  shown  in  Loeb's  remarkable  results  on  the 
production  of  parthenogenesis  in  sea-urchin  eggs  by  chemical  stimulus 
as  described  at  pages  215  and  308.  While  these  experiments  by  no 
means  show  that  division  is  itself  merely  a  chemical  process,  thev 
strongly  suggest  that  it  cannot  be  adequately  analyzed  without  reckon- 
ing with  the  chemical  changes  involved  m  it. 

Resume.     A  review  of  the  foregoing  facts  and  theories  shows  how 
far  we  still  are  from  any  real  understanding  of  the  process  involved 
either  in  the  origin  or  in  the  mode  of  action  of  the  mitotic  figure.    The 
evidence  seems  well-nigh  demonstrative,  in  case  of  the  mantle-fibres 
and  the  astral  rays,  that  Van  Beneden's  hypothesis  contains  an  element 
of  truth,  but  we  must  now  recognize  that  it  was  formulated  in  too 
simple  a  form  for  the  solution  of  so  complex  a  problem.     No  satisfac- 
tory hypothesis  can,  I  believe,  be  reached  that  does  not  reckon  with 
the  chemical  changes  occurring  at  the  spindle-poles  and  in  the  nucleus  ; 
and  these  changes  are  probably  concerned  not  only  with  the  origin  of 
the  amphiaster,  but  also  with  the  movements  of  the  chromosomes.     In 
cases  where  the  centrosome  persists  from  cell  to  cell  we  may  perhaps 
regard  it  as  the  vehicle  of  specific  substances  (ferments  ?)  which  become 
active  at  the  onset  of  mitosis,  and  run  through  a  definite  cycle  of 
changes,  to  initiate  a  like  cycle  in  the  following  generation ;    and  it  is 
quite  conceivable  that  such  substances  may  persist  at  the  nuclear  poles, 
or  may  be  re-formed  there  as  an  after-effect,  even  though  the  formed 
centrosome  disappears.^      In  this  consideration  we  may  find  a  clue  to 
the  strange  fact  —  should  it  indeed  prove  to  be  a  fact — that  the  cen- 
trosome may  divide,  yet  afterward   disappear   without  discoverable 
connection  with  the  centrosomes  of  the  succeeding  mitosis,  as  several 
recent  observers  have  maintained.  ^     When  all  is  said,  we  must  admit 
that  the  mechanism  of  mitosis  in  every  phase  still  awaits  adequate 
physiological  analysis.     The  suggestive  experiments  of  Biitschli  and 
Heidenhain  lead  us  to  hope  that  a  partial  solution  of  the  problem  may 
be  reached  along  the  lines  of  physical  and  chemical  experiment.     At 
present  we    can  only  admit  that  none  of   the  conclusions   thus  far 
reached,  whether  by  observation  or  by  experiment,  are  more  than  the 
first  7iazz'e  attempts  to  analyze  a  group  of  most  complex  phenomena 
of  which  we  have  little  real  understanding. 

1  See  pp.  306,  308.  2  QT  p.  215.  SQT  p.  213. 


I  12 


CELL-DIVISION 


2.    Division  of  tJie  CJiroviosovics 

In  de\'eloping  his  theory  of  fibrillar  contractility,  Van  Benedcn 
expressed  the  view  —  only,  however,  as  a  possibility  —  that  the 
splitting  of  the  chromosomes  might  be  passively  caused  by  the  con- 
tractions of  the  two  sets  of  opposing  s{)indle-fibres  to  which  each  is 
attached.^  Later  observations  have  demonstrated  that  this  sugges- 
tion cannot  be  sustained  ;  for  in  many  cases  the  chromatin-thread 
splits  before  division  of  the  centrosome  and  the  formation  of  the 
achromatic  figure  —  sometimes  during  the  spireme-stage,  or  even  in 
the  reticulum,  while  the  nuclear  membrane  is  still  intact.  Boveri 
showed  this  to  be  the  case  in  Ascaris,  and  a  similar  fact  has  been 
observed  by  many  observers  since,  both   in   plants  and  in  animals. 


Fig.  54.  —  Nuclei  in  the  spireme-stage. 

A.  From  the  endosperm  of  the  lily,  showing  true  nucleoli.     [FLEMMING.] 

B.  Spermatocyte  of  salamander.  Segmented  double  spireme-thread  composed  of  chromo- 
meres  and  completely  split.     Two  centrosomes  and  central  spindle  at  s.     [Hkrmann.] 

C.  Spireme-thread  completely  split,  with  six  nucleoli.  Endosperm  of  Fritillaria.  [FLEM- 
MING.] 

The  splitting  of  the  chromosomes  is  therefore,  in  Bovcri's  words, 
"  an  independent  vital  manifestation,  an  act  of  reproduction  on  the  part 
of  tJic  chromosomes'''^ 

All  of  the  recent  researches  in  this  field  point  to  the  conclusion 
that  this  act  of  division  must  be  -referred  to  the  fission  of  the 
chromatin-granules  or  chromomeres  of  which  the  chromatin-thread 
is  built.  These  granules  were  first  clearly  described  by  Balbiani 
('76)  in  the  chromatin-network  of  epithelial  cells  in  the  insect- 
ovary,  and  he  found  that  the  spireme-thread  arose  by  the  linear 
arrangement  of  these  granules  in  a  single  row  like  a  chain  of  bacte- 
ria."^    Six  years  later  Pfitzner  ('82)  added  the  interesting  discovery 


1  '87,  p.  279. 


2 '88,  p.  113. 


8  See  '81,  p.  638. 


THE  MECHANISM   OF  MITOSIS 


113 


that  during  the  mitosis  of  various  tissue-cells  of  the  salamander,  the 
granules  of  the  spireme-thread  divide  by  fission  and  thus  determine  the 
longitudinal  splitting  of  the  entire  cJironiosoine.  This  discovery  was 
confirmed  by  Flemming  in  the  following  year  ('82,  p.  219),  and  a  simi- 
lar result  has  been  reached  by  many  other  observers  (Fig.  54).  The 
division  of  the  chromatin-granules  may  take  place  at  a  very  early 
period.     Flemming  observed  as  long  ago  as  1881  that  the  chromatin- 


B 


^^\ 


^\m 


Fig-  55-  —  Formation  of  chromosomes  and  early  splitting  of  the  chromatin-granules  in  sperma- 
togonia oi  As  car  is  megalocephala,  var.  bivalens.     [BraUER.] 

A.  Ver}^  early  prophase;  granules  of  the  nuclear  recticulum  already  divided.  B.  Spireme; 
the  continuous  chromatin-thread  split  throughout.  C.  Later  spireme.  A  Shortening  of  the 
thread.  E.  Spireme-thread  divided  into  two  parts.  F.  Spireme-thread  segmented  into  four  si)lit 
chromosomes. 


thread  might  spHt  in  the  spireme-stage  (epithelial  cells  of  the  sala- 
mander), and  this  has  since  been  shown  to  occur  in  many  other  cases ; 
for  instance,  by  Guignard  in  the  mother-cells  of  the  pollen  in  the 
lily  ('91).  Brauer's  recent  work  on  the  spermatogenesis  of  Ascaris 
shows  that  the  fission  of  the  chromatin-granules  here  takes  place  even 
before  the  spireme-stage,  when  the  chromatin  is  still  in  the  form  of  a 
reticulum,  and  long  before  the  division  of  the  centrosome  (Fig.  55). 
He  therefore  concludes :  "  With  Boveri  I  regard  the  splitting  as  an 


114 


CELL-DIVISIOX 


independent  reproductive  act  of  the  chromatin.  The  reconstruction 
of  the  nucleus,  and  in  particular  the  breaking  up  of  the  chromosomes 
after  division  into  small  granules  and  their  uniform  distribution 
through  the  nuclear  cavity,  is,  in  the  first  place,  for  the  purpose  of 
allowing  a  uniform  growth  to  take  place ;  and  in  the  second  place, 
after  the  granules  have  grown  to  their  normal  size,  to  admit  of  tJicir 
precisely  equal  gnautitative  and  qualitative  division.  I  hold  that  all 
the  succeeding  phenomena,  such  as  the  grouping  of  the  granules 
in  threads,  their  union  to  form  larger  granules,  the  division  of  the 
thread  into  segments  and  finally  into  chromosomes,  are  of  secondary 
importance  ;  all  these  are  only  for  the  purpose  of  bringing  about  in 
the  simplest  and  most  certain  manner  the  transmission  of  the  daugh- 
ter-granules (Spalthalften)  to  the  daughter-cells."^  "  In  my  opinion 
the  chromosomes  are  not  independent  individuals,  but  only  groups  of 
numberless  minute  chromatin-granules,  which  alone  have  the  value 
of  individuals."  ^ 

These  obser\'ations  certainly  lend  strong  support  to  the  view  that 
the  chromatin  is  to  be  regarded  as  a  morphological  aggregate  —  as 
a  congeries  or  colony  of  self-propagating  elementary  organisms 
capable  of  assimilation,  growth,  and  division.  They  prove,  more- 
over, that  mitosis  involves  two  distinct  though  closely  related  factors, 
one  of  which  is  the  fission  of  the  chromatic  nuclear  substance,  while 
the  other  is  the  distribution  of  that  substance  to  the  daughter-cells. 
In  the  first  of  these  it  is  the  chromatin  that  takes  the  active  part ; 
in  the  second  it  would  seem  that  the  main  role  is  played  by  the 
amphiastcr. 

E.     Direct  or  Amitotic  Division 

I.    General  Sketch 

We  turn  now  to  the  rarer  and  simpler  mode  of  division  known 
as  amitosis  ;  but  as  Flemming  has  well  said,  it  is  a  somewhat  trying 
task  to  give  an  account  of  a  subject  of  which  the  final  outcome  is 
so  unsatisfactory  as  this  ;  for  in  spite  of  extensive  investigation,  we 
still  have  no  very  definite  conclusion  in  regard  either  to  the  mechan- 
ism of  amitosis  or  its  biological  meaning.  Amitosis,  or  direct  division, 
differs  in  two  essential  respects  from  mitosis.  First,  the  nucleus 
remains  in  the  resting  state  (reticulum),  and  there  is  no  formation 
of  a  spireme  or  of  chromosomes.  Second,  division  occurs  without 
the  formation  of  an  amphiaster;  hence  the  centrosome  is  not  con- 
cerned with  the  nuclear  division,  which  takes  place  by  a  simple 
constriction.     The  nuclear  substance,  accordingly,  undergoes  a  divi- 

1  '93,  pp.  203,  204.  2  /.^.^  p.  205. 


DIRECT   OR   AMITOTIC  DIVISIOX 


115 


sion  of  its  total  mass,  but  not  of  its  individual  elements  or  chromatin- 
granules  (Fig.  56). 

Before  the  discovery  of  mitosis,  nuclear  division  was  generally 
assumed  to  take  place  in  accordance  with  Remak's  scheme  (p.  63). 
The  rapid  extension  of  our  knowledge  of  mitotic  division  between 
the  years  1875  and  1885  showed,  however,  that  such  a  mode  of 
division  was,  to  say  the  least,  of  rare  occurrence,  and  led  to  doubts 
as  to  whether  it  ever  actually  took  place  as  a  normal  process.  As 
soon,  however,  as  attention  was  especially  directed  to  the  subject, 
many  cases  of  amitotic  division  were  accurately  determined,  though 


Fig.  56,—  Group  of  cells  with  amitotically  dividing  nuclei ;  ovarian  follicular  epithelium  of  the 
cockroach.     [WHEELER.] 

very  few  of  them  conformed  precisely  to  Remak's  scheme.  One 
such  case  is  that  described  by  Carnoy  in  the  follicle-cells  of  the 
Qgg  in  the  mole-cricket,  where  division  begins  in  the  fission  of  the 
nucleolus,  followed  by  that  of  the  nucleus.  Similar  cases  have 
been  since  described,  by  Hoyer  ('90)  in  the  intestinal  epithelium  of 
the  nematode  Rhabdonema,  by  Korschelt  in  the  intestine  of  the 
annelid  Ophryotrocha,  and  in  a  few  other  cases.  In  many  cases,  how- 
ever, no  preliminary  fission  of  the  nucleolus  occurs;  and  Remak's 
scheme  must,  therefore,  be  regarded  as  one  of  the  rarest  forms  of 
cell-division  (!). 

2.    Ccntrosome  and  Attraction-sphere  in  Amitosis 

The  behaviour  of  the  centrosome  in  amitosis  forms  an  interesting  question 
on  account  of  its  bearing  on  the  mechanics  of  cell-division.  Fk^iiming  obser\-ed 
('91 )  that  the  nucleus  of  leucocytes  might  in  some  cases  divide  directly  without 


1 1 6  CELL-Di  visioy 

the  formation  of  an  amphiaster,  the  attraction-sphere  remaining  undivided  mean- 
while. Heidenhain  showed  in  the  following  year,  however,  that  in  some  cases 
leucocytes  containing  two  nuclei  (doubtless  formed  by  amitotic  division)  might 
also  contain  two  asters  connected  by  a  si)indle.  Both  Heidenhain  and  Flemming 
drew  from  this  the  conclusion  that  direct  division  of  the  uulU'hs  is  in  this  case  inde- 
pendent of  the  centrosome,  but  that  the  latter  might  be  concerned  in  the  division 
of  the  cell-body,  though  no  such  process  was  observed.  A  little  later,  however, 
Meves  jniblished  remarkable  observations  that  seem  to  indicate  a  functional  activity 
of  the  attraction-sphere  during  amitotic  nuclear  division  in  the  "spermatogonia"' 
of  the  salamander.!  Krause  and  Flemming  observed  that  in  the  autumn  many 
of  these  cells  show  peculiarly  lobed  and  irregular  nuclei  (the  "polymorphic  nuclei  "  of 
Bellonci).  These  were,  and  still  are  by  some  writers,  regarded  as  degenerating 
nuclei.  Meves,  however,  asserts  —  and  the  accuracy  of  his  observations  is  in  the 
main  vouched  for  by  Flemming  —  that  in  the  ensuing  spring  these  nuclei  become 
uniformly  rounded,  and  may  then  divide  amitotically.  In  the  autumn  the  attraction- 
sphere  is  represented  by  a  diffused  and  irregular  granular  mass,  which  more  or  less 
completelv  surrounds  the  nucleus.  In  the  spring,  as  the  nuclei  become  rounded, 
the  granular  substance  draws  together  to  form  a  definite  rounded  sphere,  in  which 
a  distinct  centrosome  may  sometimes  be  made  out.  Division  takes  place  in  the 
following  extraordinary  manner:  The  nucleus  assumes  a  dumb-bell  shape,  while 
the  attraction-si)here  becomes  drawn  out  into  a  band  which  surrounds  the  central 
part  of  the  nucleus,  and  finally  forms  a  closed  ring,  encircling  the  nucleus.  After 
this  the  nucleus  divides  into  two,  while  the  ring-shaped  attraction-sphere  (-archo- 
plasm")  is  again  condensed  into  a  sphere.  The  appearances  suggest  that  the  ring- 
shaped  sphere  actually  compresses  the  nucleus  and  cuts  it  through.  In  a  later 
paper  (94)  Meves  shows  that  the  diffused  "archoplasm"  of  the  autumn-stage 
arises  by  the  breaking  down  of  a  definite  spherical  attraction-sphere,  which  is 
re-formed  again  in  the  spring  in  the  manner  described,  and  in  this  condition  the 
cells  mav  divide  cither  uiitotically  or  amitotically.  He  adds  the  interesting  observa- 
tion, since  confirmed  by  Rawitz  ('94),  that  in  the  spermatocytes  of  the  salamander 
the  attraction-spheres  of  adjoining  cells  are  often  connected  by  intercellular  bridges, 
but  the  meaning  of  this  has  not  yet  been  determined. 

It  is  certain  that  the  remarkable  transformation  of  the  sphere  into  a  ring  during 
amitosis  is  not  of  universal,  or  even  of  general,  occurrence,  as  shown  by  the  later 
studies  of  \'om  Rath  (95,  3).  In  leucocytes,  for  example,  the  sphere  persists  in 
its  typical  form,  and  contains  a  centrosome,  during  every  stage  of  the  division:  but 
it  is  an  interesting  fact  that  during  all  these  stages  the  sphere  lies  on  the  concave 
side  of  the  nucleus  in  the  bay  which  finally  cuts  through  the  entire  nucleus.  Again, 
in  the  liver-cells  of  the  isopod  Porcellio,  the  nucleus  divides,  not  by  constriction,  as 
in  the  leucocyte,  but  by  the  appearance  of  a  nuclear  plate,  in  the  formation  of  which 
the  attraction  sphere  is  apparently  not  concerned.'-^  The  relations  of  the  centro- 
some and  archoplasm  in  amitosis  are,  therefore,  still  in  doubt;  but,  on  the  whole, 
the  evidence  goes  to  show  that  they  take  no  essential  part  in  the  process. 

3 .    Biologica I  Sign  ifica nee  of  A  m  itos is 

A  survey  of  the  known  cases  of  amitosis  brings  out  the  following 
siirnificant' facts.  It  is  of  extreme  rarity,  if  indeed  it  ever  occurs  in 
embryonic  cells  or  such  as  are  in  the  course  of  rapid  and  contniued 

1  '91,  p.  62S. 

2  Such  a  mode  of  amitotic  division  was  first  described  by  Saliatier  in  the  Crustacea  ('89), 
and  a  similar  mode  has  been  observed  by  Carnoy  and  Van  der  Stricht. 


DIRECT   OR   AMITOTIC  DIVISION 


117 


multiplication.  It  is  frequent  in  pathological  growths  and  in  cells 
such  as  those  of  the  vertebrate  decidua,  of  the  embryonic  envelopes 
of  insects,  or  the  yolk-nuclei  (periblast,  etc.),  zvJiicJi  arc  on  the  luay 
toivard  degeneration.  In  many  cases,  moreover,  direct  nuclear  divi- 
sion is  not  followed  by  fission  of  the  cell-body,  so  that  multinuclear 
cells  and  polymorphic  nuclei  are  thus  often  formed.  These  and 
many  similar  facts  led  Flemming  in  1891  to  express  the  opinion  that 
so  far  as  the  higher  plants  and  animals  are  concerned  amitosis  is  "a 
process  which  does  not  lead  to  a  new  production  and  multiplication 
of  cells,  but  wherever  it  occurs  represents  either  a  degeneration  or  an 
aberration,  or  perhaps  in  many  cases  (as  in  the  formation  of  multi- 
nucleated cells  by  fragmentation)  is  tributary  to  metabolism  through 
the  increase  of  nuclear  surface."  ^  In  this  direction  Flemming 
sought  an  explanation  of  the  fact  that  leucocytes  may  divide  either 
mitotically  or  amitotically  (/.  Peremeschko,  Lowit,  Arnold,  Flemming). 
In  the  normal  lymph-glands,  where  new  leucocytes  are  continually 
regenerated,  mitosis  is  the  prevalent  mode.  Elsewhere  (wandering- 
cells)  both  processes  occur.  '*  Like  the  cells  of  other  tissues  the 
leucocytes  find  their  normal  physiological  origin  (Neubildung)  in 
mitosis ;  only  those  so  produced  have  the  power  to  live  on  and  repro- 
duce their  kind  through  the  same  process."^  Those  that  divide  ami- 
totically are  on  the  road  to  ruin.  Amitosis  in  the  higher  forms  is 
thus  conceived  as  a  purely  secondary  process,  not  a  survival  of  a 
primitive  process  of  direct  division  from  the  Protozoa,  as  Strasburger 
('82)  and  Waldeyer  i^^'^)  had  conceived  it. 

This  hypothesis  has  been  carried  still  further  by  Ziegler  and  \o\Vi 
Rath  ('91).  In  a  paper  on  the  origin  of  the  blood  in  fishes,  Ziegler 
i^'^y)  showed  that  the  periblast-nuclei  in  the  ^gg  of  fishes  divide  ami- 
totically, and  he  was  thus  led  like  Flemming  to  the  view  that  amitosis 
is  connected  with  a  high  specialization  of  the  cell  and  may  be  a  fore- 
runner of  degeneration.  In  a  second  paper  ('91),  published  shortly 
after  Flemming's,  he  points  out  the  fact  that  amitotically  dividing 
nuclei  are  usually  of  large  size  and  that  the  cells  are  in  many  cases 
distinguished  by  a  specially  intense  secretory  or  assimilative  activity. 
Thus,  Riige  ('90)  showed  that  the  absorption  of  degenerate  eggs  in 
the  Amphibia  is  effected  by  means  of  leucocytes  which  creej)  into  the 
egg-substance.  The  nuclei  of  these  cells  become  enlarged,  divide  ami- 
totically, and  then  frequently  degenerate.  Other  observers  (  Korschelt, 
Carnoy)  have  noted  the  large  size  and  amitotic  division  of  the  nuclei 
in  the  ovarian  follicle-cells  and  nutritive  cells  surrounding  the  ovum  in 
insects  and  Crustacea.  Chun  found  in  the  entodermic  cells  of  the 
radial  canals  of  siphonophores  huge  cells  filled  with  nests  of  nuclei 
amitotically  produced,  and  suggested  ('90)  that  the  multiplication  of 

1  '91,  2,  p.  291. 


1 1 8  CELL-DI VI SI  OX 

nuclei  was  for  the  purpose  of  increasing  the  nuclear  surface  as  an  aid 
to  metabolic  interchanges  between  nucleus  and  cytoplasm.  Amitotic 
division  leading  to  the  formation  of  multinuclear  cells  is  especially  com- 
mon in  gland-cells.  Thus,  Klein  has  described  such  divisions  in  the 
mucous  skin-glands  of  Amphibia,  and  more  recently  Vom  Rath  has 
carefully  described  it  in  the  huge  gland-cells  (probably  salivary)  of  the 
isopod  Aiiilocm  ('95).  Many  other  cases  are  known.  Dogiel  ('90) 
has  ob.served  exceedingly  significant  facts  in  this  field  that  place  the 
relations  between  mitosis  and  amitosis  in  a  clear  light.  It  is  a  well- 
known  fact  that  in  stratified  epithelium  new  cells  are  continually 
formed  in  the  deeper  layers  to  replace  those  cast  off  from  the  super- 
ficial layers.  Dogiel  finds  in  the  lining  of  the  bladder  of  the  mouse 
that  the  nuclei  of  the  superficial  cells,  which  secrete  the  mucus  cover- 
ing the  surface,  regularly  divide  amitotically,  giving  rise  to  huge  mul- 
tinuclear cells,  which  finally  degenerate  and  are  cast  off.  The  new 
cells  that  take  their  place  are  formed  in  the  deeper  layers  by  mitosis 
alone.  Especially  significant,  again,  is  the  case  of  the  ciliate  Infu- 
soria, which  possess  two  kinds  of  nuclei  in  the  same  cell,  a  macro- 
nucleus  and  a  micronucleus.  The  former  is  known  to  be  intimately 
concerned  with  the  processes  of  metabolism  {cf.  p.  342).  During  con- 
jugation the  macronucleus  degenerates  and  disappears  and  a  new  one 
is  formed  from  the  micronucleus  or  one  of  its  descendants.  The  macro- 
nucleus  is  therefore  essentially  metabolic,  the  micronucleus  genera- 
tive in  function.  In  view  of  this  contrast  it  is  a  significant  fact  that 
while  both  nuclei  divide  during  the  ordinary  process  of  fission  the 
mitotic  phenomena  are  as  a  rule  less  clearly  marked  in  the  macronu- 
cleus than  in  the  micronucleus,  and  in  some  cases  the  former  appears  to 
divide  directly  while  the  latter  always  goes  through  a  process  of  mitosis. 
These  conclusions  received  a  very  important  support  in  the  work  of 
Vom  Rath  on  amitosis  in  the  testis  ('93).  On  the  basis  of  a  compara- 
tive study  of  amitosis  in  the  testis-cells  of  vertebrates,  mollusks,  and 
arthropods  he  concludes  that  amitosis  never  occurs  in  the  sperm-pro- 
ducing cells  (spermatogonia,  etc.),  but  only  in  the  supporting  cells 
( Randzellen,  Stutzzellen).  The  former  multiply  through  mitosis  alone. 
The  two  kinds  of  cells  have,  it  is  true,  a  common  origin  in  cells  which 
divide  mitotically.  When,  however,  they  have  once  become  differen- 
tiated, they  remain  absolutely  distinct ;  amitosis  never  takes  place  in 
the  series  vvhich  finally  results  in  the  formation  of  spermatozoa,  and 
the  amitotically  dividing  "supporting-cells"  sooner  or  later  perish. 
Vom  Rath  thus  reached  the  remarkable  conclusion  that  "  when  once 
a  cell  has  undergone  amitotic  division  it  has  received  its  death- 
warrant ;  it  may  indeed  continue  for  a  time  to  divide  by  amitosis,  but 
inevitably  perishes  in  the  end."  ^ 

^  '91.  p.  Zl^- 


SUMMARY  AND    CONCLUSION  Iig 

There  is,  however,  strong  evidence  that  this  conckision  is  too 
extreme.  Meves  ('94)  has  given  good  reason  for  the  conckision  that 
in  the  salamander  the  nuclei  of  the  sperm-producing  cells  (spermato- 
gonia) may  divide  by  amitosis  yet  afterward  undergo  normal  mitotic 
division,  and  Preusse  ('95)  has  reached  a  similar  result  in  the  case  of 
insect-ovaries.  Perhaps  the  most  convincing  evidence  in  this  direc- 
tion is  afforded  by  Pfeffer's  ('99)  recent  experiments  on  Spirogyra. 
If  this  plant  be  placed  in  water  containing  0.5  to  1.0%  of  ether,  active 
growth  and  division  continue,  but  only  by  amitosis.  If,  however,  the 
same  individuals  be  replaced  in  water,  viitotic  division  is  resmiied  and 
entirely  normal  growth  continues.  This  seems  to  show  conclusively 
that  amitosis,  in  lower  forms  of  Hfe  at  least,  does  not  necessarily  mean 
the  approach  of  degeneration,  but  is  a  result  of  special  conditions. 
Nevertheless,  there  can  be  no  doubt  that  Flemming's  hypothesis  in  a 
general  way  represents  the  truth,  and  that  in  the  vast  majority  of  cases 
amitosis  is  a  secondary  process  which  does  not  fall  in  the  generative 
series  of  cell-divisions. 

F.    Summary  and  Conclusion 

All  cells  arise  by  division  from  preexisting  cells,  cell-body  from 
cell-body,  nucleus  from  nucleus,  plastids  (when  these  bodies  are  pres- 
ent) from  plastids,  and  in  some  cases  centrosomes  from  centrosomes. 
The  law  of  genetic  continuity  thus  applies  not  merely  to  the  cell  con- 
sidered as  a  whole,  but  also  to  some  of  its  structural  constituents. 

In  mitosis,  the  usual  and  typical  mode  of  division,  the  nucleus  under- 
goes a  complicated  transformation,  and,  together  with  some  of  the 
cytoplasmic  material,  gives  rise  to  the  mitotic  figure.  Of  this,  the 
most  characteristic  features  are  the  chromatic  figure,  consisting  of 
chromosomes  derived  from  the  chromatin,  and  the  achromatic  figure, 
derived  from  the  cytoplasm,  the  nucleus,  or  from  both,  and  consisting 
of  a  spindle,  at  each  pole  of  which,  as  a  rule,  is  a  centrosome  and 
aster.  There  is,  however,  strong  evidence  that  both  these  latter  struc- 
tures may  in  some  cases  be  wanting,  and  the  spindle  is  therefore  prob- 
ably to  be  regarded  as  the  most  essential  element. 

The  chromosomes,  always  of  the  same  number  in  a  given  species 
(with  only  apparent  exceptions),  arise  by  the  transformation  of  the 
chromatin-reticulum  into  a  thread  which  breaks  into  segments  and 
splits  lengthwise  throughout  its  whole  extent.  The  two  halves  arc 
thereupon  transported  in  opposite  directions  along  the  spindle  to 
its  respective  poles  and  there  enter  into  the  formation  of  the  two 
corresponding  daughter-nuclei.  The  spireme-thread,  and  hence  the 
chromosome,  arises  from  a  single  series  of  chromatin-granules  or 
chromomeres  which,  by  their  fission,  cause  the  splitting  of  the  thread. 


1 20  CELL-DIVISIOX 

Every  individual  chromatin-j^ranule  therefore  contributes  its  quota 
to  each  of  the  daughter-nuclei,  but  it  is  uncertain  whether  they  are 
persistent  bodies  or  only  temporary  structures  like  the  chromosomes 
themselves. 

The  spindle  may  arise  from  the  achromatic  substance  of  the 
nucleus,  from  the  cytoplasmic  substance,  or  from  both.  When  cen- 
trosomes  are  present  it  is  they,  as  a  rule,  that  lead  the  way  in  divi- 
sion. About  the  daughter-centrosomes  as  foci  are  formed  the  asters 
and  between  them  stretches  the  spindle,  forming  an  auipJiiastcr 
which  is  the  most  highly  developed  form  of  the  achromatic  figure. 
When  centrosomes  are  absent,  as  now  appears  to  be  the  case  in  the 
higher  plants,  the  spindle  is  formed  from  fibrous  protoplasmic  ele- 
ments that  gradually  group  themselves  into  a  spindle. 

The  mechanism  of  mitosis  is  imperfectly  understood.  Experi- 
mental studies  give  ground  for  the  conclusion  that  the  changes 
undergone  by  the  chromatic  and  the  achromatic  figures  respectively 
are  parallel  but  in  a  measure  independent  processes,  which  are  how- 
ever so  correlated  that  both  must  cooperate  for  complete  cell-division. 
Thus  there  is  strong  evidence  that  the  fission  of  the  chromatin-gran- 
ules,  and  the  splitting  of  the  thread,  is  not  caused  by  division  of  the 
centrosome  or  the  formation  of  the  spindle,  but  only  accompanies  it 
as  a  parallel  phenomenon.  The  divergence  of  the  daughter-chromo- 
somes, on  the  other  hand,  is  in  some  manner  determined  by  the 
spindle-fibres.  There  are  cogent  reasons  for  the  view  that  some  of 
these  fibres  are  contractile  elements  which,  like  muscle-fibres,  drae: 
the  daughter-chromosomes  asunder ;  while  other  s])indle-fibres  act  as 
supporting  and  guiding  elements,  and  probably  by  their  elongation 
push  the  spindle-poles  apart.  The  adequacy  of  this  explanation  is, 
however,  doubtful,  and  it  is  not  improbable  that  the  centrosome  or 
spindlc-poles  are  centres  of  chemical  or  other  physiological  activities 
that  play  an  essential  part  in  the  process  and  are  correlated  with 
those  taking  place  in  the  chromatin.  The  functions  of  the  astral 
rays  are  likewise  still  involved  in  doubt,  the  rays  being  regarded  by 
some  investigators  as  contractile  elements  like  muscle-fibres,  by  others 
as  rigid  supporting  fibres,  or  even  as  actively  pushing  elements  like 
those  of  the  central  spindle.  It  is  generally  believed  further  that  they 
play  a  definite  part  in  division  of  the  cell-body  —  a  conclusion  sup- 
ported by  the  fact  that  the  size  of  the  aster  is  directly  related  to  that 
of  the  resulting  cell.  On  the  other  hand  division  of  the  cell-body 
may  apparently  occur  in  the  absence  of  asters  (as  in  amitosis,  or 
among  the  Infusoria). 

These  facts  show  that  mitosis  is  due  to  the  coordinate  play  of  an 
extremely  complex  system  of  forces  which  are  as  yet  scarcely  com- 
prehended.    Its  general  significance  is,  however,  obvious.      The  effect 


LITER  A  TURE  I  o  i 

of  mitosis  is  to  produce  a  meristic  division,  as  opposed  to  a  viere  mass- 
division,  of  the  chromatin  of  the  mother-cell,  and  its  equal  distribution 
to  the  nuclei  of  the  daughter-cells.  To  this  result  all  the  operations 
of  mitosis  are  tributary;  and  it  is  a  significant  fact  that  this  process 
is  characteristic  of  all  embryonic  and  actively  growing  cells,  while 
mass-division,  as  shown  in  amitosis,  is  equally  characteristic  of  highly 
specialized  or  degenerating  cells  in  which  development  is  approaching 
its  end. 


LITERATURE.     II  i 

Auerbach,  L.  —  Organologische  Studien.     Breslau,  1874. 

Van  Beneden,  E.  —  Recherches  sur  la  maturation  de  I'oeuf.  la   fecondation    et   la 

division  cellulaire  :  Arch,  de  Biol..,  IV.      1883. 
Van  Beneden  and  Neyt.  —  Nouvelles   recherches  sur  la  fecondation  et  la  division 

mitosque  chez  TAscaride  megalocephale  :  Bull.  Acad.  roy.  de  Belgigue,  III.  14, 

No.  8.     1887. 
Boveri,  Th.  — Zellenstudien:  I.  Jena.  Ze2tschr.,XX\.     1887;   II .//;/>/.  XX II .    1888; 

III.  Ibid.  XXIV.     1890. 
Driiner,    L.  —  Studien  liber  den    Mechanismus  der  Zelltheilung.     Jena.  Zeitschr.^ 

XXIX.,  II.     1894. 
Erlanger,  R.  von.  —  Die  neuesten  Ansichten  liber  die  Zelltheilung  und  ihre  Mechanik  : 

Zo'dl.  Centra  lb..  III.  2.     1896. 
Id.  —  Uber  die  Befruchtung  und  erste  Teilung  des  Ascariseies :  Arcli.  mik.  Anat., 

XLIX.     1897. 
Flamming,    W.,    '92.  —  Entwicklung   und    Stand    der   Kenntnisse   liber   Amitose : 

Merkel  und  Bofinefs  Ergebnisse,  II.     1892. 
Id.  —  Zelle.     (See  Introductory  list.     Also  general  list.) 
Fol,  H.  —  (See  List  IV.) 

Heidenhain,  M, — Cytomechanische  Studien:  Arch.  f.  Entiuickmcch.,  I.  4.     1895. 
Id.  —  Neue  Erlauterungen  zum  Spannungsgesetz  der  centrirten  Svsteme :  Morph. 

Arb.,  VII.      1897. 
Hermann,  F.  —  Beitrag  zur  Lehre  von  der  Entstehung  der  karyokinetischen  Spindel : 

Arch.  7mk.  Anat.,XXXY\l.     1891. 
Hertwig,  R.  —  Uber  Centrosoma  und  Centralspindel :  Sit:;. -Berg.  (Jes.  Morph.  und 

Phys.     MiincJioi.  1^95'  Heft  I. 
Kostanecki  and  Siedlecki.  —  Uber  das  Verhalten  der  Centrosomen  zum  Protoplasma  : 

Arch.  niik.  Anaf.,  XL\'1I1.      1896. 
Mark,  E.  L.  —  (See  List  IV.) 

Meves,  Fr.  —  Zellteilung:  Merkel  und  Bontiefs  Ergebnisse,  W.     1897. 
Reinke,  F.  —  Zellstudien  :   I.  Arch.  mik.  Afiat.,  XLIII.     1894  :  II.  /bid.  WAV.     1S94. 
Strasburger,  E.  —  Karyokinetische  Probleme  :  Jahrb.f.  l\ 'iss.  Jiotan..  XX\'I II.     i S95. 
Strasburger,  Osterhout,  Mottier,  and  Others.  —Cytologische  Studien  aus  dem  Bonner 

Institut:  JaJirb.  luiss.  Bot..  XXX.      1897. 
Waldeyer.  W.  — iJber  Karyokinese  und  ihre  Beziehungen  zu  den  Befruchtungsvor- 

gangen:  Arch.  7tiik.  Anal,  XXXII.  1888.     QJ.M.S..  XXX.  1889-90. 

1  See  also  Literature,  IV.,  p.  231. 


CHAPTER    III 

THE     GERM-CELLS 

"  Not  all  the  propjeny  of  the  primary  impregnated  germ-cells  are  required  for  the  forma- 
tion of  the  liudy  in  all  animals;  certain  of  the  derivative  germ-cells  may  remain  unchanged 
and  become  included  in  that  body  which  has  been  composed  of  their  metamorphosed  and 
diversely  combined  or  confluent  brethren;  so  included,  any  derivative  germ-cell  may  com- 
mence and  repeat  the  same  processes  of  growth  by  imbibition  and  of  propagation  by  spon- 
taneous fission  as  those  to  which  itself  owed  its  origin;  followed  by  metamorjihoses  and 
combinations  of  the  germ-masses  so  produced,  which  concur  to  the  development  of  another 
individual."  RlCJiAKU  UWEN.i 

"  Es  theilt  sich  demgemass  das  befruchtete  Ei  in  das  Zellenmaterial  des  Individuums  und 
in  die  Zellen  fiir  die  Erhaltung  der  Art."  M.  NUbSBAUM.'^ 

The  germ  from  which  every  living  form  arises  is  a  single  cell,  de- 
rived by  the  division  of  a  parent-cell  of  the  preceding  generation. 
In  the  unicellular  plants  and  animals  this  fact  appears  in  its  simplest 
form  as  the  fission  of  the  entire  parent-body  to  form  two  new  and 
separate  individuals  like  itself.  In  all  the  multicellular  types  the 
cells  of  the  body  sooner  or  later  become  differentiated  into  two  groups, 
which  as  a  matter  of  practical  convenience  may  be  sharply  distin- 
guished from  one  another.  These  are,  to  use  Wcismann's  terms  :  (i) 
the  somatic  cells,  which  are  differentiated  into  various  tissues  by 
which  the  functions  of  individual  life  are  performed  and  which  col- 
lectively form  the  **  body,"  and  (2)  \hQ  (^ej'm-cells,  which  are  of  minor 
significance  for  the  individual  life  and  are  destined  to  give  rise  to 
new  individuals  by  detachment  from  the  body.  It  must,  however,  be 
borne  in  mind  that  the  distinction  between  germ-cells  and  somatic 
cells  is  not  absolute,  as  some  naturalists  have  maintained,  but  only 
relative.  The  cells  of  both  groups  have  a  common  origin  in  the 
parent  germ-cell ;  both  arise  through  mitotic  cell-division  during  the 
cleavage  of  the  ovum  or  in  the  later  stages  of  development ;  both  have 
essentially  the  same  structure  and  both  may  have  the  same  power  of 
development,  for  there  are  many  cases  in  which  a  small  fragment 
of  the  body  consisting  of  only  a  few  somatic  cells,  perhaps  only  of 
one,  may  give  rise  by  regeneration  to  a  complete  body.  The  dis- 
tinction  between   somatic   and    germ-cells    is    an    expression    of   the 

1  Parthenogenesis,  p.  3,  1849. 

2  Arch.  Mik.  Anat.,  XVIIL,  p.  112,  1880. 

122 


THE    GERM-CELLS 


123 


physiological  division  of  labour;  and  while  it  is  no  doubt  the  most 
fundamental  and  important  differentiation  in  the  multicellular  body, 
it  is  nevertheless  to  be  regarded  as  differing  only  in  degree,  not  in 
kind,  from  the  distinctions  between  the  various  kinds  of  somatic  cells. 
In  the  lowest  multicellular  forms,  such  as  Volvox  (Fig.  57),  the 
differentiation  appears  in  a  very  clear  form.  Here  the  body  consists 
of  a  hollow  sphere,  the  walls  of  which  consist  of  two  kinds  of  cells. 
The  very  numerous  smaller  cells  are  devoted  to  the  functions  of  nutri- 


:?t^Vi    \/,-i    -^-'n-^  .,  ,-■- 


\ 


\ 


^ 


Fig.  57.  —  Volvox,  showing  the  small  ciliated  somatic  cells  and  eight  large  germ-cells  (drawn 
from  life  by  J.  H.  Emerton). 

tion  and  locomotion,  and  sooner  or  later  die.  A  number,  usually  eight, 
of  larger  cells  are  set  aside  as  germ-cells,  each  of  which  by  progressive 
fission  may  form  a  new  individual  like  the  parent.  In  this  case  the 
germ-cells  are  simply  scattered  about  among  the  somatic  cells,  and  no 
special  sexual  organs  exist.  In  all  the  higher  types  the  germ-cells 
are  more  or  less  definitely  aggregated  in  groups,  supported  and  nour- 
ished by  somatic  cells  specially  set  apart  for  that  purpose  and  forming 
distinct  sexual  organs,  the  ovaj^es  and  spcrmarics  or  their  equivalents. 
Within  these  organs  the  germ-cells  are  carried,  protected,  and  nour- 
ished;  and  here  they  undergo  various  differentiations  to  prepare 
them  for  their  future  functions. 

In  the  earlier  stages  of  embryological  development  the  progenitors 
of  the  germ-cells  are  exactly  alike  in  the  two  sexes  and  are  indistin- 


124 


THE    GERM-CELLS 


guishable  from  the  surrounding  somatic  cells.     As  development  pro- 
ceeds, they  are  first  differentiated  from  the  somatic  cells  and  then 
diverge  very  widely  in  the  two  sexes,  undergoing  remarkable  trans- 
formations of  structure  to  fit  them  for  their  specific  functions.     The 
structural  difference  thus  brought  about  between  the  germ-cells  is, 
however,  only  the   result  of   physiological  division   of   labour.     The 
female  germ-cell,   or   ovum,   supplies   most   of   the   material   for  the 
body  of  the  embryo  and  stores  the  food  by  which  it  is  nourished.     It 
is  therefore  very  large,  contains  a  great  amount  of  cytoplasm  more  or 
less  laden  with  food-matter  (j7^/X'  or  dciitoplasui),  and  in  many  cases 
becomes  surrounded  by  membranes  or  other  envelopes  for  the  pro- 
tection of  the  developing  embryo.     On  the  whole,  therefore,  the  early 
life  of  the  ovum  is  devoted  to  the  accumulation  of  cytoplasm  and  the 
storage  of    potential  energy,  and  its  nutritive  processes  are  largely 
constructive  or  anabolic.     On  the  other  hand,  the  male  germ-cell  or 
spermatozoon  contributes   to   the   mass   of    the  embryo  only  a   very 
small  amount  of  substance,  comprising  as  a  rule  only  a  single  nucleus 
and  a  very  small  quantity  of  cytoplasm.      It  is  thus  relieved  from  the 
drudgery  of  making  and  storing  food  and  providing  protection  for 
the  embryo,  and  is  provided  with  only  sufficient  cytoplasm  to  form   a 
locomotor  apparatus,  usually  in  the  form  of    one  or  more  cilia,  by 
which  it  seeks  the  ovum.     It  is  therefore  very  small,  performs  active 
movements,  and  its  metabolism  is  characterized  by  the  predominance 
of  the  destructive  or  katabolic  processes  by  which  the  energy  neces- 
sary for  these  movements  is  set  free.^     When   finally  matured,  there- 
fore, the  ovum  and  spermatozoon  have  no  external  resemblance  ;  and 
while    Schwann    recognized,  though    somewhat    doubtfully,  the  fact 
that  the  ovum  is  a  cell,  it  was  not  until  many  years  afterward  that 
the  spermatozoon  was  proved  to  be  of  the  same  nature. 


A.     The  Ovum 

The  animal  (i^^^^  (Figs.  5CS-59)  is  a  huge  spheroidal  cell,  sometimes 
naked,  but  more  commonly  surrounded  by  one  or  more  membranes 
which  may  be  perforated  by  a  minute  opening,  the  inicropylc,  through 
which  the  spermatozoon  enters  (Fig.  63).  It  contains  an  enormous 
nucleus  known  as  the  germinal  vesicle,  within  which  is  a  very  con- 
spicuous nucleolus  known  to  the  earlier  observers  as  the  germinal 
spot.     In  many  eggs  the  latter  is  single,  but  in  other  forms  many 

1  The  metaliolic  contrast  between  the  germ-cells  has  been  fully  discussed  in  a  most  sug- 
gestive manner  by  Geddes  and  Thompson  in  their  work  on  the  Evolution  of  Sex ;  and  these 
authors  regard  this  contrast  as  but  a  particular  manifestation  of  a  metabolic  contrast  charac- 
teristic of  the  sexes  in  general. 


THE    OVUM  125 

nucleoli  are  present,  and  they  are  sometimes  of  more  than  one  kind 
as  in  tissue-cells.i  In  many  forms  no  centrosome  or  attraction-sphere 
is  found  in  the  ^gg  until  the  initial  stages  in  the  formation  of  the 
polar  bodies,  though  Mertens  ('93)  describes  a  centrosome  and  attrac- 
tion-sphere in  the  young  ovarian  eggs  of  a  number  of  vertebrates 
(Fig.  79),  while  Platner  ('89)  and  Stauffacher  ('93)  find  what  they 
believe  to  be  centrosomes  in  much  later  stages  of  AulostoDiuvi  and 
Cyclas,  lying  outside  the  nuclear  membrane.  Beside  these  cases 
should  be  placed  those  described  by  Balbiani,  Munson,  Nemec,  and 
others  in  which  a  body  closely  resembling  an  attraction-sphere  is 
identified  as  a  "yolk-nucleus"  or  "vitelline  body,"  as  described  at 
page  158.  In  none  of  these  cases  is  the  identification  of  this 
body  wholly  satisfactory,  nor  is  it  known  to  have  any  connection  with 
the  polar  mitoses.  Most  observers  find  no  centrosome  until  the 
prophases  of  the  first  polar  mitosis.  Its  origin  is  still  problematical, 
some  observers  believing  it  to  arise  de  novo  in  the  cytoplasm  (Mead), 
others  concluding  that  it  is  of  nuclear  origin  (Mathews,  Van  der 
Stricht,  Riickert),  still  others  that  it  persists  in  the  cytoplasm  hidden 
among  the  granules.  In  any  case  it  is  again  lost  to  view  after  forma- 
tion of  the  polar  bodies,  to  be  replaced  by  the  cleavage-centrosomes 
which  arise  in  connection  with  the  spermatozoon  (p.  187). 

The  egg-cytoplasm  almost  always  contains  a  certain  amount  of 
nutritive  matto^,  the  yoik  or  deiUoplasin,  in  the  form  of  liquid  drops, 
solid  spheres  or  other  bodies  suspended  in  the  meshwork  and  varying 
greatly  in  different  cases  in  respect  to  amount,  distribution,  form,  and 
chemical  composition. 

I.    TJie  Nucleus 

The  nucleus  or  germinal  vesicle  occupies  at  first  a  central  or  nearly 
central  position,  though  it  shows  in  some  cases  a  distinct  eccentricity 
even  in  its  earliest  stages.  As  the  growth  of  the  agg  proceeds,  the 
eccentricity  often  becomes  more  marked,  and  the  nucleus  may  thus 
come  to  lie  very  near  the  periphery.  In  some  cases,  however,  the 
peripheral  movement  of  the  germinal  vesicle  occurs  only  a  very  short 
time  before  the  final  stages  of  maturation,  which  may  coincide  with 
the  time  of  fertilization.  Its  form  is  typically  that  of  a  spherical  sac, 
surrounded  by  a  very  distinct  membrane  (Fig.  58);  but  during  the 
growth  of  the  Q.gg  it  may  become  irregular  or  even  amceboid  (Fig.  j'j), 
and,  as  Korschelt  has  shown  in  the  case  of  insect-eggs,  may  move 
through  the  cytoplasm  toward  the  source  of  food.      Its  structure  is 

1  Hacker  ('95,  p.  249)  has  called  attention  to  the  fact  that  the  nucleolus  is  as  a  rule 
single  in  small  eggs  containing  relatively  little  deutoplasm  (crulenterates,  echinodernis, 
many  annelids,  and  some  copepods),  while  it  is  multiple  in  large  eggs  heavily  laden  with 
deutoplasm  (lower  vertebrates,  insects,  many  Crustacea). 


126  THE    GERM-CELLS 

on  the  whole  that  of  a  typical  cell-nucleus,  but  is  subject  to  very  great 
variation,  not  only  in  different  animals,  but  also  in  different  stages  of 
ovarian  growth.  Sometimes,  as  in  the  echinoderm  ovum,  the  chro- 
matin forms  a  beautiful  and  regular  reticulum  consisting  of  numer- 
ous chromatin-granules  suspended  in  a  network  of  linin  (Fig.  58). 
In  other  cases,  no  true  reticular  stage  exists,  the  nucleus  containing 
throughout  the  whole  period  of  its  growth  the  separate  daughter-chro- 
mosomes of  the  preceding  division  (copepods,  selachians,  Amphibia),^ 


6- 

Fig.  58.  —  Ovarian  egg  of  the  sea-urchin,  Toxopneustes  (x  750). 

g.v.  Nucleus  or  germinal  vesicle,  containing  an  irregular  discontinuous  network  of  chromatin; 
g.i.  nucleolus  or  germinal  spot,  intensely  stained  with  haematoxylin.  The  naked  cell-body  con- 
sists of  a  very  regular  alveolar  meshwork,  scattered  through  which  are  numerous  minute  granules 
or  microsomes.  {Cf.  Figs.  11,  12.)  Below,  at  s,  is  an  entire  spermatozoon  shown  at  the  same 
enlargement  (both  middle-piece  and  fiagellum  are  slightly  exaggerated  in  size). 


and  these  chromosomes  may  undergo  the  most  extraordinary  changes 
of  form,  bulk,  and  staining-reaction  during  the  growth  of  the  Q^gg^ 
It  is  a  very  interesting  and  important  fact  that  during  the  growth 
and  maturation  of  the  ovum  a  large  part  of  the  chromatin  of  the 
germinal  vesicle  may  be  lost,  either  by  passing  out  bodily  into  the 
cytoplasm,  by  conversion  into  supernumerary  or  accessory  nucleoli 
which  finally  degenerate,  or  by  being  cast  out  and  degenerating  at 
the  time  the  polar  bodies  are  formed  (Figs.  97,  128). 

The  nucleolus  of  the  egg-cell  is,  as  elsewhere,  a  variable  quantity 
and  is  still  imperfectly  understood.  It  often  attains  an  enormous 
development,  forming  the  "  Keimfleck  "  or  "germinal  spot"  of  the 

^  p-  273.  2  p.  338. 


THE    OVUM 


1^7 


early  observers.  There  are  some  cases  {e.g.  echinoderm  eggs)  in 
which  it  is  always  a  single  large  spherical  body  (Fig.  58),  and  this 
condition  appears  to  be  characteristic  of  the  very  young  ovarian  e^'-f^s 
of  most  animals.  As  a  rule,  however,  the  number  of  nucleoli  ^in- 
creases with  the  growth  of  the  ovum,  until,  in  such  forms  as  Amphibia 
and  reptiles,  they  may  be  numbered  by  hundreds. 

In  a  large  number  of  cases  the  nucleoli  are  of  two  quite  distinct 
types,  which  Flemming  has  distinguished  as  the  *'  principal  nucleolus  " 


Fig.  59. —  Ovum  of  the  cat,  within  the  ovary,  directly  reproduced  from  a  photograph  of  a 
preparation  by  Dahlgren.  [Enlarged  235  diameters.]  The  ovum  lies  in  the  Graafian  follicle 
within  the  discus  proligerus,  the  latter  forming  the  immediate  follicular  investment  {corona 
radiata)  of  the  Qg^.  Within  the  torona  is  the  clear  zona  pellucida  or  egg-membrane.  {Cf. 
Fig.  92.) 

{Hajiptmicleolus)  and  ''accessory  nucleoli"  {Ncbennuclcoli).  These 
differ  widely  in  staining-reaction  ;  but  it  does  not  yet  clearly  appear 
whether  they  definitely  correspond  to  the  plasmosomes  and  karyo- 
somes  of  tissue-cells  (p.  34).  The  principal  nucleolus,  which  alone 
is  present  in  such  eggs  as  those  of  echinoderms,  often  stains  deeply 
with  chromatin-stains,  yet  differs  more  or  less  widely  from  the 
chromatin-network,!  and  in  some  cases  at  least  it  does  not  contribute 

1  Cf.  List,  '96,  Montgomery,  '98,  2,  and  Obst.,  '99. 


128  THE    GERM-CELLS 

to  the  formation  of  chromosomes.     It  cannot  therefore  be  directly  com- 
pared to  the  net-knots  or  karyosomcs  of  tissue-cells.     This  nucleolus  is 
often  vacuolated  and  sometimes  assumes  the  form  of  a  hollow  vesicle. 
It  is  rarely  double  or  multiple.     The  accessory  nucleoli,  on  the  other 
hand,  are  in  general  coloured  by  plasma-stains,  thus  resembling  the 
plasmosomes  of  tissue-cells  ;  they  arc  often  multiple,  and  as  a  rule 
they  arise  secondarily  during  the  growth  of  the  (t^g  (Fig.  6i).     The 
accessory  nucleoli  often  have  no  connection  with  the  principal ;  but 
in  some  mollusks  and  annelids  an  accessory  and  a  principal  nucleolus 
are  closely  united  to  form  a  single  compound  body  (Figs.  60,  61). 
The  numerous  nucleoli  of  the  amphibian  or  reptilian  ^gg  appear  to  be 
of  the  "accessory"  type.     The  singular  inconstancy  of  the  nucleolus 
is  evidenced  by  the  fact  that  even  closely  related  species  may  differ 
in  this  regard.     Thus,  in  Cyclops  brcviconiis,  according  to  Hacker,  the 
very  young  ovum  contains  a  single  intensely  chromatic  nucleolus ;  at 
a  later  period  a  number  of  paler  accessory  nucleoli  appear ;  and  still 
later  the  principal  nucleolus  disappears,  leaving  only  the  accessory 
ones.     In  C.  strenuus,  on  the  other  hand,  there  is  throughout  but  a 
single  nucleolus. 

The  physiological  meaning  of  the  nucleoli  is  still  involved  in  doubt. 
Many  cases   are,  however,  certainly  known  in  which  the  nucleolus 
plays  no  part  in  the  later  development  of  the  nucleus,  being  cast  out 
or  degenerating  /;/  situ  at  the  time  the  polar  bodies  are  formed.     It 
is,  for  example,  cast  out  bodily  in  the  medusa  Alquorca  (Hacker)  and 
in  various  annelids  and  echinodcrms,  afterward  lying  for  some  time 
as  a '' metanucleus  "  in  the  egg-cytoplasm  before  degenerating.     In 
these  cases  the  chromosomes  are  formed  in  the  germinal  vesicle  inde- 
pendently of  the  nucleoli  (Fig.  125),  which  degenerate  in  j//;/ when 
the  membrane  of  the  germinal  vesicle  disappears.     In  such  cases  it 
seems  quite  certain  that  the  nucleoli  do  not  contribute  to  the  forma- 
tion of  the  chromosomes,  and  that  their  substance  represents  passive 
material  which  is  of  no  further  direct  use.      Hence  we  can  hardly 
doubt  the  conclusion  of  Hacker,  that  the  nucleoli  of  the  germ-cells 
are,  in  some  cases  at  least,  accumulations  of  by-products  of  the  nuclear 
action,  derived  from  the  chromatin  cither  by  direct  transformation  of 
its  substance,  or  as  chemical  cleavage-products  or  secretions.     It  will 
be   shown   in   Chapter  V.  that   in   some   cases   a   large   part  of    the 
chromatic  reticulum  is  cast  out,  and  degenerates  at  the  time  the  polar 
bodies  are  formed.     The  immense  growth  of  the  chromatin  during 
the  ovarian  development  is  probably  correlated  in  some  way  with  the 
intense  constructive  activity  of  the  cytoplasm  (p.  339);  and  when  this 
latter  process  has   ceased  a  large  part   of   the  chromatin-substance, 
having  fulfilled  its  functions,  is  cast  aside.     It  seems  not  improbable 
that  the  nucleoli  are  tributary  to  the  same  general  process,  perhaps 


THE    OVUM 


129 


Fig.  60.  — Eggs  of  the  annelid  ^V^^rm,  before  and  after  fertilization,  X  400  (for  int.^rn..^di.itt' 
stages  see  Fig.  95). 

A.  Before  fertilization.  The  large  germinal  vesicle  occupies  a  nearly  central  position.  It  con- 
tains a  network  of  chromatin  in  which  are  seen  five  small  darker  bodies;  these  are  the  quadruple 
chromosome-groups,  or  tetrads,  in  process  of  formation  (not  all  of  them  are  shown)  ;  those  alone 
persist  in  later  stages,  the  principal  mass  of  the  network  being  lost;  g.s.  double  germinal  spot. 
consisting  of  a  chromatic  and  an  achromatic  sphere.  This  egg  is  heavily  laden  with  yolk,  in  the 
form  of  clear  deutoplasm-spheres  {d)  and  fat-drops  (/).  uniformly  distributed  through  the  cyto- 
plasm. The  peripheral  layer  of  cytoplasm  (peri-vitelline  layer)  is  free  from  deutoplasni.  Outside 
this  the  membrane.  B.  The  egg  some  time  after  fertilization  and  about  to  divide.  The  deuto- 
plasm  is  now  concentrated  in  the  lower  hemisphere,  and  the  peri-vitelline  layer  has  disappeared. 
Above  are  the  two  polar  bodies  {p.b.).  Below  them  lies  the  mitotic  figure,  the  chromosomes 
dividing. 


130 


THE    GERM-CELLS 


serving  as  storehouses  of  material  formed  incidentally  to  the  general 
nuclear  activity,  but  not  of  further  direct  use. 

Carnoy  and  Le  Brun  ('97,  '99)  reach,  however,  the  conclusion  that 
in  the  germinal  vesicle  of  Amphibia  the  chromosomes  are  derived 
not  from  the  chromatin-network,  but  solely  from  the  nucleoli.  The 
apparent  contradiction  of  this  result  with  that  of  other  observers  is, 


■-.  \*. 


V 


¥: 


•V*       ''"'^.^^^    ''-'''''V.      '' 


\ 


IS 


•-»r  !!?^" 


.^4.  "  *-f 


•J* 


■'•% 


«     e. 


J- 


Fig.  61.  —  Germinal  vesicles  of  growing  ovarian  eggs  ut  the  iaiiiLininiincli,  L^f/io  {A-D),  and 
the  spider,  Epeira  {E-F).     [Ohst.] 

A.  Youngest  stage  with  single  (principal)  nucleolus.  B.  Older  egg,  showing  accessory  nucle- 
olus attached  to  the  principal.  C.  The  two  nucleoli  separated.  D.  Much  older  stage,  showing 
the  two  nucleoli  united.  E.  (ierminal  vesicle  of  Epeira,  showing  one  accessory  nucleolus  at- 
tached to  the  principal,  and  one  free.  /•"■.  Later  stage ;  several  accessory  nucleoli  attached  to  the 
principal. 

perhaps,  only  a  verbal  one;  for  the  ''nucleoli"  are  here  evidently 
chromatin-masses,  and  the  disappearance  of  the  chromatic  network  is 
comparable  with  what  occurs  at  a  later  period  in  the  annelid  Qgg 
(Figs.  97,  128). 

2.     T/ic  Cytoplasm 

The  egg-cytoplasm  varies  greatly  in  appearance  with  the  varia- 
tions of  the  deutoplasm.     In  such  eggs  as  those  of  the  echinoderm 


THE    OVUM  131 

(Fig.  58),  which  have  Httle  or  no  deutoplasm,  the  cytoplasm  forms  a 
regular  meshwork,  which  is  in  this  case  an  undoubted  alveolar  struc- 
ture, the  structure  of  which  has  already  been  described  at  p.  28.  In 
eggs  containing  yolk  the  deutoplasm-spheres  or  granules  are  laid 
down  in  the  spaces  of  the  meshwork  and  appear  to  correspond  to  the 
alveolar  spheres  of  the  echinoderm  ^^^g  (p.  50).  If  they  are  of  large 
size  the  cytoplasm  assumes  a  *' pseudo-alveolar"  structure  (Fig.  60), 
much  as  in  plant-cells  laden  with  reserve  starch;  but  reasons  have 
already  been  given  (p.  50)  for  regarding  this  as  only  a  modification 
of  the  "primary"  alveolar  structure  of  Butschli.  There  is  good 
reason  to  believe,  however,  that  the  egg-cytoplasm  may  in  some  cases 
form  a  true  reticular  structure  with  the  yolk-granules  lying  in  its 
interstices,  as  many  observers  have  described.  In  many  cases  a  pe- 
ripheral layer  of  the  ovum,  known  as  the  cortical  or  peri-vitelline  layer, 
is  free  from  deutoplasm-spheres,  though  it  is  continuous  with  the 
protoplasmic  meshwork  in  which  the  latter  lie  (Fig.  60).  Upon 
fertilization,  or  sometimes  before,  this  layer  may  disappear  by  a 
peripheral  movement  of  the  yolk,  as  appears  to  be  the  case  in 
Nereis.  In  other  cases  the  peri-vitelline  substance  rapidly  flows 
toward  the  point  at  which  the  spermatozoon  enters,  where  a  proto- 
plasmic germinal  disc  is  then  formed;  for  example,  in  many  fish-eggs. 

The  character  of  the  yolk  varies  so  widely  that  it  can  here  be  con- 
sidered only  in  very  general  terms.  The  deutoplasm-bodies  are  com- 
monly spherical,  but  often  show  a  more  or  less  distinctly  rhomb(^idal 
or  crystalloid  form  as  in  Amphibia  and  some  fishes,  and  in  such  cases 
they  may  sometimes  be  split  up  into  parallel  lamellae  known  as  yolk- 
plates.  Their  chemical  composition  varies  widely,  judging  by  the 
staining-reactions;  but  we  have  very  little  definite  knowledge  on  this 
subject,  and  have  to  rely  mainly  on  the  results  of  analysis  of  the  total 
yolk,  which  in  the  hen's  ^gg  is  thus  shown  to  consist  largely  of  pro- 
teids,  nucleo-albumins,  and  a  variety  of  related  substances  which  are 
often  associated  with  fatty  substances  and  small  quantities  of  car- 
bohydrates (glucose,  etc.).  In  some  cases  the  deutoplasm-spheres 
stain  intensely  with  nuclear  dyes,  such  as  hsematoxylin ;  e.i:^.  in  many 
worms  and  mollusks;  in  other  cases  they  show  a  greater  affinity  for 
plasma-stains,  as  in  many  fishes  and  Amphibia  and  annelids  ( Fig.  60). 
Often  associated  with  the  proper  deutoplasm-spheres  are  drops  of  oil, 
either  scattered  through  the  yolk  (Fig.  60)  or  united  to  form  a  .jingle 
large  drop,  as  in  many  pelagic  fish-eggs. 

The  deutoplasm  is  as  a  rule  heavier  than  the  protoplasm  ;  and  in 
such  cases,  if  the  yolk  is  accumulated  in  one  hemisphere,  the  ^gg 
assumes  a  constant  position  with  respect  to  gravity,  the  egg-axis 
standing  vertically  with  the  animal  pole  turned  upward,  as  in  the 
frog,  the  bird,  and    many  other  cases.     There  are,  however,  many 


132 


THE    GERM-CELLS 


cases  in  which  the  o.^^  may  lie  in  any  position.     When  fat-drops  are 
present  thev  usually  He  in  the  vegetative  hemisphere,  and  since  they 

are  lighter  than  the  other  constituents 
they  usually  cause  the  egg  to  lie  with 
the  animal  pole  turned  downwards,  as 
is  the  case  with  some  annelids  ( .AV/'^/.v) 
and  many  pelagic  fish-eggs. 


pb  — 


cn 


Fig.  62.  —  Schematic  figure  of  a 
median  longitudinal  section  of  the  egg 
of  a  fly  ^.Musca),  showing  axes  of  the 
bilateral  egg  and  the  membranes. 
[From  KORSCHEI.T  and  Hkidkr, 
after  Hf:nkin(;  and  Blochmann.] 

e.n.  The  germ-nuclei  uniting; 
VI.  micropyle;  p.b.  the  polar  bodies. 
The  fiat  side  of  the  egg  is  the  dorsal, 
the  convex  side  the  ventral,  and  the 
micropyle  is  at  the  anterior  end. 
The  deutoplasm  (small  circles)  lies 
in  the  centre  surrounded  by  a  periph- 
eral or  peri-vitelline  layer  of  proto- 
plasm. The  outer  heavy  line  is  the 
chorion,  the  inner  lighter  line  the 
vitelline  membrane,  both  being  per- 
forated by  the  micropyle,  from  which 
exudes  a  mass  of  jelly-like  substance. 


3.     The  Egi^-ciivclopcs 

The  egg-envelopes   fall    under   three 
catetcories.     These  are  :  — 

{(I)   The  ritclli}iL    vnnibrauc,  secreted 

bv  the  ovum  itself. 
{b)    The  chorion,  formed  outside  the 
ovum    bv    the    activity   of   the 
maternal  follicle-cells. 
{c)    Accessory  envelopes,    secreted    by 
the  walls  of  the  oviduct  or  other 
maternal    structures    after    the 
ovum  has  left  the  ovary. 
Only  the  first  of    these  properly  be- 
lonirs  to  the  ovum,  the  second  and  third 
being  purely  maternal  products.     There 
are  some  e^^i^s,  such  as  those  of  certain 
coelenterates     {e.g:     Rcnilla\    that    are 
naked  throughout  their   whole  develop- 
ment.      In  many   others,   of  which   the 
sea-urchin  is  a  type,  the  fresh-laid  ^<^^  is 
naked  but  forms  a  vitelline   membrane 
almost  instantaneously  after  the  sperma- 
tozocin  touches  it.^      In  other  forms  (in- 
sects, birds)  the  vitelline  membrane  may 
be   present   before  fertilization,   and    in 
such  cases  the  Q.^^g  is  often  surrounded 
by  a    chorion    as    well.      The    latter   is 
usually   very   thick   and   firm    and    may 
have  a  shell-like  consistency,  its  surface 
sometimes     showing     various     peculiar 
markings,    prominences,    or    sculptured 
patterns    characteristic    of    the    species 
(insects).^ 


1  That  the  vitelline  membrane  does  not  preexist  seems  to  be  estal)lished  by  the  fact  that 
egg-frai^ments  likewise  surround  themselves  vlth  a  membrane  when  fertilized.     [Hertwig.] 

-  In  some  cases,  according  to  Wheeler,  the  insect-egg  has  only  a  chorion,  the  vitelline 
membrane  being  absent. 


THE    OVUM  133 


The  accessory  envelopes  are  too  varied  to  be  more  than  touched 
upon  here.  They  inchide  not  only  the  products  of  the  oviduct  or 
uterus,  such  as  the  albumin,  shell-membrane,  and  shell  of  birds  and 
reptiles,  the  gelatinous  mass  investing  amphibian  ova,  the  capsules 
of  molluscan  ova  and  the  like,  but  also  nutritive  fluids  and  capsules 
secreted  by  the  external  surface  of  the  body,  as  in  leeches  and  earth- 
worms. 

When  the  ^gg  is  surrounded  by  a  membrane  before  fertiUzation  it 
is  often  perforated  by  one  or  more  openings  known  as  micropylcs, 
through  which  the  spermatozoa  make  their  entrance  (Figs.  62,  63). 
Where  there  is  but  one  micropyle,  ^ 

it  is  usually  situated  very  near  the 
upper  or  anterior  pole  (fishes, 
many  insects),  but  it  may  be  at  the 
opposite  pole  (some  insects  and 
mollusks),  or  even  on  the  side 
(insects).       In  many  insects  there 

is  a  group  of  half  a  dozen  or  more         pig.  63.- Upper  pole  of  the  egg  of  Ar^o- 
micropyles  near  the  upper  pole  of     nanta.    [Ussow.] 

the     ^gZ',     and     perhaps    correlated  The   egg   is  surrounded   by  a   very  thick 

with    this    is    the    fact  that    several       "-.embrane.    perforated  at   m   by  the   funnel- 

shaped  micropyle;    below  the   latter  lies   the 
spermatozoa  enter  the  ^gg,  though       egg-nucleus  in 'the  peri-vitelline  layer  of  pro- 

only    one   is    concerned    with    the     topiasm ;  M  the  polar  bodies, 
actual  process  of  fertiUzation. 

The  plant-ovum,  w^hich  is  usually  know^n  as  the  oospJicrc  ( Figs.  64, 
107),  shows  the  same  general  features  as  that  of  animals,  being  a 
relatively  large,  quiescent,  rounded  cell  containing  a  large  nucleus. 
It  never,  however,  attains  the  dimensions  or  the  complexity  of  struc- 
ture shown  in  many  animal  eggs,  since  it  always  remains  attached  to 
the  maternal  structures,  by  which  it  is  provided  with  food  and  invested 
wdth  protective  envelopes.  It  is  therefore  naked,  as  a  rule,  and  is 
not  heavily  laden  with  reserve  food-matters  such  as  the  deutoplasm 
of  animal  ova.  A  vitelline  membrane  is,  however,  often  formed  soon 
after  fertilization,  as  in  echinoderms.  The  most  interesting  feature 
of  the  plant-ovum  is  the  fact  that  it  often  contains  phistids  (leuco- 
plasts  or  chromatophores)  which,  by  their  division,  give  rise  to  tho.se 
of  the  embryonic  cells.  These  sometimes  have  the  ionii  ot  typical 
chromatophores  containing  pyrenoids,  as  in  Volvox  and  many  other 
Algse  (Fig.  64).  In  the  higher  forms  (archegoniate  plants),  according 
to  the  researches  of  Schmitz  and  Schimper,  the  ^gg  contains  numer- 
ous minute  colourless  'Meucoplasts,"  which  afterward  develop  into 
green  chromatophores  or  into  the  starch-building  amyloplasts.  This 
is  a  point  of  great  theoretical  interest ;  for  the  researches  of  Schmitz, 
Schimper,  and  others  have  rendered    it   highly  probable  that  these 


134 


THE    GERM-CELLS 


plastids  are  persistent  niorpholo^c^ncal  bodies  that  arise  only  by  the 
division  of  preexisting  bodies  of  the  same  kind,  and  hence  may  be 
traced   continuously   from   one    generation    to    another   through   the 


A  C    -' 

Fig.  64.  —  Germ-cells  of  Volvox.     [OVERTON.] 

A.  Ovum  (oosphere)  containing  a  large  central  nucleus  and  a  peripheral  layer  of  chromato- 
phores  ;  /.  pyrenoid.  B.  Spermatozoid  ;  c.v.  contractile  vacuoles  ;  e.  "  eye-spot  "  (chromoplastid)  ; 
/.  pyrenoid.     C.  Spermatozoid  stained  to  show  the  nucleus  («). 


germ-cells.  In  the  lower  plants  (Algae)  they  may  occur  in  both  germ- 
cells  ;  in  the  higher  forms  they  are  found  in  the  female  alone,  and  in 
such  cases  the  plastids  of  the  embryonic  body  are  of  purely  maternal 
origin. 


B.     The    Spermatozoon 

Although  spermatozoa  were  among  the  first  of  animal  cells  ob- 
served by  the  microscope,  their  real  nature  was  not  determined  for 
more  than  two  hundred  years  after  their  discovery.  Our  modern 
knowledge  of  the  subject  may  be  dated  from  the  year  1841,  when 
Kolliker  proved  that  they  were  not  parasitic  animalcules,  as  the  early 
observers  supposed,  but  the  products  of  cells  preexisting  in  the 
parent  body.  Kolliker,  however,  did  not  identify  them  as  cells,  but 
believed  them  to  be  of  purely  nuclear  origin.  We  owe  to  Schweigger- 
Seidel  and  La  Valette  St.  George  the  ]:)roof,  simultaneously  brought 
forward  by  these  authors  in  1865,^  that  the  spermatozoon  is  a  com- 
plete cell,  consisting  of  nucleus  and  cytoplasm,  and  hence  of  the  same 
morphological  nature  as  the  ovum.  It  is  of  extraordinary  minute- 
ness, being  in  many  cases  less  than   t;ooVoo'  ^^^  bulk  of  the  ovum.^ 

1  Arc/i.  Mik.  Atiat.,  I.  '65. 

2  In  the  sea-urchin,  Toxopiieus/es,  I  estimate  its  bulk  as  being  between  4Tnnro7  ^^^  5 ^ q^q ^ ,) 
the  volume  of  the  ovum.     The  inequality  is  in  many  cases  very  much  greater. 


THE   SPERMATOZOON 


135 


Its  precise  study  is  therefore  difficult,  and  it  is  not  surprisin^^  that  our 
knowledge  of  its  structure  and  origin  is  still  far  from  complete. 


—  Apical  body  or  acrosome. 


Nucleus. 


End-knob. 


Middle-piece. 


Envelope  of  the  tail. 


.Axial  filament. 


I.    Flagellate  Sperjrtatozoa 

In  its  more  usual  form  the  animal  spermatozoon  resembles  a 
minute,  elongated  tadpole,  which  swims  very  actively  about  by  the 
vibrations  of  a  long,  slender  tail  morpho- 
logically comparable  with  a  single  cilium 
or  flagellum.  Such  a  spermatozoon  con- 
sists typically  of  four  parts,  as  shown  in 
Fig.  65  :  — 

1.  The  nucleus,  which  forms  the  main 
portion  of  the  **head,"  and  consists  of  a 
very  dense  and  usually  homogeneous  mass 
of  chromatin  staining  with  great  intensity 
with  the  so-called  "nuclear  dyes"  {e.g. 
haematoxylin  or  the  basic  tar-colours  such 
as  methyl-green).  It  is  surrounded  by  a 
very  thin  cytoplasmic  envelope. 

2.  An  apical  body,  or  acrosome,  lying  at 
the  front  end  of  the  head,  sometimes  very 
minute,  sometimes  almost  as  large  as  the 
nucleus,  and  in  some  cases  terminating  in 
a  sharp  spur  by  means  of  which  the 
spermatozoon  bores  its  way  into  the  ovum. 

3.  The  middle-piece,  or  connecting 
piece,  a  larger  cytoplasmic  body  lying 
behind  the  head  and  giving  attachment  to 
the  tail,  from  which  it  is  not  always  dis- 
tinctly marked  off.  This  body  shows  the 
same  staining-reactions  as  the  acrosome, 
having  an  especial  affinity  for  "  plasma- 
stains "  (acid  fuchsin,  etc.).  At  its  front 
end  it  is  in  some  forms  (mammals)  sepa- 
rated from  the  nucleus  by  a  short  clear 
region,  the  neck.  Like  the  acrosome,  the 
middle-piece  is  in  some  cases  derived  from  an  "  archoplasmic  "  mass, 
representing  an  attraction-sphere  {Lumbricus)  or  a  portion  of  the 
Nebenkern  (insects),  and  it  contains,  or  according  to  some  authors 
actually  arises  from,  the  centrosome  (salamander,  mammals,  insects, 
etc.). 

4.  The  tail,  or  flagellum,  in  part,  at  least,  a  cytoplasmic  product 
developed    in    connection  with   the    centrosome    and    "  archoplasm " 


End-piece. 


Fig.    65.  —  Diagram    of    the 
flagellate  spermatozoon. 


136 


THE    GERM-CELLS 


(attraction-sphere  or  "Nebenkern")  of  the  mother-cell.  Tt  consists 
of  a  fibrillatcd  axial  filauioit  surrounded  by  a  cytoplasmic  envelope, 
and  in  certain  cases  (Amjihibia)  bears  on  one  side  a  fin-like  undulat- 
ing membrane  (Fig.  66).  Toward  the  tip  of  the  flagellum  the  enve- 
lope suddenlv  disappears  or  becomes  very  thin,  leaving  a  short 
cnd-piccc  which  by  some  authors  is  considered  to  consist  of  the  naked 
axial  filament.  The  a.xial  filament  may  be  traced  through  the 
middle-piece  up  to  the  head,  at  the  base  of  which  it  usually  termi- 


n 


f 


Fig.  66.  —  Spermatozoa  of  fishes  and  Amphibia,     [Ballowitz.] 
A.  Sturgeon.     //.   Pike.     ('.  D.   Leuciscus.     E.   Triton   (anterior  part).     F.    Triton  (posterior 
part  of  flagelkim).     G.  Raja  (anterior  part),     a.  apical  body;  e.  end-piece;  /.  flagellum  ;  k.  end- 
knob;  w.  middle-piece ;  //.nucleus;  j.  apical  spur, 

nates  in  a  minute  body,  single  or  double,  known  as  the  cnd-kiwb. 
Recent  research  has  proved  that  the  axial  filament  grows  out  from 
the  spermatid-centrosome,  the  latter  in  some  cases  persisting  as  the 
end-knob  (insects,  mollusks,  mammals),  in  other  cases  apparently 
enlarging  to  form  the  main  body  of  the  middle-piece  (salamander). 
The  tail-envelopes,  on  the  other  hand,  arise  either  from  the  *'archo- 
plasm  "  of  the  Nebenkern  (insects)  together  with  a  small  amount 
of  unmodified  cytoplasm,  or  from  the  latter  alone  (salamander,  rat). 


THE   SPERMATOZOON  I  37 

From  a  physiological  point  of  view  we  may  arrange  the  parts  of 
the  spermatozoon  under  two  categories  as  follows  : 

I.    The  essential  structures  which  play  a  direct  part  in  fertilization. 
These  are :  — 

{a)  The  uucleus,  which  contains  the  chromatin. 

(b)  The  middle-piece,  which  either  contains  a  formed  centrosome 
or  pair  of  centrosomes  (end-knob),  or  is  itself  a  meta- 
morphosed centrosome.  This  is  probably  to  be  regarded 
as  the  fertilizing  element  par  excellence,  since  there  is  reason 
to  believe  that  when  introduced  into  the  Q.gg  it  gives  the 
stimulus  to  division. 

2.    The  accessory  structures,  which  play  no  direct  part  in  fertilization, 
viz. : — 
{a)  The  apex  or  spur,  by  which  the  spermatozoon  attaches  itself 
to  the  ^gg  or  bores  its  way  into  it,  and  which  also  serves 
for  the  attachment  of  the  spermatozoon  to  the  nurse-cells 
or  supporting  cells  of  the  testis. 
{b)   The  tail,  a  locomotor  organ  which  carries   the   nucleus  and 
centrosome,  and,  as  it  were,  deposits  them  in   the  <tgg  at 
the  time  of    fertilization.       There  can  be  little  doubt  that 
the  substance  of  the  flagellum  is  contractile,  and   that  its 
movements  are  of  the  same  nature    as    those  of   ordinary 
cilia.     Ballowitz's    discovery    of   its    fibril lated  structure  is 
therefore  of   great   interest,  as   indicating  its  structural   as 
well    as    physiological    similarity    to    a    muscle-fibre.      The 
outgrowth   of  the    axial   filament    from   the   centrosome   is 
probably  comparable  to  the  formation  of  spindle-fibres  or 
astral  rays,  a  conclusion  of  especial  interest  in  its  relation 
to  Van  Beneden's  theory  of  mitosis  (p.  lOO). 
Tailed  spermatozoa  conforming  more   or   less  nearly  to  the  type 
just  described  are  with  few  exceptions  found  throughout  the  Metazoa 
from  the  coelenterates  up  to  man  ;  but  they  show  a  most  surprising 
diversity  in  form  and  structure  in  different  groups  of  animals,  and 
the  homologies  between  the  different  forms  have  not  yet  been  fully 
determined.     The  simpler  forms,  for  example,  those  of  echinoderms 
and  some  of  the  fishes  (Figs.  66  and  100),  conform  very  nearly  to  the 
foregoing  description.     Fvery  part  of  the  spermatozoon  may.  how- 
ever, vary   more   or   less   widely   from    it  (Figs.  66-6(S).     The    head 
(nucleus)  may  be  spherical,  lance-shaped,  rod-shaped,  spirally  twisted, 
hook-shaped,  hood-shaped,  or  drawn  out  into  a  long  filament ;    and 
it  is  often  divided  into  an  anterior  and  a  posterior  piece  of  different 
staining-capacity,    as    is    the    case    with   many   birds   and   mammals, 
but  it  is  probable  that  the  anterior  of  these  may  represent  the  acro- 
some.     An  interesting  form  of  head  is  described  by  Wheeler  ('97)  in 


138 


THE    GERM-CELLS 


n        U 

C 


// 


K 


Fig.  67.  —  Spermatozoa  of  various,  animals.  [./-/,  Z.,  from  Rallowitz;  J,  K,  from  von 
Brunn.] 

A  (At  the  left).  Beetle  {Copris),  partly  macerated  to  show  structure  of  flagellum  ;  it  con- 
sists of  a  supporting  fibre  {s./.)  and  a  fin-like  envelope  (/)  ;  n.  nucleus;  a.  a.  apical  body  divided 
into  two  parts  (the  posterior  of  these  is  perhaps  a  part  of  the  nucleus).  /A  Insect  {Calathus), 
with  barbed  head  and  fin-membrane.  C.  Bird  (Phyllopucnste).  D.  Bird  { Muscicapa), -.how'mg 
spiral  structure;  nucleus  divided  into  two  parts  («i.  «-)  ;  no  distinct  middle-piece,  E.  Bulfinch  ; 
spiral  membrane  of  head.  F.  Gull  (Larus)  with  spiral  middle-piece  and  apical  knob.  G.  H.  Giant 
spermatozoon  and  ordinary  form  of  Tadorna.  /.  Ordinary  form  of  the  same  stained,  showing 
apex,  nucleus,  middle-piece  and  flagellum.  J.  "  Vermiform  spermatozoon  "  and,  K.  ordinary 
spermatozoon  of  the  snail  Paludina.  L.  Snake  {Coluber),  showing  apical  body  (a),  nucleus, 
greatly  elongated  middle-piece  {in),  and  flagellum  f /). 


THE   SPERMATOZOON-  I -jg 

the  spermatozoon  of  Myzostoma,  where  it  is  a  greatly  elon^^atcd 
fusiform  body,  passing  insensibly  into  the  tail  without  distinct  middle- 
piece  and  containing  a  single  series  of  chromatin-discs.  The  num- 
ber of  these  in  M.  glabrnm  is  24,  which  is  the  somatic  number 
of  chromosomes  in  this  species.  In  j\L  cirrifcruvi  the  number  of 
chromatin-discs  is  more  than  60.  Somewhat  similar  spermatozoa 
occur  in  the  acoelous  Turbellaria.^  The  acrosome  sometimes  appears 
to  be  wanting,  e.g.  in  some  fishes  (Fig.  66).  When  present,  it  is 
sometimes  a  minute  rounded  knob,  sometimes  a  sharp  stylet,  and  in 
some  cases  terminates  in  a  sharp  barbed  spur  by  which  the  sperma- 
tozoon appears  to  penetrate  the  ovum  {Triton).  In  the  mammals  it 
is  sometimes  very  small  (rat),  sometimes  very  large  (guinea-pig),  and 
in  some  forms  is  surrounded  by  a  cytoplasmic  layer  forming  the 
"head-cap"  (Figs.  6'^,  ^6).  It  is  sometimes  divided  into  two  distinct 
parts,  a  longer  posterior  piece  and  a  knob-like  anterior  piece  (insects, 
according  to  Ballowitz). 

The  middle-piece  or  connecting-piece  shows  a  like  diversity  (Figs. 
66-68).  In  many  cases  it  is  sharply  differentiated  from  the  flagellum, 
being  sometimes  nearly  spherical,  sometimes  flattened  like  a  cap 
against  the  nucleus,  and  sometimes  forming  a  short  cylinder  of  the 
same  diameter  as  the  nucleus,  and  hardly  distinguishable  from 
the  latter  until  after  staining  (newt,  earthworm).  In  other  cases  it 
is  very  long  (reptiles,  some  mammals),  and  is  scarcely  distinguishable 
from  the  flagellum.  In  still  others  (birds,  some  mammals)  it  passes 
insensibly  into  the  flagellum,  and  no  sharply  marked  limit  between 
them  can  be  seen.  In  many  of  the  mammals  the  long  connecting- 
piece  is  separated  from  the  head  by  a  narrow  "  neck  "  in  which  the 
end-knobs  lie,  as  described  below. 

Internally,  the  middle-piece  consists  of  an  axial  filament  and  an 
envelope,  both  of  which  are  continuous  with  those  of  the  flagellum. 
In  some  cases  the  envelope  shows  a  distinctly  spiral  structure,  like 
that  of  the  tail-envelope ;  but  this  is  not  always  visible.  The  most 
interesting  part  of  the  middle-piece  is  the  *' end-knob  "  in  which  the 
axial  filament  terminates,  at  the  base  of  the  nucleus.  In  some  cases 
this  appears  to  be  single.  More  commonly  it  consists  of  two  or  more 
minute  bodies  lying  side  by  side  (Fig.  6%,  B,  D). 

The  flagellum  or  tail  is  merely  a  locomotor  organ  which  i)hiys  no 
part  in  fertilization.  It  is,  however,  the  most  complex  part  of  the 
spermatozoon,  and  shows  a  very  great  diversity  in  structure.  Its 
most  characteristic  feature  is  the  axial  filament,  which,  as  Ballowitz 
has  shown,  is  composed  of  a  large  number  of  parallel  fibrillx,  like  a 
muscle-fibre.  This  is  surrounded  by  a  cytoplasmic  envelope,  which 
sometimes   shows  a  striated  or  spiral  structure,  and  in  which,  or  in 

1  Cf.  Wheeler,  p.  7. 


140 


THE    GERM-CELLS 


connection  with  which,  may  be  developed  secondary  or  accessory  fila- 
ments and  other  structures.  At  the  tip  the  axial  filament  may  lose 
its  envelope  and  thus  give  rise  to  the  so-called  "end-piece"  (Retzius). 
In  Trito)i,  for  example  (Fig.  66,  /•  ),  the  envelope  of  the  axial  fila- 
ment ('*  ])rincipal  filament  ")  gives  attachment  to  a  remarkable  fin-like 
- — -  membrane,  having  a  frilled  or 

^  —  ■'■■- 

-/6 


\ 


7 


ft 


undulating  free  margin  along 
which  is  developed  a  "mar- 
ginal filament."  Toward  the 
tip  of  the  tail  the  fin,  and 
finally  the  entire  envelope, 
disapj)ears,  leaving  only  the 
axial  filament  to  form  the  end- 
piece.  After  maceration  the 
envelope  shows  a  conspicuous 
cross-striation,  which  perhaps 
indicates  a  spiral  structure 
such  as  occurs  in  the  mam- 
mals. The  marginal  filament, 
on  the  other  hand,  breaks 
up  into  numerous  parallel 
fibrillar,  while  the  axial  fila- 
ment remains  unaltered  (Bal- 
lowitz). 

A  fin-membrane  has  also 
been  observed  in  some  insects 
and  fishes,  and  has  been  as- 
serted to  occur  in  UKinimals 
(man  included).  Later  ob- 
servers have,  however,  failed 
to  find  the  fin  in  mammals, 
and  their  observations  indi- 
cate that  the  axial  filament  is 
merely  surrounded  by  an 
envelope     which     sometimes 

Fig.  68.  — Spermatozoa  of  mammals.     {A-Fixom     showS      traCCS     of      the      same 
Ballowitz.I  .      ,  , 

,  „  ,      „•  •    N     „  ^,  c  sj3n-al    arrangement    as   that 

A.  Badger  (living).     /?.  The  same  after  staining.  '                 .              ^                 . 

C.'^A\.   {Vesper ago).       Z^.    The  same,  flagellum  and  which     is     SO     COUSpicUOUS     in 

middle-piece  or  connecting-piece,  showing  end-knobs.  4-Uq  COnnectin<^^-Diece        In  the 

E.  Head   of  the    spermatozoon   of  the   bat  {Rhino-  ^i     u                      a\ 

lophus)   showing  details.      F.   Head  of  spermatozoon  SkatC     the     tail     has    tWO    hla- 

of  the  pig.     G.  Opossum  (after  staining).    //.Double  mcntS,      both       COmpOSCd       of 

spermatozoa   from  the  vas  deferens  of  the  opossum.  n    i    /~i     -n                         .      i   i 

/  R^^^  parallel  nbnlloe,  connected  by 

h.c.  head-cap  (acrosome)  ;   >(•.  end-knob ;  w.  mid-     a      membrane       and       Spirally 

dlffe'^rem'par'tsr''"'  ^'"  ^'  ^'  ^'  '""''"''"^  ^^  ^'^^    V.^\^l^^  about  cach  Other;    a 


THE  SPERM  A  TO  ZOOM 


141 


somewhat  similar  structure  occurs  in  the  toad.  In  some  beetles  there 
is  a  fin-membrane  attached  to  a  stiff  axial  "supporting  fibre  "( F"io-. 
6-],  A).  The  membrane  itself  is  here  composed  of  four  parallel  fibres, 
which  differ  entirely  from  the  supporting  fibre  in  staining-capacity 
and  in  the  fact  that  each  of  them  may  be  further  resolved  into  a 
large  number  of  more  elementary  fibrillae. 


Fig.  69. —  Unusual  forms  of  spermatozoa. 

A.  B.  C.  Living  amoeboid  spermatozoa  of  the  crustacean  Polyp  hem  us.     [Z.xcH.VKl 
D.  E.  Spermatozoa  of  crab,  Droniia.    P.  Oi  Et/insa,  G.  oi  Maja.  //.  of  hi.uhus. 
I.  Spermatozoon  of  lobster,  Homarus.      [HERRlCK.] 
J.  Spermatozoon  of  crab,  Porcellana.     [GROBBEN.] 


[(■•UfillBKN."! 


Many  interesting  details  have  necessarily  been  passed  over  in  the  foregoing 
account.  One  of  these  is  the  occurrence,  in  some  mammals,  birds.  Amphibia  (frog). 
and  mollusks,  of  two  kinds  of  spermatozoa  in  the  same  animal.  In  the  birds  and 
Amphibia  the  spermatozoa  are  of  two  sizes,  but  of  the  same  form,  the  larger  i)eing 
known  as  -giant  spermatozoa^'  (Fig.  67,  G,  II).  In  the  gasteropod  Paluiima  the 
two  kinds  differ  entirelv  in  structure,  the  smaller  form  being  of  the  usual  type  and 
not  unlike  those  of  biVds.  while  the  larger,  or  "vermiform,"  spermatozoa  have  a 
worm-like  shape  and  bear  a  tuft  of  cilia  at  one  end,  somewhat  like  the  spermatozoids 
of  plants  (Fig.  67,  J,  K).  In  this  case  only  the  smaller  .spermatozoa  are  functional 
(von  Brunn). 


142 


THE    GERM-CELLS 


No  less  remarkable  is  the  conjugation  of  spermatozoa  in  pairs  (Fig.  68,  //).  which 
takes  place  in  the  I'as  dtferens  in  the  opossum  (Selenka)  and  in  some  insects 
(Hallowitz.  Auerbach).  IJallowitz's  researches  ('95)  on  the  double  spermatozoa 
of  beetles  {Dyiisciiia:)  prove  that  the  union  is  not  primary,  but  is  the  result  of  an 
actual  conjugation  of  previously  separate  spermatozoa.  Not  merely  two,  but  three 
or  more  spermatozoa  may  thus  unite  to  form  a  ••  spermatozeugma,"'  which  swims  like 
a  single  spermatozoon.  Whether  the  spermatozoa  of  such  a  group  separate  before 
fertilization  is  unknown;  but  Hallowitz  has  found  the  groups,  after  copulation,  in 
the  female  receptaculum.  and  he  believes  that  they  may  enter  the  egg  in  this  form. 
The  physiological  meaning  of  the  process  is  unknown. 


2.    OtJicr  Fonus  of  Spermatozoa 

The  principal  deviations  from  the  flagellate  type  of  spermatozoon 
occur  among  the  arthropods  and  nematodes  (Fig.  69).  In  many  of 
these  forms  the  spermatozoa  have  no  flagellum,  and  in  some  cases  they 
are  actively  amoeboid ;  for  example,  in  the  daphnid  PolypJicnuis  ( Fig. 
69,  A,  By  C)  as  described  by  Ley  dig  and  Zacharias.  More  commonly 
they  are  motionless  like  the  ovum.  In  the  chilognathous  myriapods 
the  spermatozoon  has  sometimes  the  form  of  a  bi-convex  lens  {Poly- 
(hs}fii(s),  sometimes  the  form  of  a  hat  or  helmet  having  a  double  brim 
{Jiihis).  In  the  latter  c^se  the  nucleus  is  a  solid  disc  at  the  base  of 
the  hat.     In  many  decapod  Crustacea  the  spermatozoon  consists  of  a 

cylindrical  or  conical  body  from 
one  end  of  which  radiate  a  num- 
ber of  stiff  spine-like  processes. 
The  nucleus  lies  near  the  base. 
In  none  of  these  cases  has  the 
centrosome  been  identified. 


^A.i !!  MiB* 


:r^\M 


'!&• 


/,;.vsy 


\ 


Spermatozoids  of   Chara.      [Bela- 


3.    Paternal   Genn-cells   of 
Plants 

In  most  of  the  flowering 
plants  the  male  germ-cells  are 
represented  by  two  "generative 
nuclei,"  lying  at  the  tip  of  the 
]:)ollen  tube  (Fig.  106).  On  the 
other  hand,  in  the  cycads  (Figs. 
87,  108)  and  in  a  large  number 
of   the   lower   plants   (pterido- 


Fig.  70. 

JEFF.] 

y^.  Mother-cells  with  reticular  nuclei.      /A  Later      phvtCS,     Muscineae,     and   many 

stage,  with  spermatozoids  forming     C  Mature  sper-      ^th^j-s),  the  male  gCrm-CCll  is  a 
matozoid  (the  elongate  nucleus  black).  ''         ^  *^ 

minute  actively  swimming  cell, 
known  as  the  spennatozoiel,  which  is  closely  analogous  to  the  sper- 
matozoon. The  spermatozoids  are  in  general  less  highly  differenti- 
ated than  spermatozoa,  and  often  show  a  distinct  resemblance  to  the 


THE   SPERMATOZOON 


143 


asexual  swarmers  or  zoospores  so  common  in  the  lower  plants  (Fi^^s. 
70,  71).  They  differ  in  two  respects  from  animal  spermatozoa  :  first 
in  possessing  not  one  but  tw^o  or  several  flagella ;  second,  in  the  fact 
that  these  are  attached  as  a  rule  not  to  the  end  of  the  cell,  but  on 
the  side.  In  the  lower  forms  plastids  are  present  in  the  form  of 
chromatophores,  one 
of  which  may  be  dif- 
ferentiated into  a  red 
*'  eye-spot,"  as  in 
Volvox  and  Fiicus 
(Figs.  57,  71,  A),  and 
they  may  even  contain 
contractile    vacuoles  '^^ 

(  Volvox )  ;  but  both 
these  structures  are 
wanting  in  the  higher 
forms.  These  con- 
sist only  of  a  nucleus 
with  a  very  small 
amount  of  cytoplasm, 
and  have  typically  a 
spiral  form.  In  Chai^a, 
where  their  structure 
and  development 
have  recently  been 
carefully  studied  by 
Belajeff,  the  sperma- 
tozoids  have  an  elon- 
gated spiral  form  with 
two  long  flagella  at- 
tached  near  the 
pointed  end,  which  is 
directed  forward  in 
swimming  (Fig.  70). 
The  main  body  of  the 
spermatozoid  is  oc- 
cupied by  a  dense, 
apparently  homoge- 
neous nucleus  sur- 
rounded by  a  very  delicate  layer  of  cytoplasm.  Behind  the  nucleus  lies 
a  granular  mass  of  cytoplasm,  forming  one  end  of  the  cell,  while  in 
front  is  a  slender  cytoplasmic  tip  to  which  the  flagella  are  attached. 
Nearly  similar  spermatozoids  occur  in  the  liverworts  and  mosses.  In 
the  ferns  and  other  pteridophytes  a  somewhat  different  type  occurs 


Fig.  71.— Spermatozoids  of  plants.  [./.  B,  C,  E,  after 
Guignard;    a  F,  after  Strasburgek.] 

A.  Of  an  alga  {Fucus) ;  a  red  chromatophore  at  the  right 
of  the  nucleus.  B.  Liverwort  {Pcliia).  C.  Moss  (>///./<'«//w). 
D.  Mars  ilia.  E.  Fern.  {Augtoptcris).  F.  Fern,  Fhfi^opterts 
(the  nucleus  dark). 


{Cf.  Figs.  87.  88.) 


144  THE    GERM-CELLS 

(Figs.  71,  %?)).  Here  the  spermatozoid  is  twisted  into  a  conical  spiral 
and  bears  numerous  cilia  attached  along  the  upper  turns  of  the  spire. 
The  nucleus  occupies  the  lower  turns,  and  attached  to  them  is  a  large 
spheroidal  cyto])lasmic  mass,  which  is  cast  off  when  the  spermatozoid 
is  set  free  or  at  the  time  it  enters  the  archegonium.  This,  according 
to  Strasburger,  proba])ly  corresi:)onds  to  the  basal  cytoplasmic  mass  of 
Cliara.  The  upper  portion  of  the  spire  to  which  the  ciHa  are  attached 
is  composed  of  cytoplasm  alone,  as  in  CJiara.  Ciliated  spermatozoids, 
nearly  similar  in  tyj^e  to  those  of  the  higher  cryptogams,  have  recently 
been  discovered  in  the  cycads  by  Hirase  {Gingko),  Ikeno  (Cj'cas),  and 
Webber  (2'^?;;//^?).  They  are  here  hemispherical  or  pear-shaped  bodies 
of  relatively  huge  size  (in  Zamia  upward  of  250  /x  in  length),  with  a 
large  nucleus  filling  most  of  the  cell  and  a  spiral  band  of  cilia  making 
from  two  to  six  turns  about  the  smaller  end  (Figs.  8j,  108). 

As  will  be  shown  farther  on  (p.  173),  the  **  anterior  "  cytoplasmic 
region  of  the  spermatozoid,  to  which  the  cilia  are  attached,  is  probably 
the  analogue  of  the  middle-piece  of  the  animal  spermatozoon  ;  and 
the  work  of  Belajeff,  Strasburger,  Ikeno,  Hirase,  Webber,  and  Shaw 
gives  good  ground  for  the  conclusion  that  it  has  an  essentially  simi- 
lar mode  of  origin,  though  w-e  are  still  unable  to  say  exactly  how  far 
the  comparison  can  be  carried.  The  *'  posterior  "  region  of  the  sper- 
matozoid appears  to  correspond,  broadly  speaking,  to  the  acrosome. 

C.     Origin  of  the  Germ-cells 

Both  ova  and  sj^ermatozoa  take  their  origin  from  cells  known  as 
primordial  germ-cells,  which  become  clearly  distinguishable  from  the 
somatic  cells  at  an  early  period  of  development,  and  are  at  ftrst  exactly 
alike  in  the  two  sexes.  W^hat  determines  their  subsequent  sexual 
differentiation  is  unknown  save  in  a  few  special  cases.  From  such 
data  as  we  possess,  there  is  very  strong  reason  to  believe  that,  with 
a  few  exceptions,  the  primordial  germ-cells  are  sexually  indifferent, 
i.e.  neither  male  nor  female,  and  that  their  transformation  into  ova 
or  spermatozoa  is  not  due  to  an  inherent  predisposition,  but  is  a  reac- 
tion to  external  stimulus.  Most  of  the  observations  thus  far  made 
indicate  that  this  stimulus  is  given  by  the  character  of  the  food,  and 
that  the  determination  of  sex  is  therefore  in  the  last  analysis  a  prob- 
lem of  nutrition.  Thus  Mrs.  Treat  ('73)  found  that  if  caterpillars 
were  starved  before  entering  the  chrysalis  state  they  gave  rise  to  a 
preponderance  of  male  imagoes,  w^hile  conversely  those  of  the  same 
brood  that  were  highly  fed  produced  an  excess  of  females.  Yung  ('81) 
reached  the  same  result  in  the  case  of  Amphibia,  highly  fed  tadpoles 
producing  a  great  excess  of  females  (in  some  cases  as  high  as  92%) 
and  underfed  ones  an  excess  of  males.     The  same  result,  again,  is 


ORIGIN  OF   THE    GERM-CEILS 


145 


given  by  the  interesting  experiments  of  Nussbaum  ('97)  on  the  rotifer 
Hydatina,  which  is  an  especially  favourable  case  since  sex  is  here  de- 
termined at  a  very  early  period,  before  tJic  egg  is  laid,  the  eggs  being 
of  two  sizes,  of  which  the  smaller  give  rise  only  to  males,  and  the 
larger  only  to  females.  The  earlier  experiments  of  Maupas  ('91 )  on 
this  form  seemed  to  show  conclusively  that  the  decisive  factor  was 
temperature  acting  on  the  parent  organism,  since  in  a  high  tempera- 
ture an  excess  of  females  produced  male  eggs,  and  in  a  low  tempera- 
ture the  reverse  result  ensued.  Nussbaum  shows,  however,  that  this 
is  not  a  direct  effect  of  temperature,  but  an  indirect  one  due  to  the 
greater  birth-rate  and  the  greater  activity  of  the  animals  under  a 
higher  temperature,  which  result  in  a  speedier  exhaustion  of  food. 
Direct  experiment  shows  that,  under  equal  temperature-conditions, 
well-fed  females  produce  a  preponderance  of  female  offspring,  and 
vice  versa,  precisely  as  in  the  Lepidoptera  and  Amphibia.  These  cases 
show  that  sex  may  be  determined  by  conditions  of  nutrition  either 
affecting  the  embryo  itself  (Lepidoptera,  Amphibia)  long  after  the  ^^^^^ 
is  laid,  or  by  similar  conditions  affecting  the  parent-organism  and 
through  it  the  ovarian  ^'g%. 

Nutrition  is,  however,  not  the  only  determining  cause  of  sex,  as  is 
shown  by  the  long-known  case  of  the  honey-bee.  Here  sex  is  deter- 
mined by  fertilization,  the  males  arising  only  from  unfertilized  eggs 
by  parthenogenesis,  while  the  fertilized  eggs  give  rise  exclusively  to 
females,  which  develop  into  fertile  forms  (queens)  or  sterile  forms 
(workers),  according  to  the  nature  of  the  food.  This  is  a  very  excep- 
tional case,  yet  here  too  it  is  the  more  highly  fed  larvae  that  produce 
fertile  females.  It  is  interesting  to  compare  with  this  case  that  of  the 
plant-lice  or  aphides.  In  these  forms  the  summer  broods,  living 
under  favourable  conditions  of  nutrition,  produce  only  females  the 
eggs  of  which  develop  parthenogenetically.  In  the  autunni,  under 
less  favourable  conditions,  males  as  well  as  females  are  produced  ;  and 
that  this  is  due  to  the  external  conditions  and  not  to  a  fixed  cyclical 
change  of  the  organism  is  proved  by  the  fact  that  in  the  favourable 
environment  of  a  greenhouse  the  production  of  females  alone  may 
continue  for  years. ^ 

We  are  not  yet  able  to  state  whether  there  is  any  one  causal  ele- 
ment common  to  all  known  cases  of  sex-determination.  The  observa- 
tions cited  above,  as  well  as  a  multitude  of  others  that  cannot  here  be 
reviewed,  render  it  certain,  however,  that  sex  as  such  is  not  inherited. 
What  is  inherited  is  the  capacity  to  develop  into  either  male  or 
female,  the  actual  result  being  determined  by  the  combined  effect  of 
conditions  external  to  the  primordial  germ-cell. 

1  See  Geddes,  Sex,  in  Encyclopcedia  Britannua  ;  Geddes  and  Thompson,  T/ie  Evolution 
of  Sex,  1889;  Brooks,  T/ie  Law  of  Heredity,  1 883;  \yatase  ('92),  The  Phenomena  of 
Sex-differentiation. 


146 


THE    GERM-CELLS 


In  the  greater  number  of  cases  the  primordial  germ-cells  arise  in 
a  germinal  epithelium  which,  in  the  cctlenterates  (Fig.  72),  may  be  a 
part  of  either  the  ectoderm  or  entoderm,  and.  in  the  higher  types,  is  a 
modified  region  of  the  peritoneal  epithelium  lining  the  body-cavity. 
In  such  cases  the  primordial  germ-cells  may  be  scarcely  distinguish- 
able at  first  from  the  somatic  cells  of  the  epithelium.  But  in  other 
cases  the  germ-cells  may  be  traced  much  farther  back  in  the  develop- 
ment, and  they  or  their  progenitors  may  sometimes  be  identified  in 
the  gastrula  or  blastula  stage,  or  even  in  the  early  cleavage-stages. 
Thus  in  the  worm  Sagitta,  Hertwig  has  traced  the  germ-cells  back  to 


ec 


^  B 

Fig.  72.  —  Origin  of  the  germ-cells  in  a  hydro-medusa,  Cladonevia.     [WeismanN.] 
A.   Young  stage ;  section  through  wall  of  manubrium  of  the  medusa ;  ova  developing  in  the 
ectoderm    (ec).     B.   Later  stage,   showing   older  ova   {o)   and   "nutritive  cells"  (ri).     The  ova 
contain  small  nuclei  probably  derived  from  engulfed  nutritive  cells. 

two  primordial  germ-cells  lying  at  the  apex  of  the  archenteron.  In 
some  of  the  insects  they  appear  still  earlier  as  the  products  of  a  large 
"  pole-cell "  lying  at  one  end  of  the  segmenting  ovum,  which  divides 
into  two  and  finally  gives  rise  to  two  symmetrical  groups  of  germ- 
cells.  Hacker  has  recently- traced  very  carefully  the  origin  of  the 
primordial  germ-cells  in  Cyclops  from  a  "stem-cell  "  (Fig.  74)  clearly 
distinguishable  from  surrounding  cells  in  the  early  blastula  stage,  not 
only  by  its  size,  but  also  by  its  large  nuclei  rich  in  chromatin,  and  by 
its  peculiar  mode  of  mitosis,  as  described  beyond. 

The  most  beautiful  and  remarkable  known  case  of  early  differenti- 
ation of  the  germ-cells  is  that  of  Ascaris,  where  Boveri  was  able  to 
trace  them  back  continuously  thiough  all  the  cleavage-stages  to  the 


ORIGIN  OF   THE    GERM-CELIS 


147 


two-cell  stage  !  Moreover,  from  the  outset  the  progenitor  of  the  germ- 
cells  differs  from  the  somatic  cells  not  07ily  in  the  greater  size  and  rich- 
ness of  chromatin  of  its  nuclei,  but  also  in  its  mode  of  mitosis;  for  in 
all  those  blastomeres  destined  to  produce  somatic  cells  a  portion  of 


^^g- 73- — Origin  of  the  primordial  germ-cells  and  casting  out  of  chromatin  in  the  somatic 

cells  of  Ascaris.     [BoVERl.] 

A.  Two-cell  stage  dividing;  s.  stem-cell,  from  which  arise  the  germ-cells.  />'.  Tlie  same  from 
the  side,  later  in  the  second  cleavage,  showing  the  two  types  of  mitosis  and  the  casting  out  of 
chromatin  {c)  in  the  somatic  cell,  C.  Resulting  4-cell  stage;  the  eliminated  onromatin  at  c. 
D.  The  third  cleavage,  repeating  the  foregoing  process  in  the  two  upper  cells. 

the  chromatin  is  cast  out  into  the  cytoplasm,  where  it  degenerates,  and 
only  in  the  germ-cells  is  the  sum-total  of  the  chromatin  retained.  In 
Ascaris  megalocephala  univalens  the  process  is  as  follows  (Fig.  yi): 
Each  of  the  first  two  cells  receives  two  elongated  chromosomes.     As 


148 


THE    GERM-CELLS 


the  ovum  prepares  for  the  second   cleavage,  the  two  chromosomes 
reappear  in  each,  but  differ  in  their  behaviour  ( F'ig.  73,  A,  B).     In  one 
of  them,  which  is  destined  to  produce  only  somatic  cells,  the  thickened 
ends  of  each  chromosome  are  cast  off  into  the  cytoplasm  and  degen- 
erate.    Only  the  thinner  central   part  is  retained  and  distributed  to 
the  daughter-cells,  breaking  uj)  meanwhile  into  a  large  number  of 
segments    which    split    lengthwise    in    the    usual    manner.        In    the 
other    cell,   which    may  be  called   the  stem-all  (Fig.   J  I,  s),  all  the 
chromatin  is  preserved  and  the  chromosomes  do   not  segment  into 
smaller  pieces.     The  results  are  plainly  apparent  in  the  four-cell  stage, 
the  two  somatic  nuclei,  which  contain  the  reduced  amount  of  chro- 
matin, being  small  and  pale,  while  those  of  the  two  stem-cells  are  far 
larger  and  richer  in  chromatin  (Fig.  73,  C).     At  the  ensuing  division 
(Fig.   73,  D)  the    numerous    minute    segments  reappear  in   the  two 
somatic  cells,  divide,  and  are  distributed  like  ordinary  chromosomes ; 
and  the  same  is  true  of  all  their  descendants  thenceforward.     The 
other    two    cells   (containing    the    large    nuclei)   exactly    repeat   the 
history  of  the  two-cell  stage,  the  two  long  chromosomes  reappearing 
in  each   of    them,  becoming  segmented    and  casting  off   their  ends 
in  one,  but  remaining  intact  in  the  other,  which  gives  rise  to  two 
cells  with  large  nuclei    as    before.       This  process    is    repeated  five 
times   (Boveri)  or    si.\  (Zur    Strassen),  after    which    the    chromatin- 
elimination  ceases,  and  the    two  stem-cells  or  primordial  germ-cells 
thenceforward    give    rise    only  to    other    germ-cells    and    the    entire 
chromatin  is  preserved.     Through  this  remarkable  process  it  comes 
to    pass    that    in    this    animal    only    tJic    ij^crni-cclls    receive  the  s/n/i- 
total  of  the  cgg-cJiromatin  Jianded  dozen  from  the  parent.     All  of  the 
somatic  cells  contain  only  a  portion   of  the  original  gcrm-substanee. 
"  The  original  nuclear  constitution  of  the  fertilized  (t^^  is  transmitted, 
as  if  by  a  law  of  primogeniture,  only  to  one  daughter-cell,  and  by 
this  again  to  one,  and  so  on  ;  while  in  the  other  daughter-cells  the 
chromatin  in  part  degenerates,  in  ])art  is  transformed,  so  that  all  of 
the  descendants  of  these  side-branches  receive  small  reduced  nuclei."  ^ 

It  would  be  difficult  to  overestimate  the  imi)ortance  of  this  dis- 
covery ;  for  although  it  stands  at  present  an  almost  isolated  case,  yet 
it  gives  us,  as  I  believe,  the  key  to  a  true  theory  of  differentiation 
development,^  and  may  in  the  end  prove  the  means  of  explaining 
many  phenomena  that  are  now  among  the  unsolved  riddles  of  the  cell. 

Hacker  ('95)  has  shown  that  the  nuclear  changes  in  the  stem- 
cells  and  primordial  eggs  of  Cyclops  show  some  analogy  to  those  of 
Ascaris,  though  no  casting  out  of  chromatin  occurs.  The  nuclei  are 
very  large  and  rich  in  chromatin  as  compared  with  the  somatic  cells, 
and  the  number  of  chromosomes,  though  not  precisely  determined, 

1  Boveri, '91,  p.  437-  2  qt  p.  426. 


ORIGIN   OF   THE    GERM-CELLS 


149 


is  less  than  in  the  somatic  cells  (Fig.  74).  Vom  Rath,  workin 
in  the  same  direction,  believes  that  in  the  salamander  also  the 
number  of  chromosomes  in  the  early  progenitors  of  the  germ-cells 
is  one-half  that  characteristic  of  the  somatic  cells. ^  In  both  these 
cases,   the    chromosomes    are    doubtless   bivalent,    representing    two 


Fig.  74.  —  Primordial  germ-cells  in  Cyclops.     [Hackkk.] 

A.  Young  embryo,  showing  stem-cell  {st).  D.  The  stem-cell  lias  ciiviiled  into  two.  giMut; 
rise  to  the  primordial  germ-cell  {g).  C.  Later  stage,  in  section;  tlie  iirimordial  germ-cell  has 
migrated  into  the  interior  and  divided  into  two ;  two  groups  of  chromosomes  in  each. 

chromosomes  joined  together.  In  Ascaris,  in  Hke  manner,  each  of 
the  two  chromosomes  of  the  stem-cell  or  primordial  germ-cells  is 
probably  plurivalent,  and  represents  a  combination  of  several  units 
of  a  lower  order  which  separate  during  the  segmenLution  ol  the 
thread  when  the  somatic  mitosis  occurs. 


1  Cf.  p.  256,  Chapter  V. 


150  THE    GERM-CELLS 

D.     Growth  and  Differentiation  of  the  Germ-cells 

I.    The  Oi'it))i 

{a)  GnnvtJi  and  Xutritiou.  —  Aside  from  the  transformations  of 
the  nucleus,  which  are  considered  elsewhere,  the  story  of  the  ova- 
rian history  of  the  (tg^^  is  largely  a  record  of  the  changes  involved  in 
nutrition  and  the  storage  of  material.  As  the  primordial  germ-cells 
enlarge  to  form  the  mother-cells  of  the  eggs,  they  almost  invariably 
become  intimately  associated  with  neighbouring  cells  which  not  only 
support  and  protect  them,  but  also  serve  as  a  means  for  the  elabora- 
tion of  food  for  the  growing  egg-cell.  One  of  the  simplest  arrange- 
ments is  that  occurring  in  coelenterates,  where  the  ^^^'g  lies  loose 
either  in  one  of  the  general  layers  or  in  a  mass  of  germinal  tissue, 
and  may  crawl  actively  about  among  the  surrounding  cells  like  an 
Avia:ba.  In  such  cases  (hydroids)  the  (t'g'g  may  actually  feed  upon 
the  surrounding  cells,  taking  them  bodily  into  its  substance  or  fusing 
with  them^  and  assimilating  their  substance  with  its  own.  In  such 
cases  (  Tubiilaria,  Hydra)  the  nuclei  of  the  food-cells  long  persist  in 
the  egg-cytoplasm,  forming  the  so-called  ''  pseudo-cells,"  but  finally 
degenerate  and  are  absorbed  by  the  ^ZZ-  ^^  would  here  seem  as 
if  a  struggle  for  existence  took  place  among  the  young  ovarian  cells, 
the  victorious  individuals  persisting  as  the  eggs  ;  and  this  view  is 
probably  applicable  also  to  the  more  usual  case  where  the  ^gg  is 
only  indirectly  nourished  by  its  brethren. 

In  most  cases,  as  ovarian  development  proceeds,  a  definite  associa- 
tion is  established  between  the  Q,gg  and  the  surrounding  cells.  In 
one  of  the  most  frequent  arrangements  the  ovarian  cells  form  a 
regular  layer  ox  follicle  about  the  ovum  (Figs.  59,  79),  and  there  is 
very  strong  reason  to  believe  that  the  follicle-cells  are  immediately 
concerned  with  the  conveyance  of  nutriment  to  the  o\'um.  A  num- 
ber of  observers  have  maintained  that  the  follicle-cells  may  actually 
migrate  into  the  interior  of  the  egg,  and  this  seems  to  be  definitely 
established  in  the  case  of  the  tunicates  and  mollusks  (Fig.  75).^ 
Such  cases  are,  however,  extremely  rare;  and,  as  a  rule,  the  material 
elaborated  by  the  nutritive  cells  is  passed  into  the  o^gg  either  in  solu- 
tion or  in  the  form  of  granular  or  j^rotoplasmic  substance.'"^  An 
interesting  case  of  this  kind  occurs  in  the  cycads,  where,  according 
to  Ikeno  ('9<S),  the  egg-cell  is  connected  with  the  surrounding  cells 
by  broad  protoplasmic  bridges  through  which  cytoplasmic  material 
flows  directly  into  the  egg-cell. 

Verv  curious  and  su^-scestive  conditions  occur  among  the  annelids 
and  insects.     In  the  annelids  the  nutritive  cells  often  do  not  form 

1  Cf.  Doflein,  '97.  «  gee  Floderus,  '95,  and  Obst,  '99.  »  cf.  p.  349. 


GROWTH  AND   DIFFERENTIATION   OF  THE    GERM-CELLS 


151 


a  follicle,  but  in  some  forms  each  ej^^g  is  accompanied  by  a  single 
nurse-cell,  attached  to  its  side,  with  which  it  floats  free  in  the  body- 
cavity.  In  Ophryotrocha,  where  it  has  been  carefully  described  by 
Korschelt,  the  nurse-cell  is  at  first  much  larger  than  the  ^^^'g  itself, 
and  contains  a  large,  irregular  nucleus,  rich  in  chromatin  (Fig.  76). 
The  egg-cell  rapidly  grows,  apparently  at  the  expense  of  the  nurse- 
cell,  which  becomes  reduced  to  a  mere  rudiment  attached  to  one  side 
of  the  ^gg  and  finally  disappears.  There  can  hardly  be  a  doubt, 
as  Korschelt  maintains,  that  the  nurse-cell  is  in  some  manner  con- 
nected with  the  elaboration  of  food  for  the  growing  egg-cell ;  and 
the  intensely  chromatic 
character  of  the  nucleus  is 
well  worthy  of  note  in  this 
connection.  Still  more  in- 
teresting are  the  conditions 
observed  by  Wheeler  ('96, 
'97)  in  Myzostoma^  where 
the  young  ^g^g  is  accom- 
panied by  two  nurse-cells, 
one  at  either  end.  These 
cells  fuse  bodily  with  the 
^Zg,  one  having  "  some- 
thing to  do  in  forming  the 
vacuolated  cytoplasm  at 
the  animal  pole,  .  .  .  the 
other  in  forming  the  granu- 
lar cytoplasm  at  the  vege- 
tative pole"  ('97,  p.  42). 
The  polar  axis  thus  deter- 
mined persists  as  that  of 
the  ripe  ovum.  This 
seems  one  of    the  clearest 

cases  showing  the   establishment  of    the    egg-polarity    through    the 
relation  of  the  ^gg  to  its  environment.^ 

Somewhat  similar  nurse-cells  occur  in  the  insects,  where  they  have 
been  carefully  described  by  Korschelt.  The  eggs  here  lie  in  a  series 
in  the  ovarian  ''egg-tubes"  alternating  with  nutritive  cells  vari- 
ously arranged  in  different  cases.  In  the  butterfly  ]\vicssa,  each 
^gg  is  surrounded  by  a  regular  follicular  layer  of  cells,  a  few  of 
which  at  one  end  are  differentiated  into  nurse-cells.  These  cells 
are  very  large  and  have  huge  amoeboid  nuclei,  rich  in  chromatin 
(Fig.  -J J,  A).  In  the  ear-wig,  Forficula,  the  arrangement  is  still  more 
remarkable,  and  recalls  that  occurring  in  Ophryotrocha.     Here  each 

1  Cf.  p.  386. 


Fig.  75.  —  Ovarian  eggs  of  Helix.     [OUST.] 

A.  Earlier  stage,  surrounded  by  follicle.  />.  Later 
stage,  showing  inward  migration  and  absorption  of  fol- 
licle-cells. 


152 


THE    GERM-CELLS 


^g^  lies  in  the  egg-tube  just  below  a  very  large  nurse-cell,  which, 
when  fully  developed,  has  an  enormous  branching  nucleus  as  shown 
in  Fig.  163.  In  these  two  cases,  again,  the  nurse-cell  is  character- 
ized by  the  extraordinary  development  of  its  nucleus  —  a  fact  which 
points  to  an  intimate  relation  between  the  nucleus  and  the  metabolic 
activity  of  the  cell.^ 

In  all  these  cases  it  is  doubtful  whether  the  nurse-cells  are  sister- 
cells  of  the  egg  which  have  sacrificed  their  own  development  for  the 
sake  of  their  companions,  or  whether  they  have  had  a  distinct  origin 
from  a  verv  early  period.  That  the  former  alternative  is  possible  is 
shown  bv  the  fact  that  such  a  sacrifice  occurs  in  some  animals  after 
the  eggs  have  been  laid.     Thus  in  the  earthworm.  Liimbricns  tcrrcs- 


Fig.  76.  —  Egg  and  nurse-cell  in  the  annelid,  Ophryotrocka,     [KORSCHELT.] 
A.  Young  stage,  the  nurse-cell  («)  larger  than  the  egg  {o).     B.  Growth  of  the  ovum.     C.  Late 
stage,  the  nurse-cell  degenerating. 

tris,  several  eggs  are  laid,  but  only  one  develops  into  an  embryo,  and 
the  latter  devours  the  undeveloped  eggs.  A  similar  process  occurs 
in  the  marine  gasteropods,  where  the  eggs  thus  sacrificed  may 
undergo  certain  stages  of  development  before  their  dissolution. - 

ib)  Diffcnntiation  of  tJic  Cytoplasm  and  Deposit  of  Dcntoplasm.  — 
In  the  very  young  ovum  the  cytoplasm  is  small  in  amount  and  free 
from  deutoplasm.  As  the  egg  enlarges,  the  cytoplasm  increases 
enormously,  a  process  which  involves  both  the  growth  of  the  pro- 
toplasm and  the  formation  of  passive  deutoplasm-bodies  suspended 
in  the  protoplasmic  network.  During  the  growth-period  a  peculiar 
body  known  as  the  yolk-nucIcus  appears  in  the  cytoplasm  of  many 
ova,  and  this  is  probably  concerned  in  some  manner  with  the  growth 

1  See  p.  338.  2  See  McMurrich,  '96. 


GROWTH  AND  DIFFERENTIATION   OF   THE    GERM-CELLS 


153 


of  the  cytoplasm  and  the  formation  of  the  yolk.     Both  its  qxW\\\  and 
its  physiological  role  are,  however,  still  involved  in  doubt. 

The  deutoplasm  first  appears,  while  the  eggs  are  still  very  small, 
in  the  form  of  granules  which  seem  to  have  at  first  no  con.stant  posi- 
tion with  reference  to  the  egg-nucleus,  even  in  the  same  species. 
Thus  Jordan  ('93)  states  that  in  the  newt  {Dicniyctylus)  the  yolk  may 
be  first  formed  at  one  side  of  the  ^g^  and  afterward  spread  to  other 
parts,  or  it  may  appear  in  more  or  less  irregular  separate  patches 
which  finally  form  an  irregular  ring  about  the  nucleus,  which  at  this 
period  has  an  approximately  central   position.      In    some  Amphibia 


Fig-  77-  —  Ovarian  eggs  of  insects.     [Korschklt.J 
A,  Egg  of  the  butterfly,  Vanessa,  surrounded  by  its  follicle;  above,  thiee  nurse-cells  {>i.c.)  wiiii 
branching  nuclei;  g.v.  germinal  vesicle.     B.  Egg  of  water-beetle,  Dyliscus,  living;  the  egg  (o.v.) 
lies  between  two  groups  of  nutritive  cells  ;  the  germinal  vesicle  sends  amoeboid  processes  into  the 
dark  mass  of  food-granules. 


the  deutoplasm  appears  near  the  periphery  and  advances  inward 
toward  the  nucleus.  More  commonly  it  first  appears  in  a  zone 
surrounding  the  nucleus  (Fig.  yS,  C,  D)  and  advances  thence  toward 
the  periphery  (trout,  Henneguy  ;  cephalopods,  Ussow).  In  still  others 
{e.g.  in  myriapods,  Balbiani)  it  appears  in  irregular  jxitches  scattered 
quite  irregularly  through  the  ovum  (Fig.  78,  A).  In  Braiicliipus  the 
yolk  is  laid  down  at  the  centre  of  the  ^'^^g,  while  the  nucleus  lies  at 
the  extreme  periphery  (Brauer).  These  variations  show  in  general 
no  definite  relation  to  the  ultimate  arrangement  —  a  fact  which 
proves  that  the  eccentricity  of  the  nucleus  and  the  polarity  of  the 


154 


THE    GERM-CELLS 


^^g  cannot  be  explained  as  the  result  of  a  simple  mechanical  dis- 
placement of  the  germinal  vesicle  by  the  yolk,  as  some  authors  have 
maintained. 

The   primary  ori^nn    of   the  deutoplasm-grains  is  a  question   that 
involves   the  whole  thcorv  of  cell-action  and   the  relation  of  nucleus 


A 


B 


Fig.  78.  —  Young  ovarian  eggs,  showing  yolk-nuclei  and  deposit  of  deutoplasm. 

A.  Myriapod  {Gcophilits)  with  single  "yolk-nucleus"  (perhaps  an  attraction-sphere)  and  scat- 
tered deutoplasm.     [Bai.BIAM.] 

D.  The  same  with  several  yolk-nuclei,  and  "  attraction-sphere,"  s.     [Balbiam.] 

C.  Fish  {Scorpccna),  with  deutoplasm  forming  a  ring  al)out  the  nucleus,  and  an  irregular  mass 
of  "eliminated  chromatin"  (?  yolk-nucleus).     [\'an  Hammkkk.] 

D.  Ovarian  egg  of  young  duck  (three  months)  surrounded  by  a  follicle,  and  containing  a  "  yolk- 
nucleus,"  j.;/.     [Mertens.] 

and  cytoplasm  in  metabolism.  The  evidence  seems  perfectly  clear 
that  in  many  cases  the  deutoplasm  arises  in  situ  in  the  cytoplasm 
like  the  zymogen-granules  in  gland-cells.  But  there  is  now  also  a 
very  considerable  body  of  evidence  indicating  that  a  part  of  the 
egg-cytoplasm  is  directly  or  indirectly  derived  from  the  nucleus 
through    the    agency    of    the    yolk-nucleus    or    otherwise ;    and    the 


GROWTH  AND  DIFFERENTIATION  OF   THE    GERM-CELLS        155 


subject  can  best  be  considered  after  an  account  of  that  bodv.  It 
may  be  mentioned  here,  however,  that  a  large  number  of  observers 
have  maintained  a  giving  off  of  nuclear  substance  to  the  cytoplasm, 
in  the  form  of  actual  buds  from  the  nucleus  (Blochmann,  Scharff! 
Balbiani,  etc.)  as  separate  chromatin-rods  or  portions  of  the  chromatin 
network  (Fol,  Blochmann,  Van  Bambeke,  Erlanger,  Mertens.  Calkins, 
Nemec,  etc.)  or  as  nucleolar  substance  (Leydig,  Balbiani,  Will,  Lev- 
dig,  Henneguy),  but  nearly  all  of  these  cases  demand  reexamination. 


—  S 


C 


Fig.  79.  —  Young  ovarian  eggs  of  birds  and  mammals.  [Mertens,] 
A.  Egg  of  young  magpie  (eight  days),  surrounded  by  the  follicle  and  containing  germinal 
vesicle  and  "  attraction-sphere."  B.  Primordial  egg  (oogonium)  of  new-born  cat,  dividing.  C.  Kgg 
of  new-born  cat  containing  "  attraction-sphere  "  {s)  and  centrosome.  D.  Of  young  thrusli  sur- 
rounded by  follicle  and  containing  besides  the  nucleus  an  attraction-sphere  and  centrosome  (s). 
and  a  yolk-nucleus  (;'.«.).  E.  Of  young  chick  containing  nucleus,  attraction-sphere,  and  f.itty 
deutoplasm-spheres  (black).  F.  Egg  of  new-born  child,  surrounded  by  follicle  ami  containmg 
nucleus  and  attraction-sphere. 

{c)  Yolk-7i?(c!e?(s.—T\\Q  term  yolk-nucleus  or  vitelline  body  (  Dottcr- 
kern,  corps  vitcllin)  has  been  applied  to  various  bodies  or  masses 
that  appear  in  the  cytoplasm  of  the  growing  ovarian  ^^'^^•^  and  it 
must  be  said  that  the  word  has  at  present  no  well-defined  mean- 
ing. As  originally  described  by  von  Wittich  ('45)  in  the  eggs  of 
spiders,  and  later  by  Balbiani  ('93)  in  those  of  certain  myriapods. 
the  yolk-nucleus   has   the  form   of   a   single   well-defined  spheroidal 


156  THE    GERM-CELLS 

mass  which  appears  at  a  very  early  period  and  persists  throughout 
the  later  ovarian  history.  In  other  forms  there  are  several  so-called 
**  yolk-nuclei,"  sometimes  of  fairly  definite  form  as  described  in  the 
Amphibia  by  Jordan  ('93)  and  in  some  of  the  myriapods  by  Balbiani 
('93).  In  some  forms  the  numerous  "yolk-nuclei"  are  irre,e;ular,  ill- 
defined  granular  masses  scattered  through  the  cytoplasm,  as  described 
by  Stuhlman  {^%6)  in  the  eggs  of  insects.  In  still  others  the  "yolk- 
nucleus"  or  "vitelline  body"  closely  simulates  an  attraction-sphere, 
being  surrounded  by  distinct  astral  radiations  and  enclosing  one  or 
more  central  granules  like  centrosomes  ( 6"r<y>/'////.v,  Balbiani,  '93,  and 
Li)uuliis,  Munson,  '98).  Balbiani  is  thus  led  to  regard  the  \'olk- 
nucleus  in  general  as  being  a  metamorphosed  attraction-sphere. 
Miss  Foot  ('96)  has  brought  forward  evidence  to  show  that  the  polar 
rings,  observed  in  the  eggs  of  certain  leeches  and  earthworms,  are 
also  to  be  regarded  as  "yolk-nuclei"  (Fig.  102).  Henneguy  ('93, 
'96)  finally  compares  the  yolk-nucleus  to  the  macronucleus  of  the 
Infusoria  (!). 

In  the  present  state  of  the  subject  it  is  quite  impossible  to  reconcile 
the  discordant  accounts  that  have  been  given  regarding  the  structure, 
origin,  and  fate  of  the  "yolk-nuclei",  and  from  the  facts  thus  far 
determined  we  can  only  conclude  that  the  various  forms  of  "  yolk- 
nuclei  "  have  little  more  in  common  than  the  name.  It  is,  in  the 
first  place,  doubtful  whether  the  "  yolk-nuclei  "  simulating  an  attrac- 
tion-sphere have  anything  in  common  with  the  other  forms ;  and 
Mertens  ('93),  Munson  ('98),  have  shown  that  the  young  ovarian  ova 
of  various  birds  and  mammals  (including  man)  and  of  Liinuliis 
contain  one  or  more  "yolk-nuclei"  in  addition  to  the  "  attraction- 
sphere "("  vitelline  body"  of  Munson).  In  the  second  place  there 
seem  to  be  two  well-defined  modes  of  origin  of  the  yolk-nucleus.  In 
one  type,  illustrated  by  Jordan's  observations  on  the  newt  ('93),  the 
"  yolk-nuclei  "  arise  separately  /;/  situ  in  the  cytoplasm  without  direct 
relation  to  the  nucleus.  The  same  is  true  of  the  small  peripheral 
"  yolk-nuclei  "  of  Limulus  (Munson).  In  a  second  and  more  frequent 
type  the  "yolk-nucleus"  first  appears  very  near  to  or  in  contact  with 
the  nucleus,  suggesting  that  the  latter  is  directly  concerned  in  its 
formation.  The  latter  is  the  case,  for  example,  in  the  eggs  of  Cyma- 
too^astcr  (Hubbard,  '94)  .S"j7/<,'-;/c?'/'// /as- (Henneguy,  '96),  the  earthworm 
(Calkins,  '95,  Foot,  '96),  PolyzoiiiuDi  and  other  myriapods  (Nemec, 
'97,  Van  Bambeke,  '98),  Limulus  (Munson,  '98),  Cypris  (Woltereck, 
'98),  and  ]\[olguIa  (Crampton,  '99).  In  nearly  all  of  these  forms  the 
yolk-nucleus  first  appears  in  the  form  of  a  cap  closely  applied  to  one 
side  of  the  nucleus  (Figs.  80,  81),  sometimes  so  closely  united  to  the 
latter  that  it  is  difficult  to  trace  a  boundary  between  them.  At  a 
later  period  the  yolk-nucleus  moves  away  from  the  nucleus  and  in 


GROWTH  AND  DIFFERENTIATION  OF   THE    GERM-CELLS         157 

most,  if  not  in  all,  cases  breaks  up  into  smaller  and  smaller  fra^^ments 
which  contribute,  directly  or  indirectly,  to  the  cytoplasmic  f^rowth. 
In  all  these  cases  the  history  of  the  yolk-nucleus  is  such  as  to  indi- 
cate the  participation  of  the  nucleus  in  its  formation.  Calkins  (95) 
endeavours  to  show  that  the  yolk-nucleus  in  Luvibricus  is  directly 
derived  from  the  nucleus  by  a  casting  out  of  a  portion  of  the  chro- 


r^\ 


r. 


p 


o 


^^ 


H 


Fig.  80.— Yo'k-niicleus  in  earthworm,  spider,  and  ascidian.  [././?,  Cai.KINS  ;  C-E,  Va.n 
Bambeke;  F-L  Cka.mpton.] 

A.  Early  ovarian  egg  of  Z,?Yr;«(5r/V//^.  B.  Later  stage;  fragmentation  of  yolk-nucleus.  C.  Ova- 
rian ^gg  of  PholcHS.  D.  Later  stage;  disintegration  of  yolk-nucleus.  E.  Remains  of  the  yolk- 
nucleus  scattered  through  the  cytoplasm.  F.  Early  stage  of  yolk-nucleus  in  Mo/^^uhi.  G-I.  Dis- 
integration of  the  yolk-nucleus  and  enlargement  of  the  products  to  form  deutoplasm-spheres. 


matin-reticulum  —  a  result  agreeing  in  principle  with  earlier  obser- 
vations on  other  eggs  by  Balbiani,  Henneguy,  Leydig,  Will,  and 
other  observers.  This  conclusion  rests  partly  on  the  apparent  direct 
continuity  of  yolk-nucleus  and  chromatin,  partly  on  the  staining- 
reactions.  Thus  when  treated  with  the  Biondi-Khrlich  mixture  (basic 
methyl-green,  acid  red  fuchsin),  the  yolk-nucleus  at  first  stains  green 
like  the  chromatin,  while  the  cytoplasm  is  red,  and  this  is  the  case 


158 


THE    GERM-CELLS 


even   after  the   yolk-nucleus  has   quite   separated   from   the   nuclear 
membrane.     Later,  however,  as  the  yolk-nucleus  breaks  up,  it  changes 
its   staining   power,  and    stains   red    like   the  cytoplasm.     The   later 
observations  of   Miss  Foot  ( '96)  give  ground  to  doubt  the  conclusion 
that    the    yolk-nucleus   is    here    actually   metamorphosed    chromatin, 
for  by  the  combined  action  of  lithium  carmine  and   Lyons  blue  its 
substance  is  sharply  differentiated  from   the  chromatin.     Still  later 
studies  by  Crampton  (99)  on  Molgula  demonstrate  that  in  this  case 
the   volk-nucleus   is   not   directly  derived  from   chromatin,  but   they 
nevertheless  indicate  clearly  the  formation  of  the  yolk-nucleus  by  or 
under  the   immediate   influence  of  the   nucleus  —  a   conclusion   also 
reached  on   less  satisfactory  evidence  by  Hubbard,  Van    Bambeke, 
Woltereck,  and  Nemec.     The  general  morphological  history  of  the 
yolk-nucleus  is  here  closely  similar  to  that  of  Lumbricus  (Fig.  80), 
except  that  no  direct  continuity  between  it  and  the  nuclear  substance 
was  observed.     Stained  with   methyl-green-fuchsin  the  yolk-nucleus 
and  major  part  of  the  nuclear  substance  stain  red,  while  the  scattered 
nuclear  chromatin-granules  and  the  cytoplasm  stain  green.     Millon's 
test,  combined  with  digestion-experiments  and  the  foregoing  staining- 
reactions,  proves  that  the  yolk-nucleus  and  the  red  staining  nuclear 
substance  consist  of   albuminous   substance  and   differ  widely  from 
the   general   cytoplasm,  which   probably  consists   largely  of   nucleo- 
albumins  {cf.  p.  331).     These  reactions  give  strong  ground  for  the 
conclusion  that  the  substance  of  the  yolk-nucleus,  which  progressively 
accumulates  just  outside  the  egg-nucleus,  is  formed  through  the  direct 
activity  of  the  latter,  perhaps  arising  within  the  nucleus  and  passing 
out  into  the  cytoplasm.     It  is  possible,  further,  that 'even  the  scattered 
"  yolk-nuclei "  that  seem  to  be  of  purely  cytoplasmic  origin  may  arise 
in  a  similar  manner,  either,  as  Crampton  suggests,  through  the  early 
formation  and  breaking  up  of  a  single  yolk-nucleus,  or  in  some  less 

obvious  way. 

Interesting  questions  are  suggested  by  those  "  yolk-nuclei,"  such 
as  occur  in  GcopJiilus  and  Limulus,  that  so  closely  simulate  an 
attraction-sphere.  Munson's  observations  show  that  this  body 
("vitelline  body")  first  appears  in  the  very  young  ova  as  a  crescent 
applied  to  the  nucleus  precisely  as  in  Molo;iila  or  Lumbricus,  but 
containing  one  or  more  central  granules  (Fig.  81).  In  later  stages 
it  becomes  spherical,  moves  away  from  the  nucleus,  and  assumes  the 
form  of  a  typical  radial  attraction-sphere  with  concentric  microsome- 
circles  and  astral  rays.  It  is  hardly  possible  to  doubt  that  this  body 
in  Limulus  is  of  the  same  general  nature  as  the  yolk-nucleus  of 
Lumbricus,  Molgula,  Cypris,  Cymatogaster,  or  PJiolcus ;  and  if  it  be 
a  true  attraction-sphere  in  the  one  case  we  must  probably  so  regard 
it  in   all.     This  identification  is,  however,  by   no    means    complete; 


GROWTH  AND  DIFFERENTIATION  OF   THE    GERM-CELLS 


159 


and  even  Munson's  careful  studies  do  not  seem  definitely  to  establish 
its  connection  with  the  attraction-sphere  or  centrosomc  of  the  last 
oogonium-division.  That  a  body  simulating  an  attraction-sj)here  and 
containing  a  central  granule  may  arise  dc  ?iovo  in  the  cytoplasm 
is  shown  by  Lenhossek's  observations  on  the  spermatids  of  the 
rat  (p.  170);  and  the  central  granule  is  in  this  case  certainly  not 
a  centrosome,  since  the  true  centrosomcs  are  found  in  another 
part  of  the  cell.  It  is  quite  possible  that  the  "vitelline  body"  of 
Liviiihis  may  have  a  similar  origin.  Nemec  ('97)  finds  in  Polyzoniuvi 
in  the  earliest  stages  a  single  body  applied  to  the  nucleus  and 
later  two  bodies,  one  of  which  enlarges  to  form  a  cap-shaped  yolk- 


D 


Fig.  81.  —  Forms  of  yolk-nuclei  in  Limidus  and  Polyzonium.  [./-C,  MrNSON;  D-F,  Np:mkc.] 
A.  Very  young  ovarian  eggs  oi  Llmulus  ;  at  the  left  "vitelline  body"  (t-)  in  the  form  of  a  cap 
on  the  nucleus;  at  the  right  older  egg  showing  astral  formation.  B.  Older  stage  of  the  same; 
"vitelline  body"  in  the  form  of  an  attraction-sphere  with  central  granule.  C.  Peripheral  "yolk- 
nuclei"  {y.n.)  in  Liviulus.  D.  Very  early  ovarian  egg  of  a  myriapod.  Polyzonium,  with  yolk- 
nucleus.  E.  O  der  egg  with  yolk-nucleus  and  astral  body  ^a).  F.  Still  later  stage,  beginning 
disintegration  of  the  yolk-nucleus. 


nucleus  Hke  those  described  above,  while  the  other  assumes  the 
structure  of  a  radiating  attraction-sphere  containing  a  central 
granule  (centrosome  .''),  and  his  observations  suggest  that  the  two 
bodies  in  question  may  have  a  common  origin  (Fig.  81).  In  none 
of  these  cases  do  the  astral  radiations,  surrounding  this  body,  seem 
to  have  any  connection  with  cell-division,  and  it  is  probable  that 
a  careful  comparison  of  their  physiological  significance  here,  in 
leucocytes,  and  in  mitotic  division,  may  give  us  a  better  under*^tand- 
ing  of  the  general  significance  of  astral  formations  in  protoplasm. 

The  fate  and   physiological  significance    of    the    yolk-nucleus  arc 
still  to  a  considerable  extent   involved  in  doubt.     In  many  cases  i1 


l6o  THE    GERM-CELLS 

breaks  up  into  smaller  and  smaller  ^i^raniiles  {Lunibricus,  JlToIgida, 
Pliolciis,  some  myriapods,  Antcdou),  which  scatter  throu^^h  the  cyto- 
plasm and  are  believed  by  many  observers  ( Halbiani,  Mertens,  \Vill, 
Calkins,  Crampton,  Nemec),  following  the  earlier  views  of  Allen 
Thomson,  to  become  directly  converted  into  deutoplasm-spheres 
(Fig.  80).  Other  observers  (Van  Hambeke,  Foot,  Stiihlman,  and 
others)  adopt  the  original  view  of  Siebold,  that  the  fragments  of 
the  yolk-nucleus  are  absorbed  or  converted  into  protoplasmic 
elements  and  thus  only  indirectly  contribute  to  the  yolk.  In  still 
other  cases  {^c.g.  the  "vitelline  body"  of  Lii)iiilus)  the  yolk-nucleus 
does  not  fragment,  but  seems  to  serve  as  a  centre  about  which  new 
deutoplasmic  material  is  formed.  A  review  of  the  general  subject 
shows  that  we  are  justified  only  in  the  somewhat  vague  conclusion 
that  the  yolk-nucleus  is  probably  involved  in  some  manner  in  the 
general  cytoplasmic  growth  ;  and  that  the  facts  strongly  suggest, 
though  they  hardly  yet  prove,  that  at  least  some  forms  of  yolk-nuclei 
are  products  of  the  nuclear  activity  and  form  a  connecting  link 
between  that  activity  and  the  constructive  processes  of  the  cyto- 
plasm. That  the  yolk-nuclei  have  no  very  definite  morphological 
value,  and  that  they  are  not  necessary  to  growth,  seems  to  be  shown 
by  Henneguy's  observation,  that  in  the  eggs  of  vertebrates  it  is  in 
some  forms  invariably  present,  in  others  only  rarely,  and  in  still 
others  is  quite  wanting  ('96,  p.  162).  If  this  be  the  case,  we  must 
conclude  that  the  yolk-nucleus  consists  of  material  that  contributes  to 
the  constructive  process,  but  is  not  necessarily  localized  in  a  definite 
body.  As  to  its  exact  role  we  are,  as  Henneguy  has  said,  reduced 
to  mere  hypotheses.^  The  facts  indicate  that  this  material  is  a  prod- 
uct of  the  nuclear  activity,  and  that  it  may  in  some  cases  contribute 
directly  to  formed  elements  of  the  cytoplasm.  It  is  probable,  how- 
ever, that  beyond  this  the  yolk-nucleus  may  su])ply  materials,  perhaps 
ferments,  that  play  a  more  subtle  part  in  the  constructive  process, 
and  of  whose  physiological  significance  we  are  quite  ignorant.  The 
whole  subject  seems  a  most  interesting  and  important  one  for  further 
study  of  the  actions  of  the  cell  in  constructive  metabolism,  and  it  is  to 
be  hoped  that  further  research  will  place  the  facts  in  a  clearer  light. 


2.    Origin  of  tJic  Spermatozoon 

(a)  General.  —  The  relation  of  the  various  parts  of  the  sperma- 
tozoon to  the  structures  of  the  spermatid  is  one  of  the  most 
interesting  questions  in  cytology,  since  it  is  here  that  we  must 
look  for  a  basis  of  interpretation  of  the  part  played  by  the  sperma- 

1  '96,  p.  170. 


GROWTH  AND  DIFFERENTIATION  OF   THE    GERM-CELLS        i6l 

tozoon  in  fertilization.  Obviously  the  most  important  of  the 
questions,  thus  suggested,  is  the  source  of  the  sperm-nucleus  and 
centrosome,  though  the  relation  of  the  other  parts  to  the  spermatid- 
cytoplasm  involves  some  interesting  problems. 

Owing  to  the  extreme  minuteness  of  the  spermatozoon,  the 
changes  involved  in  the  differentiation  of  its  various  parts  have 
always  been,  and  in  some  respects  still  remain,  among  the  most 
vexed  of  cytological  questions.  The  earlier  observations  of  Kolliker, 
Schweigger-Seidel,  and  La  Valette  St.  George,  already  mentioned, 
established  the  fact  that  the  spermatozoon  is  a  cell;  but  it  required 
a  long  series  of  subsequent  researches  by  many  observers,  foremost 
among  them  La  Valette  St.  George  himself,  to  make  known  the 
general  course  of  spermatogenesis.  This  is,  briefly,  as  follows : 
From  the  primordial  germ-cells  arise  cells  known  as  spennatogonia} 
which  at  a  certain  period  pause  in  their  divisions  and  undergo  a  con- 
siderable growth.  Each  spermatogonium  is  thus  converted  into  a 
spermatocyte,  which  by  two  rapidly  succeeding  divisions  gives  rise  to 
four  spermatozoa,  as  follows.^  The  primary  spermatocyte  first 
divides  to  form  two  daughter-cells  known  as  spermatocytes  of  the 
second  order  or  sperm-mother-cells.  Each  of  these  divides  again  — 
as  a  rule,  without  pausing,  and  without  the  reconstruction  of  the 
daughter-nuclei — to  form  two  speimatids  or  sperm-cells.  Each  of 
the  four  spermatids  is  then  directly  transformed  into  a  single  sperma- 
tozoon, its  nucleus  becoming  very  small  and  compact,  its  cytoplasm 
giving  rise  to  the  tail  and  to  certain  other  structures.  The  number 
of  chromosomes  entering  into  the  nucleus  of  each  spermatid  and 
spermatozoon  is  always  one-half  that  characteristic  of  the  tissue-cells, 
and  this  reduction  in  number  is  in  most,  if  not  in  all,  cases  effected 
during  the  two  divisions  of  the  primary  spermatocyte.  The  reduction 
of  the  chromosomes,  which  is  the  most  interesting  and  significant 
feature  of  the  process,  will  be  considered  in  the  following  chapter, 
and  we  are  here  only  concerned  with  the  transformation  of  the  sper- 
matid into  the  spermatozoon. 

All  observers  are  now  agreed  that  the  nucleus  of  the  spermatid  is 
directly  transformed  into  that  of  the  spermatozoon,  the  chromatin 
becoming  extremely  compact  and  losing,  as  a  rule,  all  trace  of  its 
reticular  structure.  It  is  further  certain  that  in  some  cases  at  least 
the  spermatid-centrosome  passes  into,  or  gives  rise  to,  a  part  of  the 
middle-piece,  and  that  from  it  the  axial  filament  grows  out  into  the 
tail.  The  remaining  structures  arise,  as  a  rule,  from  the  cytoplasm, 
and  both  the  acrosome  and  the  envelope  of  the  axial  filament  otten 
show  a  direct  relation  to  the  remains  of  the  achromatic  figure  ( "  ar- 

1  The  terminology,  now  almost  universally  adopted,  is  due  to  La  \'alette  St.  George.  Cj. 
Fig.  1 1 8.  2  See  Fig.  119. 

M 


1 62 


THE    GERM-CELLS 


choplasm  "  or  "  kinoplasm  ")  which  is  found  in  the  spermatid  in  the 
form  of  a  sphere  (sometimes  an  attraction-sphere)  or  *' Nebenkern  " 
or  both.  Apart  from  the  nuclear  history,  these  facts  have  been 
definitely  determined  in  only  a  few  cases,  and  much  confusion  still 
exists  in  the  accounts  of  different  observers.  Thus  a  number  of 
investigators  (r.^.  Platner,  I'^ield,  l^enda,  Julin,  Prenant,  Xiessinor) 
have  asserted  that  the  centrosome  passes  into  the  acrosome,  instead  of 


M 


N 


Fig.  82.  —  Formation  of  the  spermatozoon  in  an  insect,  Anasa.  [Paulmier.] 
A.  Telophase  of  secondary  spermatocyte-division,  showing  extra  chromosome  (small  dyad  of 
Fig.  127)  below.  B.  Reconstitution  of  the  nuclei.  C.  Spermatid  witli  Nebenkern  (A^)  and 
acrosome  ia).  D.  Nebenkern  double,  with  centrosome  between  the  two  halves.  E.  F.  G.  Elon- 
gation of  the  spermatid,  outgrowth  of  axial  filament,  migration  of  acrosome,  H.  Giant  spermatid 
(double  size)  with  two  centrosomes  and  axial  filaments.  /.  Giant  spermatid  (quadruple  size) 
with  four  centrosomes  and  axial  filaments. 


the  middle-piece  —  a  result  which  .stands  in  contradiction  with  the  fact 
that  durinc;  fertilization  in  a  lar^^^e  number  of  accurately  known  cases 
the  centrosome  arises  from  or  in  immediate  relation  to  the  middle- 
piece  (Amphibia,  echinoderms,  tunicates,  annelids,  mollusks,  insects, 
etc.;  see  p.  212).  The  clearest  and  most  positive  evidence  on  this 
question,  afforded  by  recent  observations  on  the  spermatogenesis  of 
insects,  annelids,  mollusks.  Amphibia,  and  mammals,  leaves,  however, 
little  doubt  that  the  former  result  was  an  error  and  that,  as  the  facts 


GROWTH  AND   DIFFERENTIATION   OF  THE   GERM-CELLS         163 

of  fertilization  would  lead  us  to  expect,  the  centrosome  of  the  sper- 
matid passes  into  the  middle-piece. 

Accounts  vary  considerably  regarding  the  origin  of  the  acrosome, 
which  according  to  most  authors  is  of  cytoplasmic  origin,  while  a  few 
describe  it  as  arising  inside  or  from  the  anterior  part  of  the  nucleus. 

(/7)  Composition  of  the  Spermatid. --1:\\q  confusion  that  has  arisen 
in  this  difficult  subject  is  owing  to  the  fact  that  the  spermatid  may 
contain,  besides  the  nucleus  and  centrosome,  no  less  than  three  addi- 
tional bodies,  which  were  endlessly  confused  in  the  earlier  studies 
on  the  subject.  These  are  the  Nebejikern}  the  attraction-sphere  or 
idiozome  (Meves),  and  the  c/uvmatoid  lYebenkorper  {WQwd^). 

The  Nebenkern  (Fig.  82),  first  described  by  Biitschli  C/ijin  the 
spermatids  of  butterflies,  was  afterward  shown  by  La  Valette  ('86), 
Platner  (^S6,  '89),  and  many  later  investigators  to  arise  wholly  or  in 
part  from  the  remains  of  the  spindle  of  the  second  spermatocyte 
division.  Its  origin  is  thus  related  to  that  of  an  attraction-sphere 
(which  it  often  closely  simulates),  since  the  latter  likewise  arises 
from  the  achromatic  figure.  To  the  remains  of  the  spindle,  however, 
may  be  added  granular  elements,  probably  forming  reserve-material 
C'centro-deutoplasm  of  Erlanger),  that  are  scattered  through  the  cyto- 
plasm or  aggregated  about  the  equator  of  the  spindle  (Fig.  126). 
Thus  the  Nebenkern  may  have  a  double  origin,  though  its  basis  is 
formed  by  the  spindle-remains.  The  Nebenkern  sometimes  takes  a 
definite  part  in  the  formation  of  the  tail-envelopes  and  of  the  acro- 
some (insects),  but  in  many  cases  it  seems  to  be  wholly  wanting. - 
The  idiozome  is  in  some  cases  an  undoubted  attraction-sphere  derived 
from  the  aster  of  the  last  division  and  at  first  containing  the  centro- 
some, e.g.  in  the  earthworm  as  shown  by  Calkins  ('95)  and  Er- 
langer ('96,  4),  in  the  salamander  and  guinea-pig,  Meves  ('96,  '99), 
and  in  Helix  according  to  Korff  ('99),  though  in  later  stages  the 
centrosomes  usually  pass  out  of  the  body  of  the  idiozome.  In  some 
cases,  however  (in  the  rat,  according  to  Lenhossek,  '99),  the  idiozome 
seems  to  arise  independently  through  condensation  of  the  cytoplasmic 
substance  into  a  sphere  having  no  relation  to  the  centrosomes.  In 
some  cases  the  idiozomes  of  adjoining  cells  remain  for  a  time  con- 
nected by  bridges  of  material  (Fig.  7)  representing  the  remains  of 
the  spindle,  and  hence  corresponding  to  a  Nebenkern  {e.g.  salaman- 
der, Meves,  '96),  and  the  distinction  between  Nebenkern  and  idio- 
zome here  fades  away.  The  idiozome  is  usually  concerned  in  the 
formation  of  the  acrosome  (Amphibia,  mammals),  but  sometimes  seems 

1  The  English  equivalent  of  this  should  be  paranucleus,  but  the  latter  word  has  already 
been  used  in  various  other  senses,  and  it  seems  preferable  to  retain  Piiilschli's  original  (ier- 
man  word. 

^  For  critical  discussion,  see  Erlanger,  '97,  I. 


164 


THE    GERM-CELLS 


to  degenerate  without  contributing  directly  to  the  sperm-formation 
{Helix).  The  chromatoid  Nebenkorper,  finally,  is  a  small  rounded 
body,  staining  with  plasma-stains,  which  appear  always  to  degenerate 
without  taking  direct  part  in  the  formation  of  the  spermatozoon.  It 
is  possibly  an  extruded  nucleolus  ( Lenhossek ).  but  its  origin  and 
meaning  are  not  definitely  known. 

(r)    Traiisforniation  of  the  Spermatid  into  tJie  Spennatozobii.  —  In 
the   works  of  earlier  authors    it   is    often    impossible    to  distinguish 


Pig,  3^.  —  Formation  of  the  spermatozoon  from  the  spermatid  in  the  salamander.  [HER- 
MANN.] 

A.  Young  spermatid,  showing  the  nucleus  above,  and  below  the  colourless  sphere,  the  ring, 
and  the  chromatic  sphere.  D.  Later  stage,  showing  the  chromatic  sphere  and  ring  at  the  base 
of  the  nucleus.  C.  D.  E.  F.  Later  stages,  showing  the  transformation  of  the  chromatic  sphere  into 
the  middle-piece  (?«). 

which  of  the  various  achromatic  elements  mentioned  above  have  been 
under  observation.  We  may  therefore  confine  ourselves  mainly  to 
the  latest  works,  in  which  these  distinctions  are  clearly  recognized. 
Owing  to  their  great  size,  the  spermatozoa  of  Amphibia  have  been 
the  subject  of  most  careful  study;  yet  a  clearer  view  of  the  subject 


GROWTH  AND  DIFFERENTIATION  OF   THE    GERM-CELLS         165 

may,  perhaps,  be  obtained  by  taking  the  spermatogenesis  of  annelids 
and  insects  as  a  basis  of  comparison.  In  the  insects  (butterflies), 
Biitschli  showed,  in  1871,  that  the  tail  is  formed  by  an  elon^Mtion 
of  the  cell-body,  into  which  extends  the  elongated  Nebenkern,  now 
divided  into  two  longitudinal  halves  (Fig.  82).  Platner  ('89),  confirm- 
ing this  observation,  further  showed  that  the  Nebenkern  (in  Pyi^icra) 
consisted  of  two  parts,  stating  that  one  ("large  mitosome")  gives  rise 
to  the  investment  of  the  axial  filament,  the  other  ("small  mitosome  ") 
to  the  middle-piece ;  while  a  third  still  smaller  body,  described  as  a 
"  centrosome,"  passes  to  the  apex.  The  later  works  of  HenkingCgi) 
and  Wilcox  ('95,  '96)  render  it  nearly  certain  that  Platner  confu.sed 
the  acrosome  with  the  centrosome,  the  first-named  observer  finding  in 
Pyrrhocoris  and  the  second  in  Calopteniis  that  Platner's  "centrosome" 
is  derived  from  the  Nebenkern,  while  Wilcox  traced  the  centrosome 
directly  into  the  middle-piece.  Paulmier,  finally,  has  shown  in  Auasa 
that  the  axial  filament  grows  out  from  the  centrosome,^  proving  that 
such  is  the  case  by  the  highly  interesting  observation  that  in  giant 
spermatozoa,  arising  by  the  non-division  of  the  primary  or  secondary 
spermatocytes,  either  two  or  four  centrosomes  are  present,  each  of 
which  gives  rise  to  a  single  axial  filament,  though  only  one  Nebenkern 
is  present  (Fig.  82).  (The  bearing  of  this  important  fact  on  the 
centrosome-question  is  indicated  elsewhere.)  These  observations, 
made  on  three  widely  different  orders  of  insects,  seem  to  leave  no 
doubt  that  in  insects  the  centrosome  lies  in  the  middle-piece  {i.e.  at 
the  base  of  the  nucleus),  while  both  the  acrosome  and  the  inner  tail- 
envelopes  are  derived  from  the  Nebenkern.  The  outer  envelope  of 
the  tail  is  derived  from  unmodified  cytoplasm. 

In  the  earthworm  the  phenomena  are  slightly  different,  the  middle- 
piece  arising  from  an  idiozome  or  attraction-sphere  (Calkins,  95),  in 
which  lies  the  centrosome  (Erlanger,  '96),  while  the  Nebenkern  seems 
to  have  no  part  in  the  formation  of  either  acrosome  or  tail-envelopes.- 

We  turn  now  to  the  Amphibia,  elasmobranchs,  and  mammals,  in 
which  the  same  general  result  has  been  attained,  though  there  is  still 
some  divergence  of  opinion  regarding  the  exact  history  of  the  centro- 
some. Working  on  the  basis  laid  by  Flemming  ('87,  ^'^^X  Hermann 
('89)  traced  the  middle-piece  in  the  salamander  to  a  "  Nebenkorper," 
which  he  believed  to  be  not  a  Nebenkern  but  an  attraction-sphere. 

1  Moore  ('95)  seems  to  have  been  the  first  actually  to  describe  the  outgrowth  of  the  axial 
filament  from  the  centrosome,  in  the  elasmobranchs.  It  has  since  been  described  by  Meves 
('97,  2)  and  Hermann  ('97)  in  the  salamander,  by  Lenhossek  ('97),  Meves  ('98,  '99),  and 
Bardeleben  ('97)  in  the  rat,  guinea-pig,  and  man;  by  Godlewski  ('97)  and  Korff  ('99)  in 
Helix,  and  by  several  others. 

2  Calkins's  preparations,  which  I  have  carefully  examined,  seem  to  leave  no  doubt  that  the 
middle-piece  arises  from  a  true  attraction-sphere  derived  from  the  spindle-poles;  but 
Erlanger  believes  that  the  granular  "  centrodeutoplasm  "  also  contributes  to  the  sphere. 


1 66 


THE    GERM-CELLS 


consisting  of  three  parts,  lying  side  by  side  in  the  cytoplasm  (Fig.  83). 
These  are  {a)2i  colourless  sphere,  shown  by  Meves's  later  researches  to 
be  probably  an  attraction-sphere  ;  (/^)  a  minute,  intensely  staining  cor- 
puscle, and  ((•)  a  small,  deeply  staining  ring.  The  concurrent  results 
of  Hermann  ('89,  '92,  '97),  IkMula  ('93),  and  Meves  ('96,  '97,  2)  have 
shown  that  the  small  corpuscle  (r)  is  one  of  the  ccutrosoDies  of  tlie 
spcnnatiti,  and  all  these  observers  agree  that  it  passes  into  or  gives 


Fig.  84.  —  Formation  of  the  spermatozoon  in  Amphibia.  \^A-E.  Salaviandra,  MEVES; 
F-K.  Aiiiphiiinta,  McGkkijok,] 

A.  Spermatid  with  peripheral  pair  of  centrosomes  lying  outside  the  sphere,  and  axial  filament. 
B.  Centrosomes  near  the  nucleus,  outer  one  ring-shaped.  C.  Inner  centrosome  inside  the 
nucleus,  enlarging  to  form  middle-piece.  D.  Portion  of  much  older  spermatid,  showing  divergence 
of  two  halves  of  the  ring  (/-).  E.  Portion  of  mature  spermatozoon,  showing  upper  half  of  ring  at 
r,  and  the  axial  filament  proceeding  from  it. 

F.  Spermatid  of  ^;;///////wa,  showing  sphere-bridges  and  ring-shaped  mid-bodies.  G.  Later 
stage;  outer  centrosome  ring-shaped,  inner  one  double;  sphere  {s)  converted  into  the  acrosome. 
//.  Migration  of  the  centrosomes.  /.  Middle-piece  at  base  of  nucleus,  y.  The  inner  centrosome 
forms  the  end-knob  within  the  middle-piece,  which  is  now  inside  the  nucleus.  K.  Enlargement  of 
middle-piece,  end-knob  within  it;  elongation  of  the  ring. 


GROWTH  AND  DIFFERENTIATION  OF   THE    GERM-CELLS         i^y 

rise  to  the  middle-piece.  According  to  Meves,  who  has  most  thor- 
oughly studied  the  entire  formation  of  the  spermatozoon,  the  history 
of  these  parts  is  as  follows :  In  the  young  spermatids  the  two  centro- 
somes  lie  quite  at  the  periphery  of  the  cell  (Fig.  84),^  and  from  the 
outer  one  growls  out  the  axial  filament.  The  two  ccntrosomes,  leav- 
ing the  idiozome  by  which  they  are  first  surrounded,  now  pass 
inwards  toward  the  nucleus,  the  outer  one  meanwhile  becoming  trans- 
formed into  the  ring  mentioned  above,  while  the  axial  filament  passes 
throusfh  it  to  become  attached  to  the  inner  centrosome.  The  latter 
pushes  into  the  base  of  the  nucleus  and  enlarges  enormously  to  form 
a  cylindrical  body  constituting  the  main  body  of  the  middle-piece. 
The  ring  meanwhile  divides  into  two  parts,  the  anterior  of  which 
gives  rise  to  a  small,  deeply  staining  body  at  the  posterior  end  of  the 
middle-piece  identical  with  the  "end-knob."  The  other  half  of  the 
ring  w^anders  out  along  the  tail,  finally  lying  at  the  limit  between 
the  main  part  of  the  latter  and  the  end-piece.  The  envelope  of  the 
axial  filament,  here  confined  to  that  side  opposite  the  marginal  fin 
(i.e.  the  **  ventral  "  side  of  Czermak),  is  formed  by  an  outgrowth  of  the 
general  cytoplasm  along  the  axial  filament.  The  fin  and  marginal 
filament  are  beheved  by  Meves,  as  I  understand  him,  to  be  formed 
from  the  axial  filament  ('97,  2,  p.  127).'^  The  acrosome,  finally,  is 
formed  from  the  idiozome  which  wanders  around  the  nucleus  to  its 
anterior  pole.  McGregor's  results  on  Ampliiinmi  (99)  agree  in  their 
broader  features  with  those  of  Meves,  but  differ  on  two  points,  one  of 
which  is  of  great  importance.  The  acrosome  here  arises  from  only 
a  part  of  the  sphere  (idiozome),  while  a  second  smaller  part  passes  to 
the  base  of  the  nucleus  and  forms  the  main  part  of  the  middle-piece. 
The  inner  centrosome  passes  into  the  middle-piece  to  persist  as  the  end- 
knob  from  which  the  axial  filament  passes  out  into  the  tail  (Fig.  84). 
The  history  of  the  sphere  thus  recalls  the  phenomena  seen  in  the  Xe- 
benkern  of  the  insect-spermatid  ;  though  the  posterior  moiety  does  not 
contribute  to  the  tail-envelope,  while  the  history  of  the  inner  centro.s<Mnc 
is  somewhat  like  that  observed  in  the  mammals,  as  described  beycaui. 
In  the  elasmobranchs  Moore ('95),  Hermann  ('98),  Suzuki  ("98).  and 
Benda  ('98)  likewise  traced  the  spermatid-centrosome  into  the  middle- 
piece  (Fig.  85),  and  Moore  first  showed  that  from  it  the  axial  filament 
grows  out. 3     Moore  derived  both  middle-piece  and  acrosome  ixom  the 

1  C/;  their  position  in  epithelial  cells,  p.  =57. 

-  Hermann  ('97)  gives  a  somewhat  different  account  of  the  process,  believing-  that  the 
ring  is  derived  from  the  mid-body  of  the  last  mitosis.  Meves  and  McCIregor  have,  however, 
shown  that  the  ring  and  mid-body  coexist  in  the  early  spermatids  (Fig.  84),  which  seems 
decisive  against  Hermann's  conclusion. 

3  Hermann  finds  also  the  ring  ol)served  in  the  salamander,  and  believes  it  t..  be  the  mid- 
body.  The  middle-piece  is  regarded  by  him  as  a  product  of  the  spimlle-remams,  but  on 
both  these  points  he  is  contradicted  by  Suzuki. 


1 68 


THE    GERM-CELLS 


"archoplasm"  of  the  spermatid.  Suzuki's  studies  clearly  show,  how- 
ever, that  the  entire  axial  filament  of  the  long  middle-piece  arises  by 
the  elongation  of  the  inner  centrosome,  while  the  outer  centrosome, 
from  which  the  axial  filament  of  the  tail  grows  out,  lies  at  the  pos- 
terior limit  of  the  middle-piece  (Fig.  85).  A  nearly  similar  result  is 
reached  by  Korff  ('99)  in  the  case  of  llclix.  It  was  shown  by  God- 
lewski  ( '97)  that  in  this  form  the  axial  filament  likewise  grows  out 


Fig.  85.  —  Formation  of  the  spermatozoon  in  elasmobranchs.  [.-/-C  SuzUKi;  D,  MoORE; 
and  in  Helix,  E-G,  KoRFF.] 

A-lJ.  Outgrowth  of  axial  filament  from  peripheral  centrosome  (r^),  which  persists  at  the 
posterior  limit  of  the  middle-piece  or  connecting-piece  (w).  Elongation  of  inner  centrosome  (<:'-) 
to  form  the  axial  filament  of  the  latter.  E-G  show  similar  phenomena  in  Helix,  with  casting  off 
of  the  sphere  (j). 

a.  Acrosome;  c'^.  peripheral,  and  c"^.  inner  centrosome;  /.  flagellum  ;  k.  end-knob,  derived 
from  inner  centrosome. 


from  the  centrosome.  Korff's  later  studies  show  that  here,  exactly 
as  in  the  elasmobranch,  the  axial  filament  grows  out  from  the  periph- 
eral centrosome  and  is  afterward  transformed  into  a  ring  (Fig.  85). 
The  inner  centrosome  elongates  to  form  a  rod,  which  afterward 
becomes  a  long  filament  traversing  the  elongated  middle-piece  and 
terminating  in  front  in  an  end-knob  at  the  base  of  the  nucleus,  while 
the  ring  lies  at  its  posterior  limit.  The  idiozome  (a  true  attraction- 
sphere)  degenerates  without  taking  part  in  the  formation  of  an  aero- 


GROWTH  AND  DIFFERENTIATION  OF   THE    GERM-CELLS         169 


some.     The  envelope  of  the  middle-piece  is  here  formed  out  of  the 
general  cytoplasm. 

In  the  mammals  the  recent  work  of  Lenhossck  on  the  rat  ('98)  and 
Meves  on  the  rat,  guinea-pig,  and  man  ('98,  '99)  gives  a  result  agree- 
ing in  its  broader  features  with  the  forms  already  considered.  In  all 
these  mammals  the  young  spermatids  are  closely  similar  to  those  of 
the  salamander,  containing  two  peripherally  placed  centrosomes,  from 
the  outer  one  of  which  the  axial  filament  grows  out  (Fig.  86).      Mcves 


Fig.  86.  — Formation  of  the  spermatozoon  in  mammals.     [Meves.] 
A.  Spermatid  of  man,  showing  centrosomes  and  axial  filament.     /A  Spermatid  of  guinea-pig, 

with   acrosome.       C.    Nearly   mature   spermatozoon,  showing  backward  migration  of  the  ring. 

D.  Mature  spermatozoon;  r.  final  position  of  the  ring. 

a.  Acrosome   surrounded   by  cytoplasm  of  the  cell-body,  most  of  which  is  after\vard  thrown 

off;    c.   centrosomes;     c.p.    connecting-piece;   /    flagellum ;     k.    neck,   containing    end-knobs; 

s.  remains  of  the   sphere  (idiozome). 

and  Lenhossek  differ  somewhat  in  their  accounts  of  the  later  history 
of  these  centrosomes,  though  agreeing  that  both  contribute  to  the 
formation  of  the  middle-piece.  Lenhossek  states  that  in  the  rat  both 
centrosomes  persist  at  the  base  of  the  nucleus  to  form  the  end-knob, 
which,  as  Jensen  showed  {"^jX  is  double  in  this  animal.  Meves  finds 
the  process  to  be  more  complicated,  agreeing  in  the  main  with  that 
observed  by  him  in  the  salamander.  In  man  and  the  rat  the  inner 
centrosome  passes  to  the  base  of  the  nucleus  and  flattens  against  it 
to  form  a  small  disc-shaped  body.     The  posterior  centrosome  divides 


I/O  THE    GERM-CELLS 

into  two  parts,  of  which  the  anterior  gives  rise  to  the  end-knob,  while 
the  posterior  is  transformed  into  a  rin;^,  which  wanders  back  to  its 
final  position  at  the  posterior  end  of  the  so-called  "connecting-piece." 
From  this  it  follows  that  the  latter  body  ( VerbindimgsstUck)  does  not 
correspond  to  the  middle-piece  of  the  salamander  (here  represented 
by  the  small  disc-shaped  body  at  the  base  of  the  nucleus),  but  belongs 
to  the  flagellum  ]:)roper.  The  origin  of  the  axial  filament  and  end- 
knob  is,  however,  nearly  the  same  in  the  two  cases.  In  the  guinea- 
pig  the  process  is  somewhat  more  complicated  and  is  not  quite  cleared 
up  bv  Meves ;  but  the  origin  and  fate  of  the  ring  is  the  same,  and  the 
end-knob  passes  into  the  neck  of  the  spermatozoon  as  in  the  rat. 
Taken  together,  these  observations  conclusively  show  that  in  mam- 
mals and  Amphibia  the  end-knob  is  a  derivative  of  the  centrosome, 
thus  sustaining,  though  with  some  modifications,  Hermann's  earlier 
conjecture  ('92)  as  to  the  nature  of  this  body;  and  they  overturn 
Niessing's  result  ('96)  that  the  centrosome  passes  into  the  acrosome. 
As  in  the  salamander,  the  acrosome  is  formed  from  an  idiozome 
derived  in  the  guinea-pig  from  the  remains  of  the  attraction-sphere 
(Meves),  while  in  the  rat,  according  to  Lenhossek,  it  is  independently 
formed  in  the  cytoplasm  without  relation  to  the  preceding  mitotic 
figure  or  the  centrosomes.  Within  the  sphere  appears  a  small,  deeply 
staining  body,  resembling  a  centrosome,  yet  staining  differently  from 
the  true  centrosome,  which  enlarges  to  form  the  acrosome,  while 
about  it  is  formed  a  clear  substance  forming  the  "  head-cap  "  (p.  139). 
In  the  rat  the  acrosome  remains  small  ("  Spitzcnknopfchen  "  of  Mer- 
kel;;  in  the  guinea-pig  it  becomes  nearly  as  large  as  the  nucleus 
itself  (Fig.  86).  An  interesting  feature  in  the  formation  of  the 
mammalian  spermatozoon  is  the  casting  off  of  a  portion  of  the 
spermatid-cytoplasm  in  the  form  of  a  ''cytoplasmic  vesicle"  or  **  tail- 
vesicle,"  which  degenerates  without  further  use  (Fig.  ?)G).  This  pro- 
cess, described  by  Meves  ('99)  in  the  guinea-j^ig,  is  closely  similar  to 
that  which  occurs  in  the  spermatozoid-formation  in  ferns  (p.  144). 

Resume.  In  reviewing  the  foregoing  facts  we  find,  despite  many 
variations  in  detail,  three  points  of  fundamental  agreement,  namely  : 
( I  )the  origin  of  the  sperm-nucleus  from  that  of  the  spermatid  ;  (2)  the 
origin  of  a  part  at  least  of  the  "middle-piece"  from  the  spermatid- 
centrosomes;  and  (3)  the  outgrowth  of  the  axial  filament  from  one  of 
the  spermatid-centrosomes.  It  is  clear,  however,  that  the  term  middle- 
piece  has  been  applied  to  structures  of  quite  different  morphological 
nature,  which  agree  only  in  lying  behind  the  nucleus.  Thus  in  the 
salamander  the  inner  centrosome  gives  rise  to  the  main  body  of  the 
middle-piece  ;  in  the  rat  or  in  man  it  gives  rise  only  to  the  small  disc- 
shaped body  lying  in  the  "neck"  in  front  of  the  so-called  middle- 


GROWTH  AND  DIFFERENTIA TIOX  OF   THE    GERM-CEILS         \-\ 

piece  ;  while  in  Helix  or  the  elasmobranch  it  is  transformed  into  a 
long  filament  traversing  a  cytoplasmic  "middle-piece"  which  forms 
a  considerable  part  of  the  flagellum.  The  term  viiddlc-piccc  has  thus 
become  highly  ambiguous  and  should  only  be  employed,  if  at  all,  as 
a  convenient  descriptive  term  which  has  no  definite  morpholo'^ncal 
meaning. 

A  very  striking  fact  in  the  origin  of  the  spermatozoon  is  the  promi- 
nent part  played  by  the  "  archoplasm,"  i.e.  substance  in  the  form 
of  idiozome  or  Nebenkern  derived  from  the  mitotic  figure,  l^oth  the 
source  and  the  fate  of  this  material  seem,  however,  to  vary  in  differ- 
ent cases,  the  acrosome  now  arising  from  the  Nebenkern  (insects), 
now  from  the  idiozome  (salamander),  the  envelope  of  the  flagellum 
being  formed  in  some  cases  from  the  Nebenkern  (insects;,  in  others 
from  unmodified  cytoplasm  (salamander,  snail),  while  the  idiozome 
may  form  the  acrosome  (salamander,  mammal)  or  degenerate  without 
apparent  use  (snail).  We  find  here,  I  think,  additional  reason  for 
regarding  "  archoplasm"  not  as  a  distinct  and  permanent  form  of 
protoplasm,  but  only  as  a  phase  in  the  general  metabolic  transfor- 
mation of  the  cell-substance,  which  may  or  may  not  persist  and  jdUiv  a 
definite  morphological  role  in  the  cell  according  to  circumstances. 
The  close  relation  of  this  substance  to  the  motor  phenomena  of  the 
cell  cannot,  however,  be  overlooked.^ 

The  outgrowth  of  the  axial  filament  from  the  centrosome  is  a  highly 
interesting  fact,  whether  we  compare  it  with  the  analogous  phenomena 
in  plants  (p.  172)  or  with  the  facts  observed  in  ordinary  ciliated  cells. 
In  the  latter  case  (Fig.  17),  as  has  long  been  known,  each  cilium  is 
attached  to  a  small,  highly  refracting  body  known  as  the  "  basal 
knob"  lying  near  the  cell-periphery.  These  bodies  stain  intensely 
in  iron  hsematoxyhn,  and  it  has  been  recently  suggested  by  Henneguy 
('98)  and  Lenhossek  ('98)  that  they  are  of  the  same  nature  as  centro- 
somes.  The  truth  of  this  surmise  must  be  tested  by  further  study ; 
but  it  seems  highly  probable  that  they  are  at  least  analogous  to  the 
spermatid-centrosome.  Ishikawa  ('99)  has  clearly  shown  that  in  the 
formation  of  the  swarm-spores  of  Noctiluea  the  flagellum  grows  out 
from  that  end  of  the  cell  at  which  the  centrosome  lies,  its  substance 
apparently  arising  from  the  central  spindle,  while  the  centrosome  lies 
at  its  base.  A  very  interesting  fact  discovered  by  Moore  ("95)  in 
elasmobranchs,  and  confirmed  by  Meves  ('97,  5)  and  Henneguy  (98) 
in  the  insects,  is  a  more  or  less  abortive  attempt  to  form  a  fiagellum 
by  the  spermatocytes,  i.e.  one  or  two  generations  before  the  sper- 
matozoon. In  the  insects  (Fig.  166)  Henneguy  has  found  the  cilia 
actually  attached  to  the  centrosomes  of  the  mitotic  figure,  thus  remov- 
ing every  doubt  as  to  their  nature. - 

^  0^  323.  ^  Cf-  Paulmier  on  giant  spermatozoa,  p.  165. 


172  THE    GERM-CELLS 

It  is  an  important  question  whether  the  axial  filament  actually 
arises  from  the  substance  of  the  centrosome  or  is  formed  by  differ- 
entiation from  the  cytoplasmic  substance,  after  the  fashion  of  an 
astral  ray  or  spindle-fibre.  Meves  ('97,  p.  117)  accepts  the  latter 
alternative  ;  but  the  observations  of  Korff  on  Ifclix  and  of  Suzuki 
on  elasmobranchs  seem  to  show  clearly  that,  in  these  cases  at  least, 
the  inner  centrosome  elon<;ates  bodily  to  form  an  extremely  long  fila- 
ment traversing  the  greater  part  of  the  flagellum,  and  apparently  of 
the  same  nature  as  the  true  axial  filament  developed  from  the  outer 
or  distal  centrosome.  This  seems  to  establish  a  probability  in  favour 
of  the  first  of  the  above  alternatives,  and  to  show  that  contractile 
elements  may  be  directly  derived  from  the  centrosome-substance. 
If  this  be  true,  this  substance  is  itself  nearly  related  with  "  archo- 
plasm  "  ;  and  the  origin  of  a  centrosome  dc  7iovo  may  be  brought 
under  the  same  category  with  the  formation  of  archoplasm.^ 

3.    Fonnation  of  tJic  Speiuiatozoids  in  Phuits 

While  the  origin  of  the  spermatozoids  has  not  yet  been  as  fully 
investigated  as  that  of  the  spermatozoa,  recent  researches  have  given 
good  ground  for  the  conclusion  that  essentially  similar  phenomena 
are  involved  in  the  two  cases.  All  recent  observ^ers  are  acfreed  that 
the  nucleus  of  the  spermatozoid  is  directly  derived  from  that  of  the 
spermatid,  while  the  cytoplasm  of  the  latter  gives  rise  to  the  cilia  and 
to  certain  other  structures.  The  ])rincipal  interest  of  the  subject  now 
lies  in  the  origin  of  the  cilia  and  their  relation  to  the  **  archoplasmic  " 
or  "  kinoplasmic  "  structures  of  the  mother-cell.  Belajeff  ('92,  '94) 
found  that  in  Cliara  the  cilia  grow  forth  from  a  small,  highly  refract- 
ing body,  taking  an  intense  plasma-stain,  that  lies  in  the  cytoplasma 
beside  the  nucleus.  He  afterward  found  the  same  body  "  which 
reminds  one  of  a  centrosome  "  in  the  developing  spermatozoids  of 
ferns  and  Equisetace^e  (Fig.  ^^),  where  it  grows  out  into  a  band, 
lying  in  the  anterior  part  of  the  spermatozoid,  from  which  the  cilia 
grow  forth.  Comparing  these  results  with  those  of  Hermann,  Bela- 
jeff  concluded  ''that  the  deeply  staining  corpuscle"  {i.e.  the  cen- 
trosome) "in  the  spermatids  of  the  salamander  and  the  mouse 
corresponds  completely  ta  the  deeply  staining  corpuscle  in  the  sper- 
matogenic  cells  of  the  Characere,  ferns,  and  Kquisetacece " ;  that, 
furthermore,  "  the  middle-piece  of  the  spermatozoon  represents  the 
band  which  bears  the  cilia  of  the  plant  spermatozoid,  while  the  tail- 
like   flagella^   of    the    salamander    or   mouse   represents  the  cilia." '^ 

1  QC  p.  321.     For  the  function  of  the  centrosome  in  fertilization,  see  p.  208. 

2  In  the  original  "  Faden  "  perhaps  meant  to  designate  the  axial  filament. 

*  '97.  3- 


GROWTH  AND  DIFFERENTIATION-  OF   THE    GERM-CELLS 


1/3 


This  tallies  with  Strasburger's  earlier  conclusion  that  the  cilia-bearing 
region  consists  of  "  kinoplasm  "  and  corresponds  to  the  middle-piece 
('92,  p.  139),  but  gives  a  still  more  definite  basis  of  comparison. ^ 

The  history  of  the  centrosomc-like  hod:\^?.  {hlcpharoplasts  of  Web- 
ber, '97,  3)  has  been  carefully  followed  out  in  Zcwiia  and  Giugko  by 
Webber  ('97),  and  in  Cycas  by  Ikeno  ('97,  '98)  with  nearly  similar 
results.     In  all  these  forms  (Fig.  Zj)  the  blepharoplasts  appear  in  the 


''"^^^^^mi 


.#. 


'rj!iir^- 


r'' 


Fig.    87.  —  Formation   of  the   spermatozoids   in   the   cycads.      \^A,   GlNUKO;    B-D,    /.itm.i 
Webber;  E-I,  Cycas,  iKENO.] 

A.  Developing  pollen-tube,  showing  stalk-cell  {s),  vegetative  cell  {v)  and  generative  cell  (^). 
the  latter  with  two  blepharoplasts.  B.  Generative  cell,  somewhat  later,  with  blepharoplasts  and 
asters.  C.  The  same  in  the  prophases  of  division,  showing  breaking  up  of  blepliaroplasts. 
D.  The  two  spermatids  formed  by  division  of  the  generative  cell ;  blepharoplasts  fragmented ; 
from  these  fragments  arises  the  cilia-bearing  band.  E.  Blepharoplast  of  Cycas,  at  a  stage  some- 
what later  than  Fig.  C;  cilia  developing.  F.  Later  stage;  ciliated  band  (derived  from  tlie  last 
stage)  attached  to  a  prolongation  from  the  nucleus.  G.  Cilia-bearing  band  continuous.  //.  Nearly 
ripe  spermatozoid  with  nucleus  in  the  centre;  ciliated  band,  sliown  in  section,  forming  a  spiral. 
/.    Slightly  later  stage,  viewed  from  above,  showing  the  spiral  course  of  the  band  (cilia  omitted). 


penultimate  cell-generation  lying  one  on  cither  side  the  nucleus,  and 
in  earlier  stages  surrounded  by  a.stral  radiations  very  closely  resem- 
bUng  those  of  a  typical  mitotic  aster,  and  they  lie  opposite  the  poles 

1  The  "anterior"  region  of  the  spermatozoid  thus  corresponds  to  the  "posterior"  region 
of  the  spermatozoon,  the  confusion  of  terms  having  arisen  from  the  fact  that  the  former 
svi^ims  with  the  ciUa-bearing  region  in  front,  the  latter  with  the  flagellum  directed  backward. 


1/4 


THE    GERM-CELLS 


of    the  ensuing  division-spindle.     They    seem,   however,  to   have  no 
part  in  the  formation  of  the  mitotic  fic^ure  or  in   division,  and  both 


Fig    88. -Formation   of  the   spermatozoids  in  the   vascular   crvptogams.    MarsiUa    (.-1    D 
R^'w^ttf-^'"'^''       •  '^'  ^'  ^"•''^■'>'    (^y»u>o^ram,„e   (//-A",  BeLAJF.FF),  and  Equisetum    (L-N, 

.-/.  Primary  spermatogonium  (two  generations  before  the  primary  spermatocytes)  in  diyision 
showing  centrosomes.  B.  Primary  spermatocyte  with  pair  of  "  blepharoplastoids  "  (centrosomes).' 
C.  i^pindle  of  primary  spermatocyte  (f^rst  maturation-division).  D.  Four  of  the  eight  secondary 
spermatocytes  w,th  blepharoplast.  Z^-^  Prophase  of  second  maturation-division.  H  Pair  of 
spermatids  (Gymno^^ram,„e)  with  blepharoplasts.  /-J.  Formation  of  the  ciliated  bandVrom  the 
Dlepnaroplast  A.  Nearly  ripe  spermatozoid,  showing  ciliated  band  {b),  nucleus,  and  "cyto- 
plasmic vesicle  (the  latter  is  ultimately  cast  off).  E  M.  Spermatids  of  Equisetum.  N  Ripe 
spermatozoid  from  above,  showing  spiral  ciliated  band.  O.  Ripe  spermatozoid  of  MarsiUa  with 
very  long  spiral  ciliated  band.  ".   wm 


STAINING-REACriONS    OF   THE    GERM-NUCLEI  175 

Webber  and  Ikeno  have  produced  apparently  stron^r  evidence  ^  that 
they  arise  separately  and  de  novo  in  the  cytoplasm.  After  the  ensu- 
ing division  (by  which  the  two  spermatids  are  formed)  the  astral  rays 
disappear,  and  the  blepharoplast  gives  rise  by  a  peculiar  process  to  a 
long,  spiral,  deeply  staining  band,  from  which  the  ciha  grow  forth. 
The  later  studies  of  Shaw  ('98,  i)  and  Helajeff  ('99)  on  the  blepharo- 
plasts  in  Onoclea  and  Maisilia  leave  no  doubt  that  these  bodies  are 
to  be  identified  with  centrosomes.  In  Marsilia  Shaw  first  found  the 
blepharoplasts  lying  at  the  poles  of  the  spindle  during  the  anaphase 
of  the  first  maturation-division  and  very  closely  resembling  centro- 
somes. Each  blepharoplast,  at  first  single,  divides  into  two  during  the 
late  telophase,  and  during  the  prophases  of  the  second  division  the 
halves  diverge  to  opposite  poles  of  the  nucleus  and  lie  at  the  respec- 
tive spindle-poles.  This  account  is  confirmed  by  Belajeff,  who  shows 
further  that  during  the  prophases  astral  rays  surround  the  blepharo- 
plasts, and  a  central  spindle  is  formed  between  them  (Fig.  88). 
Belajeff  also  finds  centrosomes  in  all  of  the  earher  spermatogenic 
divisions.  The  blepharoplasts  are  thus  proved  to  be,  in  one  case  at 
least,  dividing  organs  w^hich  in  every  way  correspond  to  the  centro- 
somes of  the  animal  spermatocytes;  and  the  justice  of  Belajeff's 
comparison  is  demonstrated.  Shaw  believed  that  the  primary  blepha- 
roplast, which  by  division  gives  rise  to  those  of  the  two  spermatids, 
arose  de  novo.  He  made,  however,  the  significant  observation  that 
in  Marsilia  *' blepharoplastoids,"  exactly  like  the  blepharoplasts,  ap- 
pear at  the  spindle-poles  of  the  preceding  (antepenultimate)  division, 
and  that  each  of  these  divides  into  two  in  the  late  telophase.  These 
are  said  to  disappear,  without  relation  to  the  blepharoplasts  which  at 
a  slightly  later  period  are  found  at  the  spindle-poles  of  the  first  matu- 
ration division  ;  but  in  view  of  the  demonstrated  continuitv  of  the 
blepharoplasts  during  the  second  division  we  may  well  hesitate  to 
accept  this  result,  as  well  as  Webber's  conclusion  regarding  the 
independent  and  separate  origin  of  the  blepharoplasts  in  Zauiia.  In 
any  case  the  facts  give  the  strongest  ground  for  the  conclusion  that 
the  formation  of  the  spermatozoids  agrees  in  its  essential  features 
with  that  of  the  spermatozoa,  and  for  the  expectation  that  the  history 
of  the  achromatic  structures  in  fertilization  will  yet  be  found  to  shnw 
an  essential  agreement  in  plants  and  animals. 

E.     Staining-reactions  of  the  Germ-nuclei 

It  was  pointed  out  by  Ryder  in  1883  that  in  the  oyster  the  germ- 
nuclei  stain  differently  in   the  two  sexes;  for  if  the  hermaphrodite 

1  Dr.  Webber  has  kindly  given  me  an  opportunity  to  look  through  his  beautiful  prepa- 
rations. 


176  THE    GERM-CELLS 

gland  of  this  animal  be  treated  with  a  mixture  of  saffranin  and  methyl- 
green,  the  egg-nuclei  are  coloured  red,  the  s]:)erm-nuclei  bluish  green. 
A  similar  difference  was  afterward  observed  by  Auerbach  ('91)  in 
the  case  of  many  vertebrate  germ-cells,  where  the  egg-nucleus  was 
shown  to  have  a  special  affinity  for  various  red  and  yellow  dyes 
(eosin,  fuchsin,  aurantia,  carmine),  while  the  sperm-nuclei  were  esj)e- 
cially  stained  with  blue  and  green  dyes  (methyl-green,  aniline-blue, 
hematoxylin).  He  was  thus  led  to  regard  the  chromatin  of  the  ^'^^^^ 
as  especially  "  erythrophilous,"  and  that  of  the  s])erm  as  "  cyanophi- 
lous."  That  the  distinction  as  regards  colour  is  of  no  value  has  been 
shown  bv  Zacharias,  Heidenhain,  and  others ;  for  staining-agents  can- 
not be  logically  classed  according  to  colour,  but  according  to  their 
chemical  composition  ;  and  a  red  dye,  such  as  saffranin,  may  in  a 
given  cell  show  the  same  affinity  for  the  chromatin  as  a  green  or  blue 
dye  of  different  chemical  nature,  such  as  methyl-green  or  hoema- 
toxylin.  Thus  Field  has  shown  that  the  sperm-nucleus  of  Astcrias 
may  be  stained  green  (methyl-green),  blue  (haematoxylin,  gentian 
violet),  red  (saffranin),  or  yellow  (iodine),  and  it  is  here  a  manifest 
absurdity  to  speak  of  '' cyanophilous  "  chromatin  ((/.  p.  335).  It  is 
certainly  a  very  interesting  fact  that  a  difference  of  staining-reaction 
exists  between  the  two  sexes,  as  indicating  a  corresponding  difference 
of  chemical  composition  in  the  chromatin ;  but  even  this  has  been 
shown  to  be  of  a  transitory  character,  for  the  staining-reactions  of  the 
germ-nuclei  vary  at  different  periods  and  are  exactly  alike  at  the  time 
of  their  union  in  fertilization.  Thus  Hermann  has  shown  that  when 
the  spermatids  and  immature  spermatozoa  of  the  salamander  are 
treated  with  saffranin  (red)  and  gentian  violet  (blue),^  the  chromatic 
network  is  stained  blue,  the  nucleoli  and  the  middle-piece  red  ;  while 
in  the  mature  spermatozoon  the  reverse  effect  is  produced,  the  nuclei 
being  clear  red,  the  middle-piece  blue.  A  similar  change  of  staining- 
capacity  occurs  in  the  mammals.  The  great  changes  in  the  staining- 
capacity  of  the  egg-nucleus  at  different  periods  of  its  history  are  de- 
scribed at  pages  338-340.  Again,  Watase  has  observed  in  the  newt 
that  the  germ-nuclei,  which  stain  differently  throughout  the  whole 
period  of  their  maturation,  and  even  during  the  earlier  phases  of 
fertilization,  become  more  and  more  alike  in  the  later  phases,  and  at 
the  time  of  their  union  show  identical  staining-reactions. ^  A  very 
similar  series  of  facts  has  been  observed  in  the  germ-nuclei  of  plants 
by  Strasburger  (p.  220).  These  and  many  other  facts  of  like  import 
demonstrate  that  the  chemical  differences  between  the  germ-nuclei 
are  not  of  a  fundamental  but  only  of  a  secondary  character.  They 
are  doubtless  connected  with  the  very  different  character  of  the  meta- 
bolic processes  that  occur  in  the  history  of  the  two  germ-cells ;  and 

1  By  Flemming's  triple  method.  ^  '92,  p.  492. 


LITERATURE  1 77 

the  difference  of  the  staining-reaction  is  probably  due  to  the  fact 
that  the  sperm-chromatin  contains  a  higher  percentage  of  nucleinic 
acid,  while  the  egg-chromatin  is  a  nuclein  containing  a  much  higher 
percentage  of  albumin. 


LITERATURE.     IIP 

Ballowitz,  E.  —  Untersuchungen    tiller  die    Struktur  der  Spermatozoen:    i.  {birds) 

Arch.  )nik.  Aiiat.,  XXXII.     1888;  2.  {insects)  Zeitschr.  iviss.  ZooL,  L.     1890: 

3.   {fishes,  amphibia.,  reptiles)  Arch.  niik.  Anat.,  XXXVI.      1890:   4.   {tnani- 

vtals)  Zeit.  wiss.  Zool..  LII.      1891. 
Belajeff,    W.  —  Uber    die    Centrosomen    in    den    spermatogenen    Zellen :    Ber.    d. 

deuisch.  bot.  Ges.,  XVII.,  6.     1899. 
Boveri,  Th.  —  Uber  Differenzierung  der  Zellkerne  wahrend  der  Furchung  des  Eies 

von  Ascaris  meg.:  Anat.  Anz.     1887. 
Id.  —  Die  Entwicklung  von  Ascaris  megalocephala  mit  besonderer  Rucksicht  auf  die 

Kernverhaltnisse  :  Festschr.  fur  C.  v.  Knpffer.    Jena,  1899. 
Brunn,  M.  von. —  Beitrage  zur  Kenntniss  der  Samenkorper  und  ihrer  Entwickelung 

bei  Vogeln  und  Saugethieren  :  Arch.  mik.  Anat.,  XXXIII.     1889. 
Hacker,    V.  —  Die    Eibildung    bei    Cyclops   und  Camptocanthus :   Zool.  Jahrb..   \. 

1892.     (See  also  List  V.) 
Hermann,  F.  —  Urogenitalsystem  :    Struktur   und    Histiogenese   der  Spermatozoen  : 

Merkel  nnd  Bonnefs  Ergebnisse,  II.     1892. 
Ikeno,  S.  —  Untersuchungen  Uber  die  Entwickelung  der  Geschechtsorgane,  t'/t:.,  bei 

Cycas  :  Jahrb.  wiss.  Bot..,  XXXI L,  4.      1898. 
Kolliker,  A.  —  Beitrage  zur  Kenntniss  der  Geschlechtsverhaltnisse  und  der  Samen- 

fliissigkeit  wirbelloser  Tiere.     Berlin.  1841. 
Leydig,  Fr. —  Beitrage  zur  Kenntniss  des  thierischen  Eies   im    unbefruchteten  Zu- 

stande  :  Zool.  Jahrb.,  III.     1889. 
Moves,  F.  — Uber  die  Entwicklung  der  mannlichen  Gescheclitszellen  von  Salaman- 

dra  :  Arch.  mik.  A?iat.j  XLVIII.     1896. 
Id.  —  t'ber  Struktur  und    Histogenese    der   Samenfaden    des    Meerschweinchens : 

Arch.  mik.  Anat.,  LIV.      1899. 
Schweigger-Seidel,  F. — Uber  die  Samenkorperchen  und  ihre  Entwicklun-^ :  Arch. 

7)iik.  Anat.,  I.      1865. 
Strasburger,  E. — Histologische  Beitrage;    Heft   IV.:    Das  \'erlialten    des  Pollens 

und  die  Befruchtungsvorgange  bei  den  Gymnospermen,  Schwarmsporen,  prianz- 

liche  Spermatozoiden  und  das  Wesen  der  Befruchtung.     Fischer,  Jena,  1892. 
Thomson,  Allen.  —  Article  ''  Ovum,''  in  Todd's  Cyclopedia  of  Anatomy  and  Physi- 
ology.    1859. 
Van  Beneden,  E.  —  Recherches  sur  la  composition  et  la  signification  de  I'd-'uf :  .Mem. 

cour.  de  VAcad.  roy.  de  Belgique.     1870. 
Waldeyer,  W.  —  Eierstock  und  Ei.     Leipzig,  1870. 
Id.  —  Bau  und  Entwickeluns  der  Samenfaden  :    I'erh.  d.  Anat.  Gcs.     Leipzig,  18S7. 


o 


1  See  also  Literature,  V.,  p.  287. 


N 


CHAPTER    IV 

FERTILIZATION   OF  THE   OVUM 

"  It  is  conceivable,  and  indeed  probable,  that  every  part  of  the  adult  contains  molecules 
derived  both  from  the  male  and  from  the  female  parent;  and  that,  regardcil  as  a  mass  of 
molecules,  the  entire  organism  may  be  compared  to  a  web  of  which  the  warp  is  derived  from 
the  female  and  the  woof  from  the  male."  IIUXLEY.i 

In  mitototic  cell-division  we  have  become  acquainted  with  the  means 
by  which,  in  all  higher  forms  at  least,  not  only  the  continuity  of  life, 
but  also  the  maintenance  of  the  species,  is  effected  ;  for  through  this 
beautiful  mechanism  the  cell  hands  on  to  its  descendants  an  exact  dupli- 
cate of  the  idioplasm  by  which  its  own  organization  is  determined. 
As  far  as  we  can  see  from  an  a  priori  point  of  view,  there  is  no  reason 
why,  barring  accident,  cell-division  should  not  follow  cell-division  in 
endless  succession  in  the  stream  of  life.  It  is  possible,  indeed  prob- 
able, that  such  may  be  the  fact  in  some  of  the  lower  and  simpler  forms 
of  life  where  no  form  of  sexual  reproduction  is  known  to  occur.  In 
the  vast  majority  of  living  forms,  however,  the  series  of  cell-divisions 
tends  to  run  in  cycles  in  each  of  which  the  energy  of  division  finally 
comes  to  an  end  and  is  only  restored  by  an  admixture  of  living  mat- 
ter derived  from  another  eel  I.  This  operation,  known  as  fertiliza- 
tion or  fecundation,  is  the  essence  of  sexual  reproduction  ;  and  in  it  we 
behold  a  process  by  which  on  the  one  hand  the  energy  of  division  is 
restored,  and  by  which  on  the  other  hand  two  independent  lines  of 
descent  are  blended  into  one.  Why  this  dual  process  should  take 
place  we  are  as  yet  unable  to  say,  nor  do  we  know  which  of  its  two 
elements  is  to  be  regarded  as  the  primary  and  essential  one. 

Harvey  and  many  other  of  the  early  embryologists  regarded  fer- 
tilization as  a  stimulus,  given  by  the  spermatozoon,  through  which  the 
ovum  was  "  animated  "  and  thus  rendered  capable  of  development. 
In  its  modern  form  this  conception  appears  in  the  "  dynamic  "  theories 
of  Herbert  Spencer,  Biitschli,  Hertwig,  and  others,  which  assume  that 
protoplasm  tends  gradually  to  pass  into  a  state  of  increasingly  sta- 
ble equilibrium  in  which  its  activity  diminishes,  and  that  fertilization 
restores  it  to  a  labile  state,  and  hence  to  one  of  activity,  through  mix- 
ture with  protoplasm  that  has  been  subjected  to  different  conditions. 
BiitschU  ('76)  pointed  out  that  the  life-cycle  of  the  metazoon  is  com- 

1  Evolution,  in  Science  and  Culture,  p.  296,  from  Enc.  Brit.,  1878. 

178 


FERTILIZATION   OF   THE    OVUM 


179 


parable  to  that  of  a  protozoan  race,  a  long  series  of  cell-divisions  being 
in  each  case  followed  by  a  mixture  of  protoplasms  through  conjuga- 
tion;  and  he  assumed  that,  in  both  cases,  conjugation  results  in  reju- 
venescence through  which  the  energy  of  growth  and  division  is 
restored  and  a  new  cycle  inaugurated.  The  same  view  has  been 
advocated  by  Minot,  Engelman,  Hensen,  and  many  others.  Mau- 
pas  i^'^Z,  '89),  in  his  celebrated  researches  in  the  conjugation  of  Infu- 
soria, attempted  to  test  this  conclusion  by  following  out  continuously 
the  hfe-history  of  various  species  through  the  entire  cycle  of  their  exist- 
ence. Though  not  yet  adequately  confirmed,  and  indeed  opposed  in 
some  particulars  by  more  recent  work,^  these  researches  have  yielded 
very  strong  evidence  that  in  these  unicellular  animals,  even  under 
normal  conditions,  the  processes  of  growth  and  division  sooner  or 
later  come  to  an  end,  undergoing  a  process  of  natural  **  senescence," 
which  can  only  be  counteracted  by  conjugation.  That  fertilization  in 
higher  plants  and  animals  does  in  fact  incite  division  and  growth  is  a 
matter  of  undisputed  observation.  We  know,  however,  that  in  parthe- 
nogenesis the  ^^%  may  develop  without  fertilization,  and  we  do  not 
know  whether  the  tendency  to  "  senescence  "  and  the  need  for  fer- 
tilization are  primary  attributes  of  living  matter. 

The  foregoing  views  maybe  classed  together  as  the  rejuvenescence 
theory.  Parallel  to  that  theory,  and  not  necessarily  opposed  to  or 
confirmatory  of  it,  is  the  view  that  fertilization  is  in  some  way  con- 
cerned with  the  process  of  variation.  Long  since  suggested  by  Tre- 
viranus  and  more  lately  developed  by  Brooks  ^  and  Weismann  '^  is  the 
hypothesis  that  fertilization  is  a  source  of  variation  —  a  conclusion  sug- 
gested by  the  experience  of  practical  breeders  of  plants  and  animals. 
Weismann  brings  forward  strong  arguments  against  the  rejuvenescence- 
theory,  and  regards  the  need  for  fertilization  as  a  secondaiy  acquisi- 
tion, the  mixture  of  protoplasms  to  which  it  leads  producing  variations 
—  or  rather  insuring  their  "mingling  and  persistent  renewal"-'  — 
which  form  the  material  on  which  selection  operates.  On  the  other 
hand,  a  considerable  number  of  writers,  including  Darwin.  Spencer, 
O.  Hertwig,  Hatschek,  and  others,  believe  that  although  crossing  may 
lead  to  variability  within  certain  limits,  its  effect  in  the  long  run  tends 
to  neutralize  indefinite  variability  and  thus  to  hold  the  species  true  to 

the  type. 

It  is  remarkable  that  we  should  still  remain  uncertain  as  to  the  physi- 
ological meaning  of  a  process  so  general  and  one  that  has  been  the 
subject  of  such  prolonged  research.  Both  the  foregoing  general  views 
are  in  harmony  with  the  results  of  Darwin's  work  on  variation  and 
with  the  experience   of   practical  breeders,  which   have   shown  that 

1  Cf.  Joukowskv,  '99.  '  Amphimixis,  1S91. 

2  The  Law  of  Heredity,  1883.  *  '99.  P-  326. 


i8o 


FERTIUZATIOX  OF   THE    OVUM 


crossing  produces  both  greater  vigour  and  greater  variability.  In  view 
of  all  the  facts,  however,  we  are  constrained  to  the  admission  that  the 
essential  nature  of  sexual  reproduction  must  remain  undetermined  until 
the  subject  shall  have  been  far  more  thoroughly  investigated,  espe- 
cially in  the  unicellular  forms,  where  the  key  to  the  ultimate  problem 
is  undoubtedly  to  be  sought. 


A.     Preliminary  General  Sketch 

Among  the  unicellular  plants  and  animals,  fertilization  is  effected 
bv  means  of  conjugation,  a  process  in  which  two  individuals  either 
fuse  together  permanently  or  unite  temporarily  and  effect  an  exchange 


A 


"^  ■•  "0       c      • 


Fig.  89.  —  Fertilization  of  the  egg  of  the  snail,  P/iysa.  [Kostanecki  and  Wierzejski.] 
A.  The  eniire  spermatozoon  lies  in  the  egg,  its  nucleus  at  the  right,  flagellum  at  the  left,  while 
the  minute  sperm-amphiaster  occupies  the  position  of  the  middle-piece.  The  first  polar  body  has 
been  formed,  the  second  is  forming.  B.  The  enlarged  sperm-nucleus  and  sperm-amphiaster  lie 
near  the  centre;  second  polar  body  forming  and  the  first  dividing.  The  egg-centrosomes  and 
asters  afterward  disappear,  their  place  being  taken  by  those  of  the  spermatozoon. 

of  nuclear  matter,  after  which  they  separate.  /;/  (7//  the  higher  forms 
fertilization  consists  in  the  permanent  fusion  of  tivo  germ-cells,  one  of 
paternal  and  ofie  of  maternal  origin.  We  may  first  consider  the  fer- 
tilization of  the  animal  Qgg,  which  appears  to  take  place  in  essentially 
the  same  manner  throughout  the  animal  kingdom,  and  to  be  closely 
paralleled  by  the  corresponding  process  in  plants. 


PRELIMINARY  GENERAL   SKETCH  l8i 

Leeuwenhoek,  whose  pupil  Hamm  discovered  the  spermatozoa 
(1677),  put  forth  the  conjecture  that  the  spermatozoon  must  pene- 
trate into  the  egg;  and  the  classical  experiments  of  Spallanzani  on 
the  frog's  egg  (1786)  proved  that  the  fertihzing  element  must  be  the 
spermatozoa  and  not  the  liquid  in  which  they  swim.  The  penetration 
of  the  ovum  was,  however,  not  actually  seen  until  1854,  when  Newport 
observed  it  in  the  case  of  the  frog's  Qg^\  and  it  was  described  by 
Pringsheim  a  year  later  in  one  of  the  lower  plants,  Gldigoniuui.  The 
first  adequate  description  of  the  process  was  given  by  Hermann  Fol, 
in  1879,^  though  many  earlier  observers,  from  the  time  of  Martin 
Barry  ('43)  onward,  had  seen  the  spermatozoon  inside  the  egg-enve- 
lopes, or  asserted  its  entrance  into  the  ^^g. 

In  many  cases  the  entire  spermatozoon  enters  the  (^^^^^  (mollusks, 
insects,  nematodes,  some  annehds,  Petromyson,  axolotl,  etc.),  and  in 
such  cases  the  long  flageilum  may  sometimes  be  seen  coiled  within 
the  ^gg  (Fig.  89).  Only  the  nucleus  and  middle-piece,  however,  are 
concerned  in  the  actual  fertilization ;  and  there  are  some  cases 
(echinoderms)  in  which  the  tail  is  left  outside  the  Qgg.  At  or  near 
the  time  of  fertilization,  the  Qgg  successively  segments  off  at  the  upjxT 
pole  two  minute  cells,  known  as  t\\Q  polar  bodies  {¥\gs.  89,  90,  116)  or 
directive  corpuscles,  which  degenerate  and  take  no  part  in  the  subse- 
quent development.  This  phenomenon  takes  place,  as  a  rule,  imme- 
diately after  entrance  of  the  spermatozoon.  It  may,  however,  occur 
before  the  spermatozoon  enters,  and  it  forms  no  part  of  the  process 
of  fertilization  proper.  It  is  merely  the  final  act  in  the  process  of 
maturation,  by  which  the  ^gg  is  prepared  for  fertilization,  and  we 
may  defer  its  consideration  to  the  following  chapter. 

I.    The  Genn-miclei  in  Fertilization 

The  modern  era  in  the  study  of  fertilization  may  be  said  to  begin 
with  Oscar  Hertwig  s  discovery,  in  1875,  of  the  fate  of  the  sperma- 
tozoon within  the  ^gg.  Earlier  observers  had,  it  is  true,  paved  the 
way  by  showing  that,  at  the  time  of  fertilization,  the  Q.gg  contains 
two  nuclei  that  fuse  together  or  become  closely  associated  before 
development  begins.  (Warneck,  Biitschli,  Auerbach,  Wan  IkMieden. 
Strasburger. )  Hertwig  discovered,  in  the  iigg  of  the  sea-urchin 
{Toxopnenstes  lividius\  that  one  of  these  nuclei  belofii^s  to  the  a^g, 
while  the  other  is  derived  from  the  spermatozoon.  This  result  was 
speedily  confirmed  in  a  number  of  other  animals,  and  has  since  been 
extended  to  every  species  that  has  been  carefully  investigated.  The 
researches  of  Strasburger,  De  Bary,  Schmitz,  Guignard,  and  others 
have  shown  that  the  same  is  true  of  plants.     ///  every  known  case  an 

1  See  Pllenogenie,  pp.  124  ff.,  for  a  full  historical  account. 


1 82  FERTILIZATION   OF   THE    OVUM 

essoitial  pJuiiovioiou  of  fcrtilizatioi  is  tJic  union  of  a  spciin-uucItHSy 
of  paternal  origin,  ivith  an  cgg-nuclcns,  of  maternal  origin,  to  form  tJie 
primary  niielens  of  the  emluyo.  TJiis  nucleus,  knozi'n  as  tJie  cleavage- 
or  segmentation-nucleus,  gives  rise  by  divisioii  to  all  t/w  nuclei  of  the 
body,  and  hence  ever]'  nucleus  of  the  child  )nay  contain  nuclear  substance 
y'  derived  from  both  parents.  And  thus  Hcrtwi<^  was  led  to  the  conclu- 
sion ('84),  independently  reached  at  the  same  time  by  Strasburger, 
Kolliker,  and  Weismann,  that  the  nucleus  is  the  most  essential  cle- 
ment concerned  in  hereditary  transmission. 

This  conclusion  received  a  strong  suj^port  in  the  year  1883,  through 
the  s]:)lendid  discoveries  of  Van  Heneden  on  the  fertilization  of  the 
thread-worm,  Ascaris  megalocephala,  the  <t^g  of  which  has  since  ranked 
with  that  of  the  echinoderm  as  a  classical  object  for  the  study  of  cell- 
problems.  Van  Beneden's  researches  especially  elucidated  the  struc- 
ture and  transformations  of  the  germ-nuclei,  and  carried  the  analysis 
of  fertilization  far  beyond  that  of  Hertwig.  In  Ascaris,  as  in  all 
other  animals,  the  sperm-nucleus  is  extremely  minute,  so  that  at  first 
sight  a  marked  inequality  between  the  two  sexes  appears  to  exist 
in  this  respect.  Van  Beneden  showed  not  only  that  the  inequality  in 
size  totallv  disappears  during  fertilization,  but  that  the  two  nuclei 
undergo  a  parallel  series  of  structural  changes  which  demonstrate 
their  precise  morj:)hological  equivalence  down  to  the  minutest  detail; 
and  here,  again,  later  researches,  foremost  among  them  those  of 
Boveri,  Strasburger,  and  Guignard,  have  shown  that,  essentially,  the 
same  is  true  of  the  germ-cells  of  other  animals  and  of  plants.  The 
facts  in  Ascaris  (variety  bivalens)  are  essentially  as  follows  (Fig. 
90):  After  the  entrance  of  the  spermatozoon,  and  during  the  for- 
mation of  the  polar  bodies,  the  sperm-nucleus  rapidly  enlarges  and 
finally  forms  a  typical  nucleus  exactly  similar  to  the  egg-nucleus. 
The  chromatin  in  each  nucleus  now  resolves  itself  into  two  long, 
worm-like  chromosomes,  which  are  exactly  similar  in  form,  size,  and 
staining-reaction  in  the  two  nuclei.  Next,  the  nuclear  membrane 
fades  away,  and  the  four  chromosomes  lie  naked  in  the  egg-substance. 
Every  trace  of  sexual  difference  has  now  disappeared,  and  it  is 
impossible  to  distinguish  the  paternal  from  the  maternal  chromo- 
somes (Fig.  90,  D,  E).  Meanwhile  an  amphiaster  has  been  devel- 
oped which,  with  the  four  chromosomes,  forms  the  mitotic  figure  for 
the  first  cleavage  of  the  ovum,  the  chronmtic  portion  of  zuhicJi  lias 
been  synthetically  formed  by  the  unioii  of  two  equal  germ-nuclei.  The 
later  phases  follow  the  usual  course  of  mitosis.  Each  chromosome 
splits  lengthwise  into  equal  halves,  the  daughter-chromosomes  are 
transported  to  the  spindle-poles,  and  here  they  give  rise,  in  the  usual 
manner,  to  the  nuclei  of  the  two-celled  stage.  EacJi  of  these  nuclei, 
tJici'cfore,  receives  exactly  equal  amounts  of  paternal  and  maternal 
chromatin. 


PRELIMINARY   GENERAL   SKE 


TCH 


IS3 


E  p 

iatefig'e"  ^^e^'^!":,^  ^'^  ''^  ''  ^''^^'^  ^e.aloccpnala,  var.  tr.aU„s.     rBo^•KR^]      (  For 

lar  tasT'of-'a^ToXn" •'?.»"'':''  "''^'^^  '''  ""'^"'^  '^  ^'^°^^"  «'  ^  =  '-'de  it  lies  the  granu- 
of  the  second Tolarbodv  ^t  vo  h  ''""'"^  •=  "''°''^  ^^""'^  "^"  ^^'^'^'"•'^  P'^'-^^"  '"  ''^e  formation 
reticular  sijj  Z  ,ttrt\^,  1?  chromosomes  m  each  nucleus).    B.   Germ-„uclei  (  ?.  cT)  in    he 

formmginthege'lnu"  ttfc^^^^^^^^^  contams  the  dividing  centrosome.  C.  Chromosome 
chromosomes ;  attrldonsDhee  AT  rr  "i?  'fi-  ^^  ^^'^'^  &crm-nucleus  resolved  into  two 
the  chromosomes  (0  aLLv  Lit  /t  .  ^  "'°"'  '^'"■"^  '"'"'"'"^  '^'^  ^'^^  ^-"^^  ^'^^-^g- i 
-ughter-chromosoList:::d  r^pi^^^ ^?^;-^-Sot:1hf.^r^""^^  "  ''^^ 


1 84 


FERTILIZATIOX  OF   THE    OVUM 


These  discoveries  were  confirmed  and  extended  in  the  case  of 
Ascaris  by  Boveri  and  by  Van  Beneden  himself  in  1887  and  1888 
and  in  several  other  nematodes  by  Carnoy  in  1887.  Carnoy  found 
the  number  of  chromosomes  derived  from  each  sex  to  be  in  Corouilla 
4,  in  OpJiiostomiini  6,  and  in  Filaroidcs  8.  A  little  later  Boveri 
('90)  showed  that  the  law  of  numerical  equality  of  the  paternal  and 
maternal  chromosomes  held  ^ood  for  other  groups  of  animals,  being 
in  the  sea-urchin  Echijius  9,  in  the  worm  Sagitta  9,  in  the  medusa 
Tiara  14,  and  in  the  mollusk  Ptcrotmclica  16  from  each  sex.  Similar 
results  were  obtained  in  other  animals  and  in  j^lants,  as  first  shown  by 
Guignard  in  the  lily  ('91 ),  where  each  sex  contributes  12  chromosomes. 


A 


B 


Fig.  91.  — Germ-nuclei  and  chromosomes  in  the  eggs  of  nematodes.     [Carnov.] 
A.   Egg  of  nematode  parasitic  in  ScylUum  ;  the  two  germ-nuclei  in  apposition,  each  containing 
four  chromosomes;  the  two  polar  bodies  above.     B.   Egg  of  Filaroidcs;  each  germ-nucleus  with 
eight  chromosomes ;  polar  bodies  above,  deutoplasm-spheres  below. 

In  the  onion  the  number  is  8  (Strasburger) ;  in  the  annelid  OpJiryo- 
trocJia  it  is  only  2  from  each  sex  (Korschelt).  In  all  these  cases  the 
number  contributed  by  each  is  one-Jialf  the  number  eliaracteristie  of  the 
body-cells.  The  union  of  two  germ-cells  thus  restores  the  normal 
number,  and  here  we  find  the  explanation  of  the  remarkable  fact 
commented  on  at  page  67  that  tJie  number  of  chromosomes  in  sexually 
produced  ori^anisms  is  akuays  eve7i} 

These  remarkable  facts  demonstrate  the  two  germ-nuclei  to  be  in 
a  morphological  sense  precisely  equivalent,  and  they  not  only  lend 
very  strong  support  to  Hertwig's  identification  of  the  nucleus  as  the 
bearer  of  hereditary  qualities,  but  indicate  further  that  these  qualities 

1  Cf.  p.  67. 


PRELIMINARY  GENERAL   SKETCH 


185 


must  be  carried  by  the  chromosomes  ;  for  their  precise  equivalence  in 
number,  shape,  and  size  is  the  physical  correlative  of  the  fact  that 
the  two  sexes  play,  on  the  whole,  equal  parts  in  hereditary  transmis- 
sion. 

2.    The  AcJirouiatic  Structures  in  Fertilization 

It  is  generally  agreed  that  the  amphiaster  of  the  primary  mitotic 
figure  of  the  fertilized   ovum  arises  from  the  egg-substance  precisely 


Fig.  92.  —  Maturation  and  fertilization  of  the  egg  of  the  mouse.  [SoHo ita.] 
A.  The  ovarian  Qgg  still  surrounded  by  the  follicle-cells  and  the  membnuie  (c./.  zona  pel- 
lucida)  ;  the  polar  spindle  formed.  D.  Egg  immediately  after  entrance  of  the  spermatozoon 
(sperm-nucleus  at  cf).  C.  The  two  germ-nuclei  (c?,  9)  siill  unequal;  polar  bodies  above. 
D.  Germ-nuclei  approaching,  of  equal  size.  E.  The  chromosomes  forming.  F.  The  minute 
cleavage-spindle  in  the  centre;   on  either  side  the  paternal  and  maternal  groui^s  of  chromosomes. 

as  in  the  ordinary  mitosis  of  tissue-cells,  and  its  mode  of  origin  there- 
fore involves  the  same  questions  as  those  already  discussed  at  page  72. 
It  is  quite  otherwise  with  the  centrosomes  at  the  astral  centres,  the 


1 86 


FERTILIZATIOX  OF   THE    OVUM 


oridn  of  which  still  remains  one  of  the  most  difficult,  as  it  is  one  of 
the  most  interesting,  problems  relating  to  fertilization. 

After  the  formation  of  the  polar  bodies,  the  egg-nucleus  is  recon- 
stituted near  the  upper  iK)le  of  the  (^'-^g,  and  the  entire  polar  mitotic 
apparatus  disappears.      In  the  meantime  a  new  astral  system  (sperm- 


X'^''^-^h. 


^S 


^m/ 


B 

Fig.  93.  —  Fertilization  of  the  egg  of  tlie  gasteropod,  Pterotrachea.  \  BOVERI.] 
A.  The  egg-nucleus  {E)  and  sperm-nucleus  {S)  approaching  after  formation  of  the  polar 
bodies;  the  latter  shown  above  {P.  B.)\  each  germ-nucleus  contains  sixteen  chromosomes;  the 
sperm-amphiaster  fully  developed.  D.  The  mitotic  figure  for  the  first  cleavage  nearly  established; 
the  nuclear  membranes  have  disappeared,  leaving  the  maternal  group  of  chromosomes  above  the 
spindle,  the  paternal  below  it. 


PRELIMINARY   GENERAL   SKETCH 


187 


aster  or  amphiaster)  is  developed  in  the  neighbourhood  of  the  sperm- 
nucleus,  and  this  in  a  large  number  of  cases  gives  rise  or  is  definitely 
related  to  the  cleavao:e- 


amphiaster  (coelente- 
rates,  flat-worms,  echi- 
noderms,  nematodes, 
annelids,  arthropods, 
mollusks,  tunicates,  ver- 
tebrates). In  many  of 
these  cases  the  sperm- 
aster,  which  by  divi- 
sion gives  rise  to  the 
amphiaster,  has  been 
found  to  arise  in  inti- 
mate relation  with  the 
middle -piece  of  the 
spermatozoon ;  e.g.  in 
echinoderms(Flemming, 
Hertwig,  Boveri,  Wil- 
son, Mathews,  Hill,  etc.), 
in  the  axolotl(Fick)  and 
salamander  (Michaelis), 
in  the  tunicates  (Hill), 
annelids  (Foot,  Vejdov- 
sky),  insects  (Henking), 
nematodes  (Meyer,  Er- 
langer),  and  mollusks 
(Henking,  Kostanecki, 
and  Wierzejski).  The 
agreement  between 
forms  so  diverse  is  very 
strong  evidence  that  this 
is  a  very  general  phe- 
nomenon, and  it  is  one  of 
great  interest,  owing  to 
the  fact  that  the  middle- 
piece  is  itself  derived 
from  or  contains  the 
centrosome  of  the  sper- 
matid.^ 

The  facts  may  be  il- 
lustrated by  a  brief 
description  of  the   phe- 


m  -•- 
n  -  - 


»-.<'r 


I 


% 


A 


B 


C 


Fig.  94.  —  Entrance  and  rotation  of  the  sperm-head  and 
formation  of  tlie  sperm-aster  in  the  sea-urchin,  loxopneustes 
{A-F,  X  1600 ;    G,  H,  X  800). 

A.  Sperm-head  before  entrance;  n.  nucleus;  w.  mid- 
dle-piece and  part  of  the  flagellum.  />'.  C.  Immediately 
after  entrance,  showing  entrance-cone.  />».  Rotation  of  the 
sperm-head,  formation  of  the  sperm-aster  about  the  middle- 
piece.  £.  Casting  off  of  middle-piece;  centrosome  at  focus 
of  the  ravs  (r/C  Fig.  12).  The  changes  figured  occupy  about 
eight  minutes.  F.  G.  Approach  of  the  germ-nuclei ;  growth 
of  the  aster. 

1  C/.  p.  170. 


1 88  FERTILIZATIOX   OF   THE    OVUM 

nomena  in  the  sea-urchin  Toxop)icustcs  (Fig.  94).  As  described  at 
page  197,  the  tail  is  in  this  case  left  outside,  and  only  the  head  and 
middle-piece  enter  the  egg.  Within  a  few  minutes  after  its  entrance, 
and  while  still  very  near  the  periphery,  the  lance-shaped  sperm-head, 
carrying  the  niiddle-})iece  at  its  base,  rotates  through  nearly  or  cpiite 
180°,  so  that  the  pointed  end  is  directed  outward  and  the  middle- 
piece  is  turned  inward  (  P^ig.  94,  A-F)}  During  or  shortly  after  the 
rotation  appears  a  minute  aster  centring  in  or  very  near  the  middle- 
piece.  As  it  enlarges,  the  middle-piece  itself  is  thrown  to  one  side 
(Fig.  12),  where  it  soon  degenerates,  while  in  the  centre  of  the  aster 
a  minute  intensely  staining  centrosome  may  be  seen,  l^oth  sperm- 
nucleus  and  aster  now  rapidly  advance  toward  the  centre  of  the  egg, 
the  aster  leading  the  way  and  its  rays  extending  far  out  into  the 
cytoplasm  and  finally  traversing  nearly  an  entire  hemisphere.  The 
central  mass  of  the  aster  comes  in  contact  with  the  egg-nucleus, 
divides  into  two,  and  the  daughter-asters  pass  to  opposite  poles  of 
the  egg-nucleus,  while  the  sperm-nucleus  flattens  against  the  latter 
and  assumes  the  form  of  a  biconvex  lens  (Fig.  95).  The  nuclei  now 
fuse  to  form  the  cleavage-nucleus.  Shortly  afterward  the  nuclear 
membrane  fades  away,  a  spindle  is  developed  between  the  asters,  and 
a  group  of  chromosomes  arises  from  the  cleavage-nucleus.  These 
are  36  or  38  in  number ;  and  although  their  relation  to  the  paternal 
and  maternal  chromatin  cannot  in  this  case  be  accurately  traced, 
owing  to  the  apparent  fusion  of  the  nuclei,  there  can  be  no  doubt  on 
general  grounds  that  one-half  have  been  derived  from  each  germ- 
nucleus.  The  egg  then  divides  into  two,  four,  etc.,  by  ordinary 
mitosis  (Figs.  4,  52). 

In  the  type  of  fertilization  just  described,  the  polar  bodies  are 
formed  long  before  the  entrance  of  the  spermatozoon  and  the  germ- 
nuclei  conjugate  immediately  upon  entrance  of  the  spermatozoon, 
fusing  to  form  a  true  cleavage-nucleus.  In  a  second  and  more 
frequent  type  {Ascaris,  Fig.  90;  PJiysa,  Fig.  89;  Nereis,  Fig.  97; 
Cyclops,  Fig.  98)  the  sperm-nucleus  penetrates  for  a  certain  distance, 
often  to  the  centre  of  the  egg,  and  then  pauses  while  the  polar 
bodies  are  formed.  It  then  conjugates  with  the  re-formed  egg- 
nucleus.  In  this  case  the  sperm-aster  always  divides  to  form  an 
amphiaster  before  conjugation  of  the  nuclei,  while  in  the  first  case 
the  aster  may  be  still  undivided  at  the  time  of  union.  This 
difference  is  doubtless  due  merely  to  a  difference  in  the  time 
elapsing  between  entrance  of  the  spermatozoon  and  conjugation 
of  the   nuclei,  the    amphiaster  having,  in  the   second   case,   time   to 

^  The  first,  as  far  as  I  know,  to  observe  the  rotation  of  the  sperm-head  was  Flemming 
in  the  echinoderm-egg  ('8i,  pp.  17-19).  It  has  since  been  clearly  observed  in  several  other 
cases,  and  is  probably  a  phenomenon  of  very  general  occurrence. 


PRELIMINARY  GENERAL   SKETCH 


189 


form  during  extrusion  of  the  polar  bodies.  The  two  types  just 
described  (Fig.  96)  are  connected  by  various  gradations.  Thus, 
in  the  lamprey,  the  frog,  the  rabbit,  and  in  An.pln.xus  one  polar 
body  is  expelled  before,  and  one  after,  the  entrance  of  the  sper- 
matozoon ;  in  the  annelid  Othryotrocha,  entrance  takes  place  when 
the   first    polar    spindle   is   in   the   stage    of   the   equatorial    plate ; 


/ 


T<,4»'us,e,,  X  icoo.     (For  later  stages  see  t  .g.^S^.)^    ^^^^^^^^^^  ^^  ^^^  sperm-nucleus  against  the 


A.  Union  of  the  nuclei ;  extension 
egg-nucleus ;  division  of  the  aster. 


of  the  aster.    B.  Flattening 


190 


FERTILIZATION  OF   THE    OVUM 


while  in   CJuctopterus  and  Pi  en's  the  first  polar  spindle  has  advanced 
into  the  anaphase.^ 

It  is  an  interesting  and  signiftcant  fact  that  the  aster  or  amphiaster 
always  leads  the  way  in  the  march  toward  the  egg-nucleus  ;  and  in 
many  cases  it  may  be  far  in  advance  of  the  sperm-nucleus.-  Boveri 
('87,  I)  has  observed  in  sea-urchins  that  the  sperm-nucleus  may  indeed 
be    left  entirely  behind,  the   aster   alone  conjugating  with  the  egg- 


Fig.  96. —  Diagrams  of  two  principal  types  of  fertilization.  /.  Polar  bodies  formed  after  the 
entrance  of  the  spermatozoa  (annelids,  mollusks,  flat-worms).  //.  Polar  bodies  formed  before 
entrance  (echinoderms). 

A.  Sperm-nucleus  and  centrosome  at  c^  ;  first  polar  body  forming  at  9 .  B.  Polar  bodies 
formed  ;  approach  of  the  nuclei.  C.  Union  of  the  nuclei.  D.  Approach  of  the  nuclei.  E.  Union 
of  the  nuclei.     F.  Cleavage-nucleus. 

nucleus  and  causing  division  of  the  egg  ivitJiout  union  of  tJic  gcnn- 
nuclci,  though  the  sperm-nucleus  afterward  conjugates  with  one  of 
the  nuclei  of  the  two-cell  stage.  This  process,  known  as  "  partial  fer- 
tilization," is  undoubtedly  to  be  regarded  as  abnormal.  It  affords, 
however,  a  beautiful  illustration  of  the  view  that  it  is  the  centro- 
some alone  that  incites  division  of  the  egg,  and  is  tJierefore  the  fer- 
tilizing element  proper  (Boveri,  '87,  2). 

The  foregoing  facts  lead  us  to  a  consideration  of  Boveri's  theory 
of  fertilization,  which  has  for  several  years  formed  a  central  point  of 
discussion.     The  ground  for  this  theory  had  been  prepared  by  Oscar 

^  QC  p.  181.  *  Cf.  Kostanecki  and  Wicrzejski,  '96. 


PRELIMINARY   GENERAL   SKETCH 


191 


Hertwig  and  Fol.  The  latter  ('73)  early  reached  the  conclusion  that 
the  asters  represented  "  centres  of  attraction "  lying  outside  and 
independent  of  the  nucleus.     Oscar  Hertwig  showed,  in    1^75,  that 


Fig  97.  —  Fertilization  of  the  egg  of  Nereis,  from  sections.  \  a  400.) 
A.  Soon  after  the  entrance  of  the  spermatozoon,  showing  the  minute  '>:^'''^:';^^^^J^:^'^l 
germinal  vesicle  disappearing,  and  the  first  polar  mitotic  figure  forming  ^ '^^^.^^'^ 'P*)^'' '^/'^ 
fent  deutoplasm-spheres  (slightly  swollen  by  the  reagents),  the  firm  circles  o  Wrop^  J^,\^^^Z 
nucleus  (J)  advancing,  a  minute  amphiaster  in  front  of  it;  first  polar  '"'^  .^/.f;^  .  "''^'^j  ^'^I^ ' 
polar  concentration  of  the  protoplasm.  C.  Later  stage;  second  polai  '^;^'^, '*':"  "^^.^.^jllve 
polar  bodies  formed;  conjugation  of  the  germ-nuclei;  the  egg-ccn.rosomes  and  asters  have 
disappeared,  leaving  only  the  sperm-amphiaster  {cf.  tig.  60). 

in  the  sea-urchin  egg,  the  amphiaster  arises  by  the  division  of  a 
single  aster  that  first  appears  near  the  sperm-nucleus  and  accom,ianies 
it  in  its  progress  toward  the  egg-nucleus.  A  similar  observation  was 
soon  afterward  made  by  Fol  ('79)  i"  the  eggs  of  Asl.nas  and 
Sagiita,  and  in  the  latter  case  he  determined  the  fact  that  the  astral 


192  FERTILIZATIOX   OF   THE    OVUM 

rays  do  not  centre  in  the  nucleus,  as  Hertwig  described,  but  at  a 
point  in  advance  of  it  —  a  fact  afterward  confirmed  by  Hertwig 
himself  and  by  l^overi  {^'^'S),  i  ).  Hertwig  and  Fol  afterward  found 
that  in  cases  of  polyspermy,  when  several  spermatozoa  enter  the  <iz^, 
each  sperm-nucleus  is  accompanied  by  an  aster,  and  Hertwig  proved 
that  each  of  these  might  give  rise  to  an  amphiaster  (Fig.  loi).  In 
1886-87  \^ejdovsky  brought  forward  strong  evidence  to  show  that 
in  the  fresh-water  annelid  RJiynchchnis  the  cleavage-amphiaster 
arises  directly  from  the  sperm-amphiaster,  itself  derive'd  by  the 
division  of  a  '*  periplast  "  (attraction-sphere)  imported  into  the  (t^^^  by 
the  sj^ermatozoon,  while  the  polar  amphiaster  entirely  disappears. 
It  was  Boveri  i^'^J,  2)  who  first  carefully  studied  the  facts  with 
reference  to  the  centrosome,  reaching  the  conclusion  (in  the  case  of 
Ascaris  and  the  sea-urchin)  that  a  single  centrosome  is  brought 
in  by  the  spermatozoon,  and  that  it  divides  to  form  two  centres  about 
which  are  developed  the  two  asters  of  the  cleavage-figure.  He  was 
thus  led  to  the  following  conclusion,  which  has  received  the  sup- 
port of  many  later  investigators  :  The  ripe  egg  possesses  all  of  the 
organs  and  qualities  necessary  for  division  excepting  the  centrosome, 
by  which  division  is  initiated.  The  spermatozoon,  on  the  other  Juitid, 
is  provided  zvith  a  centivsome,  but  lacks  the  substance  in  ivhich  this 
organ  of  division  may  exert  its  activity.  TJirough  the  union  of  the 
tiuo  cells  in  fertilization,  all  of  the  essential  organs  necessary  for 
division  are  brought  together ;  the  egg  7iozv  contains  a  centrosome 
ivJiich  by  its  oivn  division  leads  the  zcay  in  the  embryonic  develop- 
ment} Very  numerous  observations,  supporting  this  conclusion,  have 
been  made  by  later  observers.  Bbhm  could  find  in  Petromyzon  ('88) 
and  the  trout  ('91)  no  radiations  near  the  egg-nucleus  after  the 
formation  of  the  polar-bodies,  while  a  beautiful  sperm-aster  is  devel- 
oped near  the  sperm-nucleus  and  divides  to  form  the  amphiaster. 
Platner  ('86)  had  already  made  similar  observations  in  the  snail 
Arion,  and  the  same  result  was  soon  afterward  reached  by  Brauer 
('92)  in  the  case  of  Branchipus,  and  by  Julin  ('93)  in  Styleopsis. 
Pick's  careful  study  of  fertilization  of  the  axolotl  ('93)  proved  in 
a  very  convincing  manner  not  only  that  the  amphiaster  is  a  product 
of  the  sperm-aster,  but  also  that  the  latter  is  developed  about  the 
middle-piece  as  a  centre.  The  same  result  was  indicated  by  Foot's 
observations  on  the  earthworm  ('94),  and  it  was  soon  afterward 
conclusively  demonstrated  in  echinoderms  through  the  independent 
and  nearly  simultaneous  researches  of  myself  on  the  Qgg  of  Toxo- 
pneustes,  of  Mathews  on  Arbacia,  and  of  Boveri  on  EcJiinus.  Nearly 
at  the  same  time  a  careful  study  was  made  by  Mead  ('95,  '98,  i)  of 
the  annelid    Chcetoptei'us,  and  of  the  starfish  Asterias  by  Mathews, 

1  '87,  2,  p.  155. 


PRELIMINARY  GENERAL   SKETCH 


193 


both  observers  independently  showing  that  the  polar  spindle  contains 
distinct  centrosomes,  which,  however,  degenerate  after  the  formation 
of  the  polar  bodies,  their  place  being  taken  by  the  sperm-centrosome, 
which  divides  to  form  an  amphiaster  before  union  of  the  nuclei,  as 
in  RJiyiicJielmis.  Exactly  the  same  result  has  since  been  reached  by 
Hill  ('95)  and  Reinke  ('95)  in  SpJicerecJiiims,  by  Hill  in  the  tunicate 
Phallusia,  by  Kostanecki  and  Wierzejski  ('96)  in  Physa  (Fig.  89), 
and  by  Van  der  Stricht  ('98)  in  Thysanozodn  ;  and  in  all  of  these  the 
centrosome  is  likewise  shown  to  arise  from  the  middle-piece  or  in 
its  immediate   neighbourhood.       Among  others  who  have  produced 


Fig.  98.  —  Fertilization  of  the  egg  in  the  copepod,  Cyclops  strenuus.  [RUCKERT.] 
A.  Sperm-nucleus  soon  after  entrance,  the  sperm-aster  dividing.  B.  The  germ-nuclei  ap- 
proaching;  cf,  the  enlarged  sperm-nucleus  with  a  large  aster  at  each  pole;  9,  the  egg-nucleus 
re-formed  after  formation  of  the  second  polar  body,  shown  at  the  right.  C.  The  apposed  reticular 
germ-nuclei,  now  of  equal  size ;  the  spindle  is  immediately  afterward  developed  between  the  two 
enormous  sperm-asters ;  polar  body  at  the  left. 

evidence  that  the  cleavage-centrosome  stands  in  definite  relation  to 
the  spermatozoon,  may  be  mentioned  Oppel  ('92)  in  reptiles,  Hrauor 
('92)  in  Branchipus,  Henking  (92)  in  insects,  Riickert  ('95.2)  in 
Cyclops,  Sobotta  ('95)  in  the  mouse  and  ('98)  Ampliioxus,  Ziegler  (95) 
in  Diplogaster  and  Rhabditis,  Castle  ('96)  in  Ciomu  Korschelt 
('95)  in  OpJiryotrocha,  Meyer  (95)  in  Strougylus,  Griffin  ('96,  "99) 
in    Thalassema,  and  Coe  ('98)  in   Ccrcbratulns. 

Beside  the  foregoing  evidence  may  be  placed  the  following  addi- 
tional data  based  on  experiment  and  the  study  of  pathological  fer- 
tilization,    (i)  In  the  case  of  sea-urchin  eggs,  Hertwig,  Boveri,  and 
o 


194 


FERTILIZATION   OF   THE    OVCM 


several  later  observers  have  shown  that  egg-fragments,  obtained  by 
shaking  eggs  to  pieces,  are  readily  penetrated  by  the  spermatozoa, 
and  that  such  fragments,  though  containing  no  nuclear  matter  from 
the  egg.  may  segment  and  give  rise  to  perfect  larvae.^  (2)  Hoveri 
('88)  has  observed  that  in  ordinary  fertiUzation  the  sperm-aster  may 
separate  from  the  sperm-nucleus,  travel  through  the  cytoplasm  to  the 
egg-nucleus  and  cause  cleavage,  the  sperm-nucleus  afterward  fusing 
with  one  of  the  nuclei  of  the  two-cell  stage  ("partial  fertiHzation  "). 
(3)  Most  remaikable  of  all,  l^overi,  confirmed  by  Ziegler  (98),  has 
recently  observed  that  during  the  first  cleavage  the  whole  of  the 
chromatin  may  pass  to  one  pole,  so  that  upon  division  one  of  the 
halves  of  the  ^%^  receives  only  a  centrosome  without  a  nucleus.  In 
the  nucleated  half  cleavage  proceeds  as  usual.  In  the  enucleated 
half  the  centrosomes  and  asters  continue  for  a  considerable  period 
to  multiply  at  the  same  rate  as  the  cleavage  of  the  nucleated  half, 
though  the  cell-body  does  not  itself  divide.^  Putting  these  facts 
together  we  must  conclude  ( i )  that  something  is  introduced  into  the 
egg  by  the  middle-piece  of  each  spermatozoon  entering  it  that  is 
either  a  centrosome  or  has  the  power  to  incite  the  formation  of  one  ; 
(2)  that  the  centrosome  thus  arising  is  structurally  independent  of 
both  nuclei  and  may  divide  independently  of  them  ;  (3)  that  indepen- 
dently of  the  division  of  the  nucleus  or  cell-body  there  is  some  kind 
of  historical  continuity  between  the  centrosomes  of  successive  genera- 
tions. 

In  the  case  of  echinoderm-eggs  this  continuity  is  not  yet  known  to 
be  effected  by  actual  persistence  of  the  centrosomes.^  There  are, 
however,  a  number  of  cases  in  which  the  division  of  the  primary 
cleavage-centrosomes  and  the  persistence  of  their  descendants  as 
those  of  the  daughter-cells  seem  to  have  been  conclusively  shown  — 
for  example  on  Ascaris  (Van  Beneden,  Boveri,  Kostanecki,  and  Sied- 
lecki),  in  the  trout  (Henneguy,  '96),  in  Thalasscma  (Griffin,  '96,  '99), 
in  ClicetoptcrHs{Vi<i'3.Ci,  '95,  '98),  in  Pliysa  (Kostanecki  and  Wierzejski, 
'96),  in  Ccrcbmtulus  (Coe,  '98),  and  in  Rhynchchnis  (Vejdovsky  and 
Mrazek,  98).  In  Thalasscma  and  Ccrcbratulus  (Figs.  99,  155)  the 
centrosome  is  a  minute  granule  at  the  focus  of  the  sperm-aster, 
which  divides  to  form  an  amphiaster  soon  after  the  entrance  of  the 
spermatozoon.  During  the  early  anaphase  of  the  first  cleavage,  each 
centrosome  divides  into  two,  passes  to  the  outer  periphery  of  the 
centrosphere,  and  there  forms  a  minute  amphiaster  for  the  second 

1  Cf.  p.  353-  '  C^  P-  ^08. 

3  Erlangcr's  statement  ('98)  that  the  centrosomes  persist  through  the  first  cleavage  in 
echinoderm-eggs  is  not  supported  by  his  figures  ;  and  I  am  convinced  from  my  own  long- 
continued  studies  of  these  eggs,  as  well  as  by  an  examination  of  Erlanger's  preparations, 
kindly  placed  in  my  hands  by  Professor  Biitschli,  that  these  difficult  olijects  are  very  unfavour- 
able for  a  decision  of  the  question. 


PRELIMLYAKY   GENERAL   SKETCH 


195 


cleavage  before  the  first  cleavage  takes  place.  The  minute  centro- 
somes  of  the  second  cleavage  are  therefore  the  direct  descendants  of 
the  sperm-centrosome  ;  and  there  is  good  reason  to  believe  that  the 
continuity  is  not  broken  in  later  stages.     The  facts  are  nearly  similar 


•% 


A 


•  •   • 


»•?•••  v.. 


*  •  •• 


.e   • 


•  ••^ 


"-^^ 


B      •' 


,^- ♦  •' 


• «.; 


\ 

c 


/ 


/ 


•    •• 
*  • 


/ 


A 


/ 


••••.  •.  • 

•  • 

••    .      •  •  •   • 

••  . 

•         ..     •' 

# 

•  •• 

•  • 

• 

• 
•  • 

t  m 

• 

^ 

• 
• 
• 

• 
• 
• 

«                     ^ 

, 

• 
•  • 

V 

•  ••:•.  •• 

•/ 

• 

, ,.  •  •••  • 

7^ 

--• -•-^•.v' 

y^ 


Fig.  99.  —  Fertilization  in  an  annelid  (armed  Gephyrean),  Thalasscma.     [CiRlFFlN.] 

A.  Second  polar  body  forming;  sperm-nucleus  and  centrosome  below.  />.  Apjiroach  of  the 
egg-nucleus  and  sperm-nucleus,  the  latter  accompanied  by  the  sperm-amphiaster.  C.  Union  of 
the  nuclei.  D.  Later  stage  of  last.  E.  Prophase  of  cleavage-spindle.  /'.  Anaphasr  of  the  same; 
centrosome  divided.  G.  H.  I.  Successive  stages  in  the  nuclear  reconstitution  and  lormation  of 
the  daughter-amphiasters  for  the  second  cleavage.     J.   Two-cell  stage. 


in  the  trout,  in  Chcetoptcrus,  and  in  PJiysa.  In  Ascaris  division  of 
the  centrosome  first  occurs  at  a  somewhat  later  period  ( Figs.  90,  176). 
If  now  the  centrosomes  were  indeed  permanent  cell-organs,  wc 
should  thus  reach  the  following  result:  During  cleavage  the  cytoplasm 
of  the  blastomeres  is  derived  from  that  of  the  egg,  the  centrosomes  from 


I()5  FERTILIZATION   OF   THE    OVUM 

the  sptnnatozodn,  ivJiilc  the  nuelei  {chromatin)  are  equally  derived  from 

both  germ-cells. 

There  is  very  strong  reason  to  accept  the    first  part  of  this  con- 
clusion (applying    to    nucleus    and    cytoplasm),  but   the  question  of 
the  centrosomes  remains  an  open  one.     The  array  of  evidence  given 
above,  derived  from  the  study  of  so  many  diverse  groups,  seems  to 
place  Boveri's  lucid  and  enticing  hypothesis  upon  a  strong  foundation. 
Two  essential  points  still  remain,  however,  to  be  determined  :   first, 
whether  the  facts  observed  in  Ascaris,  l^xhinoderms,  PJiysa,  Thalas- 
sema,  and  the  like,  are  typical  of  all  forms  of  fertilization  ;  and,  second, 
whether,  if  so,  the  primary  cleavage-centrosome  is  actually  imported 
into  the  ^gg  by  the  spermatozoon  or  is  only  formed  under  its  influence 
out  of  the  egg-substance.     Both  these  questions  have  been  raised  by 
recent  inves^tigators,  apparently  on  good  evidence,  and  some  of  this 
evidence  is  directly  opposed  to  both  of  the  principal  assumptions  of 
Boveri's  theory.     Thus,  Wheeler  ('97)  has  found  that  in  Mycostoma 
both  centrosomes  are  derived  from  the  egg ;   Carnoy  and   Le  Brun 
('97)  maintain  that  in  Ascaris  one  centrosome  is  derived  from  each  of 
the  germ-nuclei;   in  some  moUusks,  according  to  MacFarland  ('97) 
and   Lillie  ('97),   both  egg-cehtrosomes   and   sperm-centrosomcs   dis- 
appear, to  be  replaced  by  two  centrosomes  of  unknown  origin  ;  while 
recent  botanical  workers  are  unable  to  find  any  centrosomes  in  fertili- 
zation.   These  and  other  divergent  results  will  be  critically  considered 
bevond  (p.  208)  in  connection  with  a  more  detailed  examination  of 
the   general   subject.      It   may   be   pointed   out   here,   however,   that 
recent  researches  on  spermatogenesis  (p.  170)  render  it  nearly  certain 
that  the  centrosome  of  the  sperm-aster  cannot  be  the  unmodified  cen- 
trosome of  the  spermatid,  since  the  latter,  in  some  cases,  enlarges  to 
form  a  "  middle-piece  "  or  analogous  structure  that  is  far  larger  than 
the  sperm-centrosome. 

W.       UxMON    OF    THE    GeRM-CELLS 

It  does  not  lie  within  the  scope  of  this  work  to  consider  the 
innumerable  modes  by  which  the  germ-cells  are  brought  together, 
further  than  to  recall  the  fact  that  their  union  may  take  place  inside 
the  body  of  the  mother  or  outside,  and  that  in  the  latter  case  both 
eggs  and  spermatozoa  are  as  a  rule  discharged  into  the  water,  where 
fertilization  and  development  take  place.  The  spermatozoa  may 
live  for  a  long  period,  either  before  or  after  their  discharge,  without 
losing  their  fertilizing  power,  and  their  movements  may  continue 
throughout  this  period.  In  many  cases  they  are  motionless  when 
first  discharged,  and  only  begin  their  characteristic  swimming  move- 
ments after  coming  in  contact  with  the  water.     There  is  clear  evi- 


UNION  OF   THE   GERM-CELLS 


197 


dence  of  a  definite  attraction  between  the  germ-cells,  which  is 
in  some  cases  so  marked  (for  example  in  the  polyp  Roiilla)  that 
when  spermatozoa  and  ova  are  mixed  in  a  small  vessel,  each  ovum 
becomes  in  a  few  moments  surrounded  by  a  dense  fringe  of  sperma- 
tozoa attached  to  its  periphery  by  their  heads  and  by  their  move- 
ments actually  causing  the  ovum  to  move  about.  The  nature  of  the 
attraction  is  not  positively  known,  but  Pfeffer's  researches  on  the 
spermatozoids  of  plants  leave  little  doubt  that  it  is  of  a  chemical 
nature,  since  he  found  the  spermatozoids  of  ferns  and  of  Sclni^iuc/ia 
to  be  as  actively  attracted  by  solutions  of  malic  acid  or  malates  (con- 
tained in  capillary  tubes)   as   by   the   substance   extruded   from   the 


H 


J  — 


\ 


'^- 


■\<i 


/VvV^;?: 


I 


Fig.  ioo<  —  Entrance  of  the  spermatozoon  into  the  egg.  A-G.  In  tlie  sea-urchin.  Toxopneustei. 
H.  In  tlie  medusa,  Mitrocoma.     [MetSCHNIKOFF.]     /.  In  the  star-fish  Astcrias.     [FOL.] 

A.  Spermatozoon  of  Toxopneustes,  X  2000;  a.  the  apical  body,  w.  nucleus,  w.  middlo-pu-ci-. 
f.  flagellum.  B.  Contact  with  the  egg-periphery.  C.  D.  Entrance  of  the  head,  formation  of  tlie 
entrance-cone  and  of  the  vitelline  membrane  {v),  leaving  the  tail  outside.  E.  F.  I^ter  st.iges. 
G.  Appearance  of  the  sperm-aster  (j)  about  3-5  minutes  after  first  contact;  entrance-cone  break- 
ing  up.  H.  Entrance  of  the  spermatozoon  into  a  preformed  depression.  /.  .Approach  of  the 
spermatozoon,  showing  the  preformed  attraction-cone. 

neck  of  the  archegonium.  Those  of  mosses,  on  the  other  hand,  are 
indifferent  to  malic  acid,  but  are  attracted  by  cane-sugar.  These 
experiments  indicate  that  the  specific  attraction  between  the  germ- 
cells  of  the  same  species  is  owing  to  the  presence  of  specific  chemical 
substances  in  each  case.  There  is  clear  evidence,  furthermore,  that 
the  attractive  force  is  not  exerted  by  the  egg-nucleus  alone,  but  by 
the  egg-cytoplasm;  for,  as  the  Hertwigs  and  others  have  shown, 
spermatozoa  will   readily  enter  egg-fragments    entirely  devoid    ot   a 

nucleus. 

In  naked  eggs,  such  as  those  of  some  echinoderms,  and  coelen- 
terates,  the  spermatozoon  may  enter  at  any  pcnnt ;  but  there  are 
some  cases  in  which  the  point  of  entrance  is  predetermined  by  the 


1 98  FERTILIZATIOX  OF  THE    OVUM 

presence  of  special  structures  through  which  the  spermatozoon 
enters  (F'ig.  lOO).  Thus,  the  starhsh-egg,  according  to  Fol,  pos- 
sesses before  fertilization  a  pecuHar  protoplasmic  "attraction-cone" 
to  which  the  head  of  the  spermatozoon  becomes  attached,  and  through 
which  it  enters  the  *^'^^^.  In  some  of  the  hydromedusas,  on  the  other 
hand,  the  entrance  point  is  marked  by  a  funnel-shaped  depression  at 
the  egg-periphery  ( Metschnikoff ).  When  no  preformed  attraction- 
cone  is  i)resent,  an  "  entrance-cone  "  is  sometimes  formed  by  a  rush 
of  protoplasm  toward  the  point  at  which  the  spermatozoon  strikes 
the  Q.^^  and  there  forming  a  conical  elevation  into  which  the  sperm- 
head  passes.  In  the  sea-urchin  (Fig.  lOO)  this  structure  persists 
only  a  short  time  after  the  spermatozoon  enters,  soon  assuming  a 
ragged  flame-shape  and  breaking  up  into  slender  rays.  In  some 
cases  the  o.^^^  remains  naked,  even  after  fertilization,  as  appears  to 
be  the  case  in  many  ccelenterates.  More  commonly  a  vitelline  mem- 
brane is  quickly  formed  after  contact  of  the  spermatozoon,  —  e.g. 
in  AnipJiioxus,  in  the  echinoderms,  and  in  many  plants,  —  and  by 
means  of  this  the  entrance  of  other  spermatozoa  is  prevented.  In 
eggs  surrounded  by  a  membrane  before  fertilization,  the  spermato- 
zoon either  bores  its  way  through  the  membrane  at  any  point,  as  is 
probably  the  case  with  mammals  and  Amphibia,  or  may  make  its 
entrance  through  a  micropyle. 

In  some  forms  only  one  spermatozoon  normally  enters  the  ovum, 
a?  in  echinoderms,  mammals,  many  annelids,  etc.,  while  in  others 
several  may  enter  (insects,  elasmobranchs,  reptiles,  the  earthworm, 
Petroniyzon,  etc.).  In  the  former  case  more  than  one  spermatozoon 
may  accidentally  enter  (pathological  polyspermy),  but  development 
is  then  always  abnormal.  In  such  cases  each  sperm-centrosome 
gives  rise  to  an  amphiaster,  and  the  asters  may  then  unite  to  form 
the  most  comj^lex  polyasters,  the  nodes  of  w^hich  are  formed  by  the 
centrosomes  (Fig.  loi).  Such  eggs  either  do  not  divide  at  all  or 
undergo  an  irregular  multi])le  cleavage  and  soon  j^erish.  If,  how- 
ever, only  two  spermatozoa  enter,  the  q,<^^j:^  may  develop  for  a  time. 
Thus  Driesch  has  determined  the  interesting  fact,  which  I  have  con- 
firmed, that  sea-urchin  eggs  into  which  two  spermatozoa  have  acci- 
dentally entered  undergo  a  double  cleavage,  dividing  into  four  at  the 
first  cleavage,  and  forming  eight  instead  of  four  micromeres  at  the 
fourth  cleavage.  Such  embryos  develop  as  far  as  the  blastula  stage, 
but  never  form  a  gastrula.^  In  cases  where  several  spermatozoa 
normally  enter  the  ^^^g  (physiological  polyspermy),  only  one  of  the 
sperm-nuclei  normally  unites  with  the  egg-nucleus,  the  supernumer- 
ary sperm-nuclei  either  degenerating,  or  in  rare  cases  —  e.g.  in  elas- 
mobranchs and  reptiles  —  living  for  a  time  and  even  dividing  to  form 

1  For  an  account  of  the  internal  changes,  see  p.  355. 


UNION  OF  THE    GERM-CELLS 


199 


"merocytes     or  accessory  nuclei.     The  fate  of   the  latter  is  still  in 
doubt ;  but  they  certainly  take  no  part  in  fertilization. 

It  is  an  interesting  question  how  the  entrance  of  supernumcrarv 
spermatozoa  is  prevented  in  normal  monospermic  fertilization.  In 
the  case  of  echinoderm-eggs  Fol  advanced  the  view  that  this  is 
mechanically  effected  by  means  of  the  vitelline  membrane  formed 
instantly  after  the  first  spermatozoon  touches  the  q^^.  This  is  indi- 
cated by  the  following  facts.     Immature  eggs,  before  the  formation 


Fig.  lOi.  —  Pathological  polyspermy. 

A.  Polyspermy  in  the  egg  of  y^j^aWj  ;  below,  the  egg-nucleus ;  above,  three  entire  spermatozoa 
within  the  &gg.     [Sala.] 

B.  Polyspermy  in  sea-urchin  egg  treated  with   0.0050''  nicotine  solution;    ton  sperm-n"  '••• 
shown,  three  of  which  have  conjugated  witli  the  egg-nucleus.      C.    Later  stage  of  an  egg  sini: 
treated,  showing  polyasters  formed  by  union  of  the  sperm-amphiasters.     [O.  and  K.  Hkkiwkj.j 

of  the  polar  bodies,  have  no  power  to  form  a  vitelline  membrane, 
and  the  spermatozoa  always  enter  them  in  considerable  numbers. 
Polyspermy  also  takes  place,  as  O.  and  R.  Hertwig's  beautiful  ex- 
periments showed  {^'^J\  in  ripe  eggs  whose  vitality  has  been  dimin- 
ished by  the  action  of  dilute  poisons,  such  as  nicotine,  strychnine, 
and  morphine,  or  by  subjection  to  an  abnormally  high  temperature 


200  FEA'TIIJZATIOX   OF   THE    OVi'M 

(31°  C);  and  in  these  cases  the  vitelline  membrane  is  only  slowly 
formed,  so  that  several  spermatozoa  have  time  to  enter.^  Similar 
mechanical  explanations  have  been  given  in  various  other  cases. 
Thus  Hoffman  believes  that  in  telcosts  the  micropyle  is  blocked  by 
the  polar  bodies  after  the  entrance  of  the  first  spermatozoon  ;  and 
Calberla  suggested  {Pcnvviycon)  that  the  same  result  might  be 
caused  by  the  tail  of  the  entering  spermatozoon.  It  is,  however, 
far  from  certain  whether  such  rude  mechanical  explanations  arc 
adequate  ;  and  there  is  considerable  reason  to  believe  that  the  egg 
may  possess  a  physiological  power  of  exclusion  called  forth  by  the 
first  spermatozoon.  Thus  Driesch  found  that  spermatozoa  did  not 
enter  fertilized  sea-urchin  eggs  from  which  the  membranes  had  been 
removed  by  shaking.-  In  some  cases  no  membrane  is  formed  (some 
ccelenterates),  in  others  several  spermatozoa  are  found  inside  the 
membrane  (nemertines),  in  others  the  spermatozoon  may  penetrate 
the  membrane  at  any  point  (mammals),  yet  monospermy  is  the  rule. 

I.    Iiiiuicdiatc  Results  of  Uiiioi 

The  union  of  the  germ-cells  calls  forth  profound  changes  in  both. 

{a)  TJic  SpcnnatozooJi. — Almost  immediately  after  contact  the  tail 
ceases  its  movements.  In  some  cases  the  tail  is  left  outside,  being 
carried  away  on  the  outer  side  of  the  vitelline  membrane,  and  only 
the  head  and  middle-piece  enter  the  ^%g  (echinoderms,  Fig.  100). 
In  other  cases  the  entire  spermatozoon  enters  (amphibia,  earthworm, 
insects,  etc.,  Fig.  89),  but  the  tail  always  degenerates  within  the 
ovum  and  takes  no  part  in  fertilization.  Within  the  ovum  the 
sperm-nucleus  rapidly  grows,  and  both  its  structure  and  staining- 
capacity  rapidly  change  (</.  p.  182).  The  most  important  and  signifi- 
cant result,  however,  is  an  iniincdiate  rcsiiniptioi  by  the  spcrm-nuclcus 
and  spenn-ccntrosoinc  of  the  power  of  division^  which  has  hitherto 
been  suspended.  This  is  not  due  to  the  union  of  the  germ-nuclei ; 
for,  as  the  Hertwigs  and  others  have  shown,  the  supernumerary 
sperm-nuclei  in  polyspermic  eggs  may  divide  freely  without  copu- 
lation with  the  egg-nucleus,  and  they  divide  as  freely  after  entering 
enucleated  egg-fragments.  The  stimulus  to  division  must  therefore 
be  given  by  the  egg-cytoplasm.  It  is  a  very  interesting  fact  that 
in  some  cases  the  cytoplasm   has  this  effect  on  the  sperm-nucleus 

1  The  Hertwigs  attribute  this  to  a  diminished  irritability  on  the  part  of  the  egg-substance. 
Normally  requiring  the  stimulus  of  only  a  single  spermatozoon  for  the  formation  of  the  vitel- 
line membrane,  it  here  demands  the  more  intense  stimulus  of  two,  three,  or  more  before  the 
membrane  is  formed.  That  the  membrane  is  not  present  before  fertilization  is  admitted  by 
Hertwig  on  the  ground  stated  at  page  132. 

-  On  the  other  hand,  Morgan  states  ('95,  5,  p.  270)  that  one  or  more  spermatozoa  will 
enter  nucleated  or  enucleated  egg-fragments  whether  obtained  before  or  after  fertilization. 


UNION  OF   THE    GERM-CELLS 


201 


only  after  formation  of  tJic  polar  bodies  ;  for  when  in  sea-urchins  the 
spermatozoa  enter  immature  eggs,  as  they  freely  do,  they  penetrate 
but  a  short  distance,  and  no  further  change  occurs. 

(^)   TJie  Ovum.  —  The  entrance  of  the  spermatozoon  produces  an 
extraordinary  effect  on  the  ^g^,  which  extends  to  every  part  of  its 
organization.     The  rapid  formation  of  the  vitelHne  membrane,  already 
described,  proves  that  the  stimulus  extends  almost  instantly  through- 
out the  whole  ovum.^     At  the  same  time  the  physical  consistency 
of  the  cytoplasm  may  greatly  alter,  as  for  instance  in  echinoderm 
eggs,  w^here,  as  Morgan  has  observed,  the  cytoplasm  assumes  immedi- 
ately    after    fertilization    a    peculiar 
viscid    character   which  it  afterward 
loses.     In  many  cases  the  egg  con- 
tracts, performs  amoeboid  movements, 
or  shows  wave-like  changes  of  form. 
Again,  the  egg-cytoplasm  may  show 
active    streaming   movements,   as    in 
the  formation  of  the  entrance-cone  in 
echinoderms,  or  in  the  flow  of  periph- 
eral  protoplasm    toward    the    region 
of   entrance    to    form    the    germinal 
disc,    as  in    many    pelagic   fish-eggs. 
An    interesting    phenomenon    is   the 
formation,     behind     the     advancing 
sperm-nucleus,  of  a  peculiar  funnel- 
shaped     mass     of     deeply     staining 

material    extending    outward    to    the   during  fertilization,    [whitman.] 
periphery.     This  has  been  carefully        A^-  P^'f  ^^^^ies-  /..polar  rings; 

ptii^jiiti;'.        X  xiio    xi«.o  J     ^.ig^yage-nucleus  near  the  centre. 

described  by  Foot  ('94)  in  the  earth- 
worm, where  it  is  very  large  and  conspicuous,  and  1  have  smcc  ob- 
served it  also  in  the  sea-urchin  (Fig.  94). 

The  most  profound  change  in  the  ovum  is,  however,  the  migration 
of  the  germinal  vesicle  to  the  periphery  and  the  formation  of  the 
polar  bodies.  In  many  cases  either  or  both  these  processes  may  occur 
before  contact  with  the  spermatozoon  (echinoderms,  some  vertebrates). 
In  others,  however,  the  ^gg  awaits  the  entrance  of  the  spermatozoon 
(annelids,  gasteropods,  etc.),  which  gives  it  the  necessary  stnmilus. 
This  is  well  illustrated  by  the  o.^^  of  Nereis.  In  the  newly  dis- 
charged ^zg  the  germinal  vesicle  occupies  a  central  position,  the 
yolk,  consisting  of  deutoplasm-spheres  and  oil-globules,  is  uniformly 
distributed,  and  at  the  periphery  of  the  egg  is  a  zone  of  clear  peri- 
vitelline  protoplasm  (Fig.  60).     Soon  after  entrance  of  the  sperma- 

1  I  have  often  observed  that  the  formation  of  the  membrane,  in  To.xopumsUs  proceeds 
Hke  a  wave  from  the  entrance-point  around  the  periphery,  but  this  is  often  irregular. 


Fig.  102. 


Eger  of  the  leech    Clef>stne 


202 


FERTILIZATIOX   OF   TJIK    OVUM 


tozoon  the  germinal  vesicle  moves  toward  the  periphery,  its  membrane 
fades  away,  and  a  radially  directed  mitotic  figure  appears,  bv  means 
of  which  the  first  polar  body  is  formed  (Fig.  97).  Meanwhile  the 
protoplasm  flows  toward  the  iipj)er  })oIe,  the  peri-vitelline  zone  disap- 
pears, and  the  <i^'^i,  now  shows  a  sharply  marked  polar  differentiation. 
A  remarkable  phenomenon,  described  by  Whitman  in  the  leech  ('78), 
and  later  by  Foot  in  the  earthworm  ('94),  is  the  formation  of  "polar 
rings,"  a  j^rocess  which  follows  the  entrance  of  the  sj^ermatozoon 
and  accompanies  the  formation  of  the  polar  bodies.  These  are  two 
ring-sha])ed  cytoplasmic  masses  which  form  at  the  periphery  of  the 
^g^  near  either  pole  and  advance  thence  toward  the  poles,  the  upper 
one  surrounding  the  point  at  which  the  polar  bodies  are  formed 
(Fig.  102).  Their  meaning  is  unknown,  but  Foot  ('96)  has  made 
the  interesting  discovery  that  they  are  probably  of  the  same  nature 
as  the  yolk-nuclei  (p.  156). 


2.    PatJis  of  tJic  Gcrm-nuclci(yPro-nuclci)^ 

After  the  entrance  of  the  spermatozoon,  both  germ-nuclei  move 
through  the  egg-cytoplasm  and  finally  meet  one  another.  The  paths 
traversed  by  them  vary  widely  in  different  forms.  In  general  two 
classes  are  to  be  distinguished,  according  as  the  polar  bodies  are 
formed  before  or  after  entrance  of  the  spermatozoon.  In  the  former 
case  (echinoderms)  the  germ-nuclei  unite  at  once.  In  the  latter  case 
the  sperm-nucleus  advances  a  certain  distance  into  the  q.%^  and  then 
pauses  while  the  germinal  vesicle  moves  toward  the  periphery,  and 
gives  rise  to  the  polar  bodies  {Ascaris,  annelids,  etc.).  This  signifi- 
cant fact  proves  that  the  attractive  force  between  the  two  nuclei  is 
only  exerted  after  the  formation  of  the  polar  bodies,  and  hence  that 
the  entrance-path  of  the  sperm-nucleus  is  not  determined  by  such 
attraction.  A  second  important  point,  first  pointed  out  by  Roux,  is 
that  the  path  of  the  sperm-nucleus  is  curved,  its  "entrance-path" 
into  the  &^^  forming  a  considerable  angle,  with  its  "copulation-path  " 
toward  the  egg-nucleus. 

These  facts  are  well  illustrated  in  the  sea-urchin  Q.gg  (Fig.  103), 
where  the  egg-nucleus  occupies  an  eccentric  position  near  the  point 
at  which  the  polar  bodies  are  formed  (before  fertilization).      Entering 

1  The  terms  fcvialc  pro-uttc/eus,  male  pro-niicleiis  (Van  I>eneden),  are  often  applied 
to  the  germ-nuclei  before  their  union.  These  should,  I  think,  be  rejected  in  favour  of 
Hertwig's  terms  egg-nucleus  and  sperm-nucleus,  on  two  grounds:  (i)  The  germ-nuclei  are 
true  nuclei  in  every  sense,  differing  from  the  somatic  nuclei  only  in  the  reduced  number  of 
chromosomes.  As  the  latter  character  has  recently  been  shown  to  be  true  also  of  the 
somatic  nuclei  in  the  sexual  generation  of  plants  (p.  275),  it  cannot  be  made  the  ground  for 
a  special  designation  of  the  germ-nuclei.  (2)  The  germ-nuclei  are  not  male  and  female 
in  any  proper  sense  (p-  243). 


UNION  OF   THE    GERM-CELLS 


203 


the  egg  at  any  point,  the  sperm-nucleus  first  moves  rapidly  inward 
along  an  entrance-path  that  shows  no  constant  relation  to  the  position 
of  the  egg-nucleus  and  is  approximately  but  never  exactly  radial, 
i.e.  toward  a  point  near  the  centre  of  the  ^gg.     After  penetrating  a 


Fig.  103.  —  Diagrams  showing  the  paths  of  the  germ-nuclei  in  four  different  eggs  of  the  sea- 
urchin,  Toxopneustes.     From  camera  drawings  of  the  transparent  living  eggs. 

In  all  the  figures  the  original  position  of  the  egg-nucleus  (reticulated)  is  shown  at  9  ;  the  point 
at  which  the  spermatozoon  enters  at  E  (entrance-cone).  Arrows  indicate  the  paths  traversed  by 
the  nuclei.  At  tlie  meeting-point  (yl/)  the  egg-nucleus  is  dotted.  The  cleavage-nucleus  in  its 
final  position  is  ruled  in  parallel  lines,  and  through  it  is  drawn  the  axis  of  the  resulting  cleavage- 
figure.  The  axis  of  the  &gg  is  indicated  by  an  arrow,  the  point  of  which  is  turned  away  from  the 
micromere-pole.  Plane  of  first  cleavage,  passing  near  the  entrance-point,  shown  by  the  curved 
dotted  line. 


certain  distance  its  direction  changes  slightly  to  that  of  the  copula- 
tion-path, which,  again,  is  directed  not  precisely  toward  the  egg- 
nucleus,  but  toward  a  meeting-point  where  it  comes  in  contact  with 
the    egg-nucleus.       The    latter    does    not    begin    to    move    until    the 


'fc>iD 


204  FERTILIZATION  OF   THE    OVUM 

entrance-path  of  the  sperm-nucleus  changes  to  the  copulation-path. 
It  then  begins  to  move  slowly  in  a  somewhat  curved  path  toward  the 
meeting-point,  often  showing  slight  amoeboid  changes  of  form  as  it 
forces  its  way  through  the  cytoplasm.  From  the  meeting-point  the 
apposed  nuclei  move  slowly  toward  the  point  of  final  fusion,  which 
in  this  case  is  near,  but  never  precisely  at,  the  centre  of  the  (t^^^. 

These  facts  indicate  that  the  paths  of  the  germ-nuclei  are  deter- 
mined bv  at  least  two  different  factors,  one  of  which  is  an  attraction 
or  other  dvnamical  relation  between  the  nuclei  and  the  cytoplasm, 
the  other  an  attraction  between  the  nuclei.  The  former  determines 
the  entrance-path  of  the  sperm-nucleus,  while  both  factors  probably 
operate  in  the  determination  of  the  copulation-path  along  which  it 
travels  to  meet  the  egg-nucleus.  The  real  nature  of  neither  factor 
is  known. 

Hertwig  first  called  attention  to  the  fact  —  which  is  easy  to  observe  in  the  living 
sea-urchin  egg  —  that  the  egg-nucleus  does  not  begin  to  move  until  the  sperm- 
nucleus  has  penetrated  some  distance  into  the  egg  and  the  sperm-aster  has  attained 
a  considerable  size;  and  Conklin  (94)  has  suggested  that  the  nuclei  are  passively 
drawn  together  by  the  formation,  attachment,  and  contraction  of  the  astral  rays. 
While  this  view  has  some  fiicts  in  its  favour,  it  is,  1  believe,  untenable,  for  many 
reasons,  among  which  may  be  mentioned  the  fact  that  neither  tlie  actual  paths  of 
the  pro-nuclei  nor  the  arrangement  of  the  rays  support  the  hypothesis ;  nor  does 
it  account  for  the  conjugation  of  nuclei  when  no  astral  rays  are  developed  (as  in 
Protozoa  or  in  plants).  I  have  often  observed  in  cases  of  dispermy  in  the  sea-urchin, 
that  both  sperm-nuclei  move  at  an  equal  pace  toward  the  egg-nucleus  ;  but  if  one  of 
them  meets  the  egg-nucleus  first,  the  movement  of  the  other  is  immediately  retarded, 
and  only  conjugates  with  the  egg-nucleus,  if  at  all,  after  a  considerable  interval ;  and 
in  polv.spermy  the  egg-nucleus  rarely  conjugates  with  more  than  two  sperm-nuclei. 
Probably,  therefore,  the  nuclei  are  drawn  together  by  an  actual  attraction  which  is 
neutralized  bv  union,  and  their  movements  are  not  improbably  of  a  chemotactic  char- 
acter. Conklin  (99)  has  recently  suggested  that  the  nuclei  are  drawn  together  by 
the  agency  of  protoplasmic  currents  in  the  egg-substance. 

3.    Union  of  tJic  Gcrm-nuclci.      The  CJiromosouics 

The  earlier  observers  of  fertilization,  such  as  Auerbach,  Stras- 
burger,  and  Hertwig,  described  the  germ-nuclei  as  undergoing  a  com- 
plete fusion  to  form  the  first  embryonic  nucleus,  termed  by  Hertwig 
the  cleavage-  or  segmentation-nucleus.  As  early  as  1881,  however, 
Mark  clearly  showed  that  in  the  slug  Li  max  this  is  not  the  case,  the 
two  nuclei  merely  becoming  apposed  without  actual  fusion.  Two 
years  later  appeared  Van  Beneden's  epoch-making  work  on  Ascaris, 
in  which  it  was  shown  not  only  that  the  nuclei  do  not  fuse,  but  that 
they  give  rise  to  two  independent  groups  of  chromosomes  which 
separately  enter  the  equatorial  plate  and  whose  descendants  pass 
separately  into  the  daughter-nuclei.  Later  observations  have  given 
the  strono-est  reason  to  believe  that,  as  far  as  the  chromatin  is  con- 


UNION  OF   THE    GERM-CELLS 


205 


cerned,  a  true  fusion  of  the  nuclei  never  takes  place  during  fertili- 
zation, and  that  the  paternal  and  maternal  chromatin  may  remain 
separate  and  distinct  in  the  later  stages  of  development  —  possibly 
throughout  life  (p.  299).  In  this  regard  two  general  classes  may  be 
distinguished.  In  one,  exemplified  by  some  echinoderms,  by  Aviplti- 
oxus,  Phallnsia,  and  some  other  animals,  the  two  nuclei  meet  each 
other  when  in  the  reticular  form,  and  apparently  fuse  in  such  a  manner 
that  the  chromatin  of  the  resulting  nucleus  shows  no  visible  distinc- 
tion between  the  paternal  and  maternal  moieties.  In  the  other  class, 
which  includes  most  accurately  known  cases,  and  is  typically  repre- 
sented by  Ascaris  (Fig.  90)  and  other  nematodes,  by  Cyclops  (Fig.  98), 
and  by  Pterotrachea  (Fig.  93),  the  two  nuclei  do  not  fuse,  but  only 
place  themselves  side  by  side,  and  in  this  position  give  rise  each  to 
its  own  group  of  chromosomes.  On  general  grounds  we  may  confi- 
dently maintain  that  the  distinction  between  the  two  classes  is  only 
apparent,  and  probably  is  due  to  corresponding  differences  in  the  rate 
of  development  of  the  nuclei,  or  in  the  time  that  elapses  before  their 
union. ^  If  this  time  be  very  short,  as  in  echinoderms,  the  nuclei 
unite  before  the  chromosomes  are  formed.  If  it  be  more  prolonged, 
as  in  Ascaris,  the  chromosome-formation  takes  place  before  unicm. 

With  a  few  exceptions,  which  are  of  such  a  character  as  not  to 
militate  against  the  rule,  tJie  number  of  cJiromosomcs  arisitij^  f'om  tJic 
germ-nuclei  is  always  the  same  in  botJi,  and  is  one-half  the  number 
characteristic  of  the  tissue-cells  of  the  species.  By  their  union,  there- 
fore, the  germ-nitclei  give  rise  to  an  equatorial^  plate  eonfanung  the 
typical  niLuiber  of  chromosomes.  This  remarkable  discovery  was  first 
made  by  Van  Beneden  in  the  case  of  Ascaris,  where  the  number  of 
chromosomes  derived  from  each  sex  is  either  one  or  two.  It  has 
since  been  extended  to  a  very  large  number  of  animals  and  plants,  a 
partial  list  of  which  follows. 

1  Indeed,  Boveri  has  found  that  in  Ascaris   both  modes  occur,  though  the  fusion  of  the 

germ-nuclei  is  exceptional.      (^Cf.  p.  296.) 


206 


FERTILIZATIOX  OF   THE   OVUM 


A  Partial  List  showing  the  Number  of  Chromosomes  Char- 
acteristic OF  THE  Germ-nuclei  and  Somatic  Nuclei  in 
Various    Plants   and   Animals  ^ 


Germ- 

Somatic 

V*             -, 

/-» 

Nuclei. 

Nuclei. 

Name. 

Group. 

Authority. 

I 

2 

Ascaris  me^alocephala. 
van  univalens. 

Nematodes. 

Van  Beneden, 
Boveri. 

2 

4 

Id.,  var.  bivalens. 

•  » 

«% 

>» 

Ophi  votrocha. 

Annelids. 

Korschelt. 

>» 

p 

St\ieop.sis. 

Tunicates. 

Julin. 

4 

Coronilla. 

Nematodes. 

Carnoy. 

» 

Pallavicinia. 

Hepatica.'. 

Farmer. 

>» 

Anthoceras. 

•  4 

Davis. 

6 

I  '* 

Spiroptera. 

Nematodes. 

Carnoy. 

» 

Prosthecerasus. 

Polyclades. 

Klinckostrom, 
Francotte. 

>» 

Nais. 

Phanerogams. 

Guignard. 

w 

Spirogyra. 

Conjugatae. 

Strasburger. 

?> 

[.." 

Grvllotalpa. 

Insects. 

Vom  Rath. 

;> 

Caloptenus. 

?» 

Wilcox. 

D.] 

/Equorea. 

Hydromedusae. 

Hacker. 

7 

H 

Pentatoma. 

Insects. 

Montgomery. 

8 

i6 

Filaroides. 

Nematodes. 

Carnoy. 

» 

~ 

Prosthiostomum. 

Polyclades. 

Francotte. 

» 

f"     n 

Leptoplana. 

M 

?> 

» 

[••1 

Cycloporus. 

?? 

*» 

» 

Hydropliilus. 

Insects. 

Vom  Rath. 

?> 

Phallusia. 

Tunicates. 

Hill. 

>» 

Li  max. 

Gasteropods. 

Vom  Rath. 

» 

r   ~ 

Rat. 

Mammals. 

Moore. 

» 

p 

0.\.  guinea-pig.  man. 

•  ^ 

Bardeleben. 

?> 

Ceratozamia. 

Cvads. 

Overton, 
Guignard. 

» 

Pinus. 

Coniferae. 

Dixon. 

»> 

Scilla.  TriticLim. 

Angiosperms. 

Overton. 

?) 

Allium. 

» 

Strasburger, 
Guignard. 

» 

Podoijhyllum. 

*« 

Mottier. 

9 

i8 

Echinus. 

Ecliinoderms. 

Boveri. 

?> 

Thysanozoon. 

Polyclades. 

Van  der  Stricht. 

» 

Sagitta. 

Chx'tognaths. 

Boveri. 

» 

Chittopterus. 

Annelids. 

Mead. 

» 

Ascidia. 

Tunicates. 

Boveri. 

lO 

20 

Lasius. 

Insects. 

Henking. 

II 

[22] 

Allolobophora. 

Annelids. 

Foot. 

12 

24 

iMyzostoma. 

» 

Wheeler. 

^  This  table  is  compiled  from  papers  both  on   fertilization  and  maturation, 
brackets  are  inferred. 


Numbers  in 


UNION  OF  THE    GEKM-CEU.S 


207 


Germ- 

Somatic 

TVT 

Nuclei. 

Nuclei. 

Name. 

Group. 

Authority. 

12 

24 

Thalassema. 

Annelids. 

Griffin. 

II  (12) 

22  (24) 

Cyclops  strenuus. 

Copepods. 

Riickert. 

12 

24 

brevicomis. 

?? 

Hacker. 

5? 

J? 

Helix. 

Gasteropods. 

PlatnerA'cjm  Rath. 

I-) 

V 

Branchipus. 

Crustacea. 

IJrauer. 

?> 

V 

Pyrrhocoris. 

Insects. 

Henkin;:. 

J> 

yy 

Salmo. 

Teleosts. 

0 

liohm. 

)7 

7? 

Salamandra. 

Ampliibia. 

F^lemminjr. 

I") 

?> 

Rana. 

«^ 

0 

Vom  Rath. 

V 

?j 

Mouse. 

Mammals. 

Sobotta. 

•>■> 

>? 

Osmunda. 

Ferns. 

Strasburs^er. 

V 

?j 

Lilium. 

Angiosperms. 

Strasbur;^er, 
Gui<(nard. 

7> 

?? 

Helleborus. 

?7 

Strasburger. 

?> 

?7 

Leucojum,  Pa^onia, 
Aconitum. 

7? 

Overton. 

M 

28 

Tiara. 

Hydromedusas. 

Boveri. 

,1? 

V 

Pieris. 

Insects. 

Henkinsr. 

i6 

32 

Cerebratulus,  Alicrura. 

Nemertines. 

Coe. 

?? 

77 

Pterotrachea,  Carinaria, 

Phyllirhoe. 

Gastropods. 

Boveri. 

)) 

[,;] 

Diaptomus,  Heterocope. 

Copepods. 

Riickert. 

?? 

-"J 

Anomalocera,  Euchaeta. 

77 

Vom  Rath. 

•  « 

.77J 

Lumbricus. 

Annelids. 

Calkins. 

i8 

36 

Torpedo,  Pristiurus. 

Elasmobranchs. 

Riickert. 

[18(19)] 

36(38) 

Toxopneustes. 

Echinoderms. 

Wilson. 

30 

[60] 

Crepidula. 

Gasteropods. 

ConkHn. 

84 

168 

Artemia. 

Crustacea. 

Brauer. 

The  above  data  are  drawn  from  sources  so  diverse  and  show  so 
remarkable  a  uniformity  as  to  establish  the  general  law  with  a  very 
high  degree  of  probability.  The  few  known  exceptions  are  almost 
certainly  apparent  only  and  are  due  to  the  occurrence  of  plurivalent 
chromosomes.  This  is  certainly  the  case  with  Asarris  {cf.  j).  ^j). 
It  is  probably  the  case  with  the  gasteropod  Ariou,  where,  as  described 
by  Platner,  the  egg-nucleus  gives  rise  to  numerous  chromosomes,  the 
sperm-nucleus  to  two  only  ;  the  latter  are,  however,  plurivalent,  for 
Garnault  showed  that  they  break  up  into  smaller  chromatin-bodies, 
and  that  the  germ-nuclei  are  exactly  alike  at  the  time  of  union.  W'c 
may  here  briefly  refer  to  remarkable  recent  observations  by  Riickert 
and  others,  which  seem  to  show  that  not  only  the  paternal  and  mater- 
nal chromatin,  but  also  the  chromosomes,  may  retain  their  individu- 
ality throughout  development.^     Van   Keneden,  the  pioneer  observer 

1  '89,  pp.  10,  2,2>' 


208  FERTILIZATIOX  OF   THE   OVUM 

in  this  direction,  was  unable  to  follow  the  paternal  and  maternal 
chromatin  beyond  the  first  cleavage-nucleus,  though  he  surmised  that 
they  remained  distinct  in  later  stages  as  well ;  but  Rabl  and  Hoveri 
broujjht  forward  evidence  that  the  chromosomes  did  not  lose  their 
identity,  even  in  the  resting  nucleus.  Riickert  ('95,  3)  and  Hacker 
('95,  I )  have  recently  shown  that  in  Cyclops  \.\\<i  paternal  and  mater- 
nal chromatin-groups  not  only  remain  distinctly  separated  during  the 
anaphase,  but  give  rise  to  double  nuclei  in  the  two-cell  stage  (Fig.  146). 
I^^ach  half  again  gives  rise  to  a  separate  grou])  of  chromosomes  at 
the  second  cleavage,  and  this  is  repeated  at  least  as  far  as  the  blas- 
tula  stage.  Herla  and  Zoja  have  shown  furthermore  that  if  in 
Ascaris  the  egg  of  variety  bivalois,  having  two  chromosomes,  be 
fertilized  with  the  spermatozoon  of  variety  univalens  having  one 
chromosome,  the  three  chromosomes  reappear  at  each  cleavage,  at 
least  as  far  as  the  twelve-cell  stage  (Fig.  145);  and  according  to  Zoja, 
the  paternal  chromosome  is  distinguishable  from  the  two  maternal  at 
each  step  by  its  smaller  size.  We  have  thus  what  must  be  reckoned 
as  more  than  a  possibility,  that  every  cell  in  the  body  of  the  child  may 
receive  from  each  parent  not  only  half  of  its  chromatin-substance, 
but  one-half  of  its  chromosomes,  as  distinct  and  individual  descendants 
of  those  of  the  parents. 

C.    The  Cextrosome  in  Fertilization 

In  examining  more  critically  the  history  of  the  centrosomes  we  may 
conveniently  take  Boveri's  hypothesis  of  fertilization  as  a  point  of 
departure,  since  it  has  long  formed  the  focus  of  discussion  of  the 
entire  subject.  Before  the  hypothesis  is  more  closely  scrutinized  we 
may  first  eliminate  two  other  views,  both  of  which  are  irreconcilable 
with  it,  though  neither  has  stood  the  test  of  later  research.  The  first 
of  these,  doubtfully  suggested  by  Van  Beneden  {''^J)  and  definitely 
maintained  by  Wheeler  ('97)  in  the  case  of  Myzostonia,  is  that  the 
cleavage-centrosomes  have  no  definite  relation  to  the  spermatozoon, 
but  are  derived  from  the  egg  —  a  conclusion  that  has  the  a  priori 
support  of  the  fact  that  in  parthenogenesis  the  centrosomes  are  cer 
tainly  of  maternal  origin. 

Van  Beneden's  early  statement  may  be  passed  by,  since  it  was  no 
more  than  a  surmise.  Wheeler,  after  a  careful  research,  found  that  no 
sperm-aster  accompanied  the  sperm-nucleus — a  fact  correlated  with 
the  absence  of  a  middle-piece  in  the  spermatozoon,  —  and  reached  the 
conclusion  that  after,  formation  of  the  polar  bodies,  the  egg-centro- 
somes  persisted  to  become  directly  converted  into  the  cleavage-centro- 
somes (Fig.  104).  That  the  absence  of  a  distinct  middle-piece  is  not  a 
valid  argument  is  shown  by  the  insect-spermatozoon,  where  the  region 


THE    CENTROSOME   IN  EERTILIZAriOX 


209 


of  the  middle-piece  is  likewise  not  marked  off  from  the  tail,  yet  as  we 
have  seen  (p.  165)  the  centrosome  passes  into  this  part  of  the  sperma- 
tozoon.    Kostanecki's   later   examination   of   the  fertilization  of  the 


Fig.  104.  —  Fertilization  of  the  &gg  of  the  parasitic  annelid,  Myzostoma.     [WUKKLHk.j 
A.  Soon  after  entrance  of  the   spermatozoon  ;    the  sperm-nucleus  at  cT  ;   at   9   the  germinal 
vesicle;  at  c  the  double  centrosome.    B.  First  polar  body  formins;  at  9  ;  ;/.  tlic  cast-out  nucle- 
olus or  germinal  spot.      C.  The  polar  bodies  formed  {p.b.)\  germ-nuclei  of  equal  size;  at  <:  the 
centrosomes.     D.  Approach  of  the  germ-nuclei ;   the  amphiaster  formed, 

same  animal  ('98),  while  inconclusive  on  the  main  point,  loaves  little 
doubt  that  Wheeler's  evidence  was  equally  so ;  for  he  has  on  the  one 
hand  shown  that  the  sperm-nucleus  is  often  accompanied  by  a  sperm- 


2IO  FERTILIZATION   OF  THE   OVUM 

aster  containing  a  pair  of  centrosomes,  on  the  other  hand  that  these, 
Hke  the  egg-centrosonies,  wholly  disappear  from  view  at  a  later  period, 
the  cleavage-centrosomes  having  only  a  conjectural  origin. 

The  second  of  the  views  in  question  is  that  the  cleavage-centro- 
somes are  derived  from  both  germ-cells  ;  and  this  in  turn  has  in  its 
favour  the  a  priori  evidence  that  in  the  Infusoria  conjugation  takes 
place  between  two  mitotic  figures  (p.  224).  It  appears  in  two  forms, 
of  which  the  first,  though  undoubtedly  erroneous,  has  had  so  interest- 
ing a  historv  as  to  deserve  a  brief  review.  It  was  predicted  by  Rabl 
in  i8;S9  that  if  the  centrosome  be  a  permanent  cell-organ,  the  con- 
jugation of  germ-cells  and  germ-nuclei  would  be  found  to  involve 
also  a  conjugation  of  centrosomes.  Unusual  interest  was  therefore 
aroused  when  Fol,  in  1891,  under  the  somewhat  dramatic  title  of  the 
"  Quadrille  of  Centres,"  described  precisely  such  a  conjugation  of 
centrosomes  as  Rabl  had  predicted.  The  results  of  this  veteran 
observer  were  very  positively  and  specifically  set  forth,  and  were  of 
so  logical  and  consistent  a  character  as  to  command  instant  accept- 
ance on  the  part  of  many  authorities.  In  the  eggs  of  the  sea-urchin 
the  sperm-centrosome  and  egg-centrosome  w^ere  asserted  to  divide 
each  into  two,  the  daughter-centrosomes  then  conjugating  two  and 
two,  paternal  with  maternal,  to  form  the  cleavage-centrosomes.  The 
same  result  was  announced  by  Guignard  ('91)  in  the  lily,  by  Conklin 
('93)  in  the  gasteropod  Crcpidnla,  less  definitely  by  Blanc  (93)  in  the 
trout,  and  still  later  by  Van  der  Stricht  (95)  in  AinpJiioxus.  None 
of  these  results  have  stood  the  test  of  later  w^ork.  Fol's  result  was 
opposed  to  the  earlier  conclusions  of  Boveri  and  Hertwig,  and  a  careful 
reexamination  of  the  fertilization  of  the  echinoderm  ^^^g,  indepen- 
dently made  in  1894-95  by  Boveri  {Ec/iinus),  by  myself  (  Toxopncustes\ 
and  Mathews  {Arbacia,  Astcrias\  and  slightly  later  by  Hill  (95)  and 
Reinke  ('95)  in  Splicerechiiius,  demonstrated  its  erroneous  character. 
Various  attempts  have  been  made  to  explain  Fol's  results  as  based  on 
double-fertilized  eggs,  on  imperfect  method,  on  a  misinterpretation  of 
the  double  centrosomes  of  the  cleavage-spindle,  yet  they  still  remain 
an  inexplicable  anomaly  of  scientific  literature. 

Serious  doubt  has  also  been  thrown  on  Conklin's  conclusions  by 
subsequent  research.  Kostanecki  and  Wierzejski  ('96)  made  a  very 
thorough  study,  by  means  of  serial  sections,  of  the  fertilization  of 
the  gasteropod  Pliysa,  and  reached  exactly  the  same  result  as  that 
obtained  in  the  echinoderms.  Here,  also,  the  egg-centres  degenerate, 
their  place  being  taken  by  a  new  pair,  arising  in  intimate  relation  with 
the  middle-piece  of  the  spermatozoon,  about  which  forms  a  sperm- 
amphiaster  (Fig.  89).  Conklin,  after  renewed  research,  himself 
admitted  that  no  quadrille  occurs  in  Cnpidula,  though  he  still  believes 
that  a  union  of  paternal  and  maternal  attraction-spheres  takes  place. 


THE   CENTROSOME  IN  FERTILIZATION 


21  r 


Guignard's  results,  too,  have  entirely  failed  of  confirmation  by  later 
observers  (p.  221),  and  in  his  own  latest  contribution  to  the  subject 
('99)  the  centrosomes  are  conspicuous  by  their  absence  in  both  the 
text  and  the  figures.  In  like  manner  Van  der  Stricht's  conclusions 
have  been  shown  by  Sobotta  ('97)  to  be  without  substantial  founda- 
tion, while  Blanc's  account,  opposed  to  the  earlier  work  of  I^ohm,  is 
too  incomplete  to  carry  any  weight.  The  entire  case  for  the  "  qua- 
drille "  has  thus  fallen  to  the  ground.  In  its  second  form  the  sup]:)o.sed 
double  origin  of  the  centrosomes  rests  upon  a  single  research  upon 
Ascaris  by  Carnoy  and  Le  Brun  ('97,  2),  who  assert  that  the  cleavage- 
centrosomes  arise  de  novo  and  separately,  one  inside  of  each  of  tlie 
germ-nuclei,  to  migrate  thence  out  into  the  cytoplasm.  At  the  close 
of  mitosis  they  wholly  disappear,  to  be  replaced  by  a  new  pair,  like- 
wise of  intranuclear  origin.  Since  this  result  is  totally  opposed  to 
those  of  Van  Beneden,  Boveri,  Erlanger,  and  Kostanecki  and  Sied- 
lecki  on  the  same  object,  and  is  contradicted  in  the  most  positive  man- 
ner by  Fiirst,!  it  may  be  received  with  some  scepticism.  The  work  of 
Kostanecki  and  Siedlecki  ('96)  demonstrates  the  division  of  the  sperm- 
centrosome  in  Ascaris  as  described  by  Boveri;  and  while  it  still 
remains  possible  that  the  daughter-centrosomes  may  for  a  very  brief 
period  disappear  (as  in  some  of  the  mollusks  described  beyond),  no 
ground  is  given  for  such  a  conclusion  as  Carnoy  has  drawn.  No  one 
familiar  with  the  object  can  repress  the  suspicion  that  Carnoy  and 
Le  Brun  have  confused  the  centrosomes  with  the  nucleoli ;  but  only 
renewed  research  can  determine  the  point. 

The  ground  is  now  clear  for  a  closer  study  of  Boveri's  hypothesis 
in  the  light  of  more  recent  research.  It  should  first  be  pointed  out 
that  that  hypothesis  is  based  upon  and  forms  a  part  of  the  more  gen- 
eral theory  of  the  autonomy  of  the  centrosome  ;  and  it  the  latter 
theory  cannot  be  sustained,  the  a  priori  side  of  Boveri's  hypothesis 
assumes  a  different  aspect.  In  point  of  fact  the  general  outcome  of 
recent  research  on  fertilization  has  been  on  the  whole  unfavourable  to 
the  view  that  the  cleavage-centrosomes  must  necessarily  be  intlividu- 
ally  identical  with  permanent  preexisting  centrosomes  —  indeed,  it  is 
in  this  very  field  that  some  of  the  most  convincing  evidence  against 
the  persistence  of  the  centrosome  has  been  produced.  The  mode  of 
origin  of  the  cleavage-centrosomes  is  nevertheless  a  question  of  high 
interest  on  account  of  the  unmistakable  genetic  relations  existing 
between  the  centrosome  of  the  spermatid  and  spermatozoon  and  those 
of  the  sperm-amphiaster  within  the  QgZ- 

There  are  two  points  of  capital  importance  to  be  determined  before 
a  definite  decision  re2:ardino:  the  origin  of  the  cleavage-centrosomes 
can  be  reached.     First,  are  the  centrosomes  of  the  sperm-aster  within 

i  '98,  p.  105. 


212  FERTILIZATJON   OF   THE    OVUM 

the  ^g^  identical  with,  or  the  descendants  of,  a  centrosome  or  pair 
of  centrosomes  in  the  middle-piece  of  the  spermatozoon  ?     Second, 
do  they  actually  persist  to  form  those  of   the  cleavao^e-amphiaster  ? 
In  the  present  state  of  knowledge  we  are  not  in  a  position  to  give  an 
affirmative  answer  to  the  first  of  these  questions.     As  has  been  shown 
in  Chapter  III.,  it   is  no  longer  possible  to  doubt  that  the  middle- 
piece  either  contains  or  is  itself  a  metamorphosed  centrosome  ;  but,  as 
pointed  out  at  page  196,  it  does  not  seem  possible  that  the  extremely 
minute  centrosome  of  the  sperm-aster  can  represent  the  entire  cen- 
trosome of  the  middle-piece  (however  we  conceive  the  origin  of  the 
latter).     At  most  we  can  only  assume  that  a  part  of  the  latter  per- 
sists as  the  sperm-centrosome  within  the  ^gg.     The  exact  origin  of 
the  latter  still  remains  problematical.     A  large  number  of  observers 
are  now  agreed  that  the  sperm-aster  is  formed  about  a  focus  that  is 
either  in  or  very  near  the  middle-piece  ;  ^  but  no  one,  I  believe,  has 
yet  succeeded  in  showing  that  the  centrosome  actually  is  the  meta- 
morphosed middle-piece,  or  escapes  from  it.-     The  possibility  there- 
fore remains  that  the  centrosome  of  the  sperm-aster  is  not  actually 
imported  as  such  into  the  Qgg,  but  is  either  only  a  portion  of   the 
original  spermatid-centrosome,  or,  as  was  first  suggested  by  Miss  Foot 
('97)  and  further  discussed  by  Mead  (98,  2),  is,  like  the  aster,  formed 
anew  in  the  egg-cytoplasm.     If  the  latter  alternative  be  the  case,  the 
original  form  of  Boveri's  hypothesis  would  have  to  be  abandoned; 


1  For  example,  in  echinoderms  (Flemming,  '81,  O.  and  R.  Hertwig,  '86,  Boveri,  '95, 
Wilson  and  Mathews,  '95,  Hill,  '95,  Reinke,  '95,  R.  Hertwig,  '96,  Doflein,  '97,  2,  Erlanger,  '98), 
in  rterotrachea  and  Pieris  (Henking,  '91,  '92),  in  the  axolotl  (Kick,  '93),  and  Triton 
(Michctlis,  '97).  in  Phalliisia  (Hill,  '95),  in  Ophryotrocha  (Korschelt,  '95),  in  Physa 
(Kostanecki  and  \Vier/.ejski, '96),  in  Strongylm  (Meyer,  '95),  in  Thysanozoon  (Van  der 
Stricht,  '98),  and  Prosthiostomum  (Francotte,  '98).  In  a  large  number  of  other  cases  the 
sperm-aster  is  found  near  the  sperm-nucleus,  but  its  relation  to  the  middle-piece  has  not 
been  demtmslrated. 

2  I  mvself  formerly  concluded  ('95,  2)  that  the  entire  middle-piece  of  echinoderms  is  the 
centnjsome  —  a  result  apparently  confirmed  in  a  most  positive  manner  by  Erlanger  ('98), 
as  well  as  by  R.  Hertwig  ('96)  and  Doflein  ('97,  2).  I  have,  however,  demonstrated  this 
to  be  an  error,  showing  that  the  extremely  minute  centrosome  is  (juite  distinct  from  the 
mi. Idle-piece,  the  latter  being  thrown  aside  and  degenerating  in  the  egg-cytoplasm  outside 
of  the  newly  formed  sperm-aster  (Figs.  12,  94).  This  fact,  of  which  the  phenomena  in 
Toxopufustes  leave  no  doubt  (see  Wilson,  '97,  '99),  is,  I  think,  fatal  to  Kostanecki's  and 
Wierzejski's  theory  of  fertilization  ('96,  pp.  374-375)'  according  to  which  the  archojilasm  of 
the  middle-piece  gives  rise  to  the  new  astral  system  and  is  thus  the  essential  fertilizmg  sub- 
stance (the  centrosome  being  merely  a  mechanical  centre  for  the  attachment  of  the  rays) ; 
but  the  most  careful  examination  has  still  failed  to  show  whether  the  centrosome  actually 
escapes  from  the  middle-piece,  nor  have  other  observers  had  l)ctter  success  with  any  animal. 
Erlanger  ('96,  2,  '97,  4)  believes  he  has  seen  the  centrosome  in  the  Ascaris  spermatozoon 
as  a  distinct  body  lying  behind  the  nucleus,  and  that  it  can  be  traced  continuously  into  the 
egg  and  after  its  division  into  the  two  poles  of  the  cleavage-figure.  Neither  the  schematic 
figures  of  his  preliminary  nor  the  photographic  ones  of  his  final  paper  seem  sufficient  to 
establish  either  the  identity  or  the  subsequent  history  of  the  granule  in  question. 


THE    CENTROSOME   I^f  FERTIIIZATJOX  21  3 

though  in  substance  it  would  still  retain  an  element  of  truth,  as  pointed 
out  beyond. 

We  may  now  examine  the  question  whether  the  sperm-centrosomcs 
are  actually  identical  with  the  cleavage-centrosomes.  That  such  is 
the  case  is  positively  maintained  in  the  case  of  Ascaris  by  I^overi 
Kostanecki,  and  Erlanger,  in  PJiysa  by  Kostanecki  and  \Vierzejski 
('96),  in  Thalassema  by  Griffin  ('96,  '99),  and  in  Chcetoptcrus  by  Mead 
('95,  '98).  The  two  last-mentioned  observers,  who  have  followed 
the  phenomena  with  especial  care,  produce  very  strong  evidence 
that  at  no  time  do  the  sperm-centrosomes  and  asters  disa])pear,  and 
that  the  former  may  be  traced  in  unbroken  continuity  from  the  time 
of  their  first  appearance  to  the  daughter-cells  resulting  from  the  first 
cleavage  (Figs.  99,  155).  On  the  other  hand,  a  considerable  number 
of  observers,  beginning  with  W^x\.\V\g{PhyllirrJioc\  Ptcrotmchca,  '75), 
have  found  that  as  the  sperm-nucleus  enlarges  the  sperm-asters  di- 
minish in  size,  until,  in  many  cases,  they  nearly  or  quite  disappear  ;  for 
example,  in  /^r^i-///^f^r^;/j-(KHnckowstrom,  '97),  in  the  mouse  (Sobotta, 
'95),  in  PlciiropJiyllidia  (MacFarland,  '97),  Pliysa  (Kostanecki  and 
Wierzejski,  '96),  Arenicola  {Q\i\\(\i,  '97),  6';//^  (Lillie,  '97),  Myzostoma 
(Kostanecki,  '98),  and  Cerebmtulus  (Coe,  '98).^  Several  of  these 
observers  (KUnckowstrom,  MacFarland,  Lillie,  Child)  have  found  that 
not  only  the  asters  but  also  the  ccntrosomes  totally  disappear  about 
the  time  the  germ-nuclei  come  together,  a  new  pair  of  cleavage- 
centrosomes  and  asters  being  afterward  developed  at  the  poles  of 
the  united  nuclei.  These  conclusions,  if  correct,  place  in  a  new 
Hght  the   disappearance   of   the  egg-centrosomes ;    for  this   process 

1  Coe  has  pointed  out  that  the  eggs  of  various  animals  may  be  arranged  in  a  scries  show- 
ing successive  graduations  in  the  disappearance  of  the  sperm-asters.  "  At  the  head  of  the 
series  we  must  place  the  eggs  of  Ascaris  and  I\[yzostouia  (according  to  Kostanecki)  and 
similar  ones  in  which  the  sperm-asters  make  their  appearance  only  a  short  time  befitrc  the 
formation  of  the  cleavage-spindle,  and  which,  consequently,  suffer  no  diminutit)n  in  si/c. 
Following  these  are  the  eggs  of  Chixtopterus  (Mead)  and  Ophryotrocha  (Korschclt)  and 
of  some  echinoderms  in  which  the  sperm-asters  develop  very  early,  but  are  not  described 
as  decreasing  in  size  before  the  formation  of  the  cleavage-spindle.  Then  cume  the  eggs  of 
Toxopnensies  (Wilson)  and  Thalassema  (Griffm),  where  the  sperm-asters  appear  early  and 
develop  to  a  very  considerable  size,  but  nevertheless  become  very  much  smaller  ant!  less 
conspicuous  after  the  germ-nuclei  have  come  together.  After  these  we  must  place  the  eggs 
of  Physa  (Kostanecki  and  Wierzejski),  for  here  the  sperm-asters,  after  becoming  very  large 
and  conspicuous,  degenerate  to  such  an  extent  that  only  a  very  few  exceeilingly  delicate 
fibres  remain.     Those  of  Cerebratulus  follow  next. 

"  Here  the  sperm-asters  increase  in  size  until  they  extend  thrt>ughout  the  whole  body 
of  the  cell,  but  at  the  time  of  fusion  of  the  germ-nuclei  they  degenerate  completely.  The 
peripheral  portions  of  their  fibres,  however,  may  be  followed,  as  stated  above  of  PUuro- 
phyllidia,  Prosthecerccus,  etc.,  where  the  sperm-asters  degenerate  soon  after  their  forma- 
tion, so  that  for  a  considerable  period  the  tg^  is  without  trace  of  aster-llbres.  Vet  in  all 
of  those  cases  where  the  sperm-asters  disappear  and  their  centrosomes  become  lost  among 
the  other  granules  of  the  cell,  we  are  justified  in  believing  that  the  sperm-centrosomes 
nevertheless  retain  their  identity,  and  later  reappear  in  the  cleavage-asters  "  ('98,  p.  455). 


214  FERTII.IZATroy  OF   THE    OVUM 

wuuld  thus  seem  to  be  of  the  same  nature  as  the  disappearance  of 
the  sperm-centrosomes,  and  both  Hoveri's  theory  of  fertilization  and 
the  f>-eneral  hypothesis  (^f  the  ]K"rmanence  of  the  centrosomes  would 
receive  a  serious  bhnv. 

The  investii^ators  to  whom  these  observations  are  due  have  ranged 
themselves  in  two  gnnips  in  the  interpretation  of  the  phenomena. 
On  the  one  hand.  Lillie  and  Child  do  not  hesitate  to  maintain  that 
the  centrosomes  actually  go  out  of  existence  as  such,  to  be  re-formed 
like  the  asters  out  of  the  egg-substance ;  and  that  such  a  new  forma- 
tion of  centrosomes  is  possible  seems  to  be  conclusively  shown  by  the 
experiments  of  Morgan  and  Loeb  described  at  pages  2 1 5  and  307.  On 
the  other  hand,  Sobotta,  .MacF'arland,  Kostanecki,  and  Coe,  relying 
partly  on  the  analogy  of  other  forms,  partly  on  the  occasional  pres- 
ence of  the  centrosomes  during  the  critical  stage,  urge  that  the  dis- 
appearance of  the  sperm-centrosomes  is  only  apparent,  and  is  due  to 
the  disappearance  of  the  asters,  which  renders  difficult  or  impossible 
the  identification  of  the  centrosomes  among  the  other  protoplasmic 
granules  of  the  c,^^.     These  authors  accordingly  still  uphold  Boveri's 

theory. 

It  is  difficult  to  sift  the  evidence  at  present,  for  it  has  now  become 
very  im])ortant  to  reexamine,  in  the  light  of  these  facts,  those  cases 
in  which  the  absolute  continuity  of  the  centrosome  has  been  main- 
tained -  for  example,  in  Ascaiis,  ChcBtopterus,  and  TJialassema  —  in 
order  to  determine  whether  there  may  not  be  here  also  a  brief  critical 
period  in  which  the  centrosomes  disappear.  There  are,  however, 
some  facts  which  tend  to  sustain  the  conclusion  that  even  though  the 
sperm-centrosomes  disappear  from  view,  there  is  some  kind  of  genetic 
continuity  between  them  and  the  cleavage-centrosomes.  First,  both 
Kostanecki  and  Wierzejski  (96)  and  Coe  ('98)  have  found  that  there 
is  some  variation  in  eggs  apparently  equally  well  preserved,  a  few 
individuals  showing  the  sperm-centrosomes  at  the  poles  of  the  united 
nuclei  at  the  same  period  when  they  are  invisible  in  other  individuals. 
Second,  both  these  observers,  Coe  most  clearly,  have  shown  that  the 
egg-centrosomes  di.sappear  considerably  earlier  than  the  sperm-cen- 
trosomes, and  Coe  has  traced  the  sperm-centrosomes  continuously  to 
the  exact  points  {tJic  poles  of  tJie  united  nuclei)  at  zvhicJi  the  cleavage- 
centrosomes  afterzvartl  appear  {V\g.  155).  This  important  observation 
leads  to  the  suspicion  that  the  apparent  disappearance  of  the  centro- 
somes may  be  due  to  a  loss  of  staining-capacity  at  the  critical  period, 
or  that  even  though  the  formed  centrosome  disappears  its  substance 
reappears  in  its  successor.  Here  again  we  come  to  the  view  sug- 
gested at  page  1 1 1,  that  the  centrosome  may  be  regarded  as  the  vehicle 
of  a  specific  chemical  substance  which  is  transported  to  the  nuclear 
poles  by  its  division,  and  may  there  persist  even  though  the  body  of  the 


FERTILIZATION  IN  PLANTS 


215 


centrosome  be  no  longer  visible.  On  such  a  basis  we  may  perhaps 
find  a  reconciliation  between  these  observations  and  Boveri's  theory, 
and  may  even  bring  the  fertilization  of  plants  into  relation  with  it 
(p.  221).  Even  in  case  of  the  nucleus,  universally  recognized  as  a 
permanent  cell-organ,  it  is  not  the  whole  structure  that  persists  as 
such  during  division,  but  only  the  chromatin-substance  —  in  some 
cases  only  a  small  fraction  of  that  substance.  The  law  of  genetic 
continuity  therefore  would  not  fail  in  case  of  the  centrosome,  though 
only  a  portion  of  its  substance  were  handed  on  by  division  ;  and  even 
if  we  take  the  most  extreme  negative  position,  assuming  that  the 
sperm-centrosome  is  wholly  formed  anew  under  the  stimulus  of  the 
spermatozoon,  we  should  still  not  escape  the  causal  nexus  between 
it  and  the  centrosome  of  the  spermatid. 

Boveri  himself  has  suggested  ^  that  the  ^gg  may  be  incited  to 
development  by  a  specific  chemical  substance  carried  by  the  sperma- 
tozoon, and  the  same  view  has  been  more  recently  urged  by  IMead,- 
while  Loeb's  recent  remarkable  experiments  on  sea-urchins  ('99)  show 
that  the  ^^g  may  in  this  case  {Arbacia)  undergo  complete  parthe- 
nogenetic  development  as  the  result  of  artificial  chemical  stimulus.-"^ 
Assuming  such  a  substance  to  exist,  by  what  part  of  the  spermato- 
zoon is  it  carried  .-*  It  is  possible  that  the  vehicle  may  be  the  nucleus, 
which  forms  the  main  bulk  of  that  which  enters  the  Q.gg ;  and  this 
view  seems  to  be  supported  by  what  is  at  present  known  of  fertiliza- 
tion in  the  plants  (p.  221).  Yet  when  we  regard  the  facts  of  fertili- 
zation in  animals,  taken  in  connection  with  the  mode  of  formation  of 
the  spermatozoon,  we  find  it  difficult  to  avoid  the  conclusion  that  the 
substance  by  which  the  stimulus  to  development  is  normally  given  is 
originally  derived  from  the  spermatid-centrosome,  is  conveyed  into 
the  Qgg  by  the  middle-piece,  and  is  localized  in  the  sperm-centro- 
somes  which  are  conveyed  to  the  nuclear  poles  during  the  am  phi- 
aster-formation.  Accepting  such  a  view,  we  could  gain  an  intelligible 
view  of  the  genetic  relation  between  spermatid-centrosome,  middle- 
piece,  sperm-centrosome,  and  cleavage-centrosomes,  without  commit- 
ting ourselves  to  the  morphological  hypothesis  of  the  persistence  of 
the  centrosome  as  an  individualized  cell-organ.  Such  a  conclusion, 
I  believe,  would  retain  the  substance  of  Boveri's  theory  while  leaving 
room  for  the  abandonment  of  the  too  simple  morphological  form  in 
which  it  was  originally  cast. 

D.     Fertilization  in  Plants 

The  investigation  of  fertilization  in  the  plants  has  always  lagged 
somewhat  behind  that  of  the  animals,  and  even  at  the  i)resent  time 

1  '91,  p.  431.  2  '98,   2,  p.    217.  8   (7:  p.   III. 


2l6 


FERTIUZATIOX  OF   THE    OVUM 


uur  knowledge  of  it  is  rather  incomplete.  It  is,  however,  sufficient 
to  show  that  the  essential  fact  is  everywhere  a  union  of  two  germ- 
j^^jclci  —  a  process  agreeing  fundamentally  with  that  observed  in 
animals.  On  the  other  hand,  almost  nothing  is  known  regarding  the 
centrosome  and  the  archoplasmic  or  kinoplasmic  structures;  and 
most  recent  observations  point  to  the  conclusion  that  in  the  lowering 
plants  and  pteridophytes  no  centrosomes  are  concerned  in  fertilization. 
Manv  earl v  observers  from  the  time  of  Pringsheim  ('55)  onward 
described  a  conjugation  of  cells  in  the  lower  plants,  but  the  union  of 
germ-Huclci,  as  far  as  I  can  find,  was  first  clearly  made  out  in  the 
Howering  plants  by  Strasburger  in  xZ'j'j-'j'^,  and  carefully  described 
by  him    in    1884.     Schmitz  observed  a  union  of    the   nuclei  of  the 


B 


Fig.  105.  —  I-'ertilization  in  Pilularia.     [Cami'HELL.] 

A.  /?.  Early  stages  in  tlie  formation  of  the  spermatozoid.     C.  The  mature  spermatozoid ;  the 

nucleus  lies  above  in  the  spiral  turns;  below  is  a  cytoplasmic  mass  containing  starch-grains  {cf. 

the  spermatozoJds  of  ferns  and  of  Marsilia,  Fig.  71).     D.  Archegonium  during  fertilization.  In 
the  centre  the  ovum  containing  the  apposed  germ-nuclei  (d",  9  ). 

conjugating  cells  of  Spirogyra  in  1879,  and  made  similar  observations 
on  other  algx  in  1S84,  Among  other  forms  in  which  the  same 
phenomenon  has  been  described  may  be  mentioned  Gidigoniuin 
(Klebahn,  '92),  Vauchcria  (Oltmanns,  '95),  Cystopus  (Wager,  '96), 
Splurrothcca  and  /:";;;'.v////r  (Harper,  '96),  /v/^/m' ( Farmer  and  Williams, 
'96,  Strasburger,  '97),  Inisidioboliis  (Fairchild,  '97),  Pilularia  (Fig. 
105,  Campbell,  '88),  Onoclca  (Shaw,  '98,  2),  Zamia  (Webber,  '97,  2), 
and  Lilitim  (Guignard,  '91,  Mottier,  '97),  Ginkgo  (Hirase,  '97).^  In 
all  of  these  forms  and  many  others  fertilization  is  effected  by  the 
union  of  a  single  paternal  and  a  single  maternal  uninucleated  cell, 
such  as  occurs  throughout  the  animal  kingdom.  There  are,  however, 
some  apparently  well-determined  exceptions  to  this  rule  occurring 
in  the  "compound"  multinucleate  oospheres  of  some  of  the  lower 


^  For  unicellular  forms  see  pp.  228,  280. 


FERTILIZATION  IN  PLANTS 


217 


plants.  In  Albugo  bliti  (one  of  the  Peronosporece),  for  example,  as 
shown  by  the  recent  work  of  Stevens  ('99),  the  mature  ovum  contains 
about  a  hundred  nuclei,  and  is  fertilized  by  a  multinucleate  proto- 
plasmic mass  derived  from  the  anthcridium,  each  nucleus  of  the  latter 
conjugating  with  one  of  the  egg-nuclei.  But  although  the  conjugat- 
ing bodies  are  here  multinucleate,  the  germ-nuclei  conjugate  two  and 
two  (as  is  also  the  case  in  the  multinucleate  cysts  of  ActinospJuEriuDi, 
p.  279);  and  the  case  therefore  forms  no  real  exception  to  the 
general  rule    that   one    paternal    nucleus   unites  with  one  maternal. 


C 


D 


Fig.  106.  —  Formation  of  the  ovum  and  penetration  of  the  pollen-tube  in  flowering  plants. 
[Strasburger.] 

A.  Embryo-sac  of  Monotropa,  showing  the  division  that  follows  the  two  maturation-divisions 
and  produces  the  upper  and  lower  "  tetrads."  B.  The  same,  ready  for  fertihzation.  sliowing  ovum 
{0),  synergidae  {s),  upper  and  lower  polar  cells  (/),  and  antipodal  cells  (<j).  C.  I'enetration  of 
the  pollen-tube  (/./.)  in  Orchis ;  o.  ovum,  with  synergidas  at  either  side,  g.n.  generative  nuclei  in 
the  pollen-tube.     D.  Slightly  later  stage  with  generative  nuclei  entering  the  micropyle. 

Whether  a  union  of  more  than  two  germ-nuclei  occurs  in  any  of  the 
lower  plants  is  a  question  still  disputed  by  botanists. ^  Such  i)lural 
fusion  is  rendered  a  priori  improbable  by  the  observations  thus  far 
made  upon  the  one-celled  forms  both  in  plants  and  in  animals ;  and 
the  known  facts  are  sufficient  to  show  that  it  must  be,  to  say  the 
least,  an  exceptional  process. 

In  cases  where  the  paternal  germ-cell  is  a  ciliated  spermatozoid,  as 
in  Fiicus,  Pilularia,  and  the  ferns  and  cycads,  the  germ-nuclei  differ 

1  Cf.  Hartog,  '91,  '96,  Trow,  '95,  Stevens,  '99,  Zimmerman,  '96,  and  literature  there  cited. 


2i8  '    FERTII.IZATIOX  OF   THE    OVUM 

more  or  less  widely  at  the  time  of  union,  the  sperm-nucleus  being 
smaller,  more  compact,  and  deeply  staining  (Figs.  105,  108),  as  is  the 
case  in  such  forms  of  fertilization  as  the  cchinoderm-egg.  In  the 
case  of  angiosperms  all  earlier  observers,  including  Strasburger  ('78, 
'84),  Guignard  ('91.  i  ),  and  Mottier  (97,  i ),  found  the  conjugating 
nuclei  to  be  closely  similar  at  the  time  of  union.  The  recent  obser- 
vations of  Guignard  ('99)  and  Nawaschin  (99)  show,  however,  that 
even  here  the  sperm-nucleus  is  smaller,  more  compact,  and  of  differ- 
ent form  (spindle-shaped)  from  the  egg-nucleua  (Fig.  107). 

The  ovum  or  oosphere  of  the  flowering  ])lant  is  a  large,  rounded 
cell  containing  a  large  nucleus  and  numerous  minute  colourless 
plastids  from  which  arise,  by  division,  the  plastids  of  the  embryo 
(chromatophores,  amyloplasts).  In  the  angiosperms  the  ovum  forms 
one  of  the  eight  cells  constituting  the  embryo-sac  which  morphologi- 
cally represents  the  female  prothallium  or  sexual  generation  of  the 
j)teridoj)hyte  and  is  itself  embedded  in  the  ovule  within  the  ovary. ^ 
The  male  germ-cells  are  represented  in  the  cycads  by  two  ciliated 
spcrmatozoids  (p.  175),  in  the  angiosperms  by  two  spindle-shaped 
"generative  nuclei"  which  are  suspected  by  Guignard  and  Nawaschin 
to  be  motile  bodies,  though  no  cilia  were  seen.  These  lie  near  the 
tip  of  the  pollen-tube  (Fig.  107),  which  is  developed  as  an  outgrowth 
from  the  pollen-grain  and  represents  a  rudimentary  male  prothallium 
or  sexual  generation. ^ 

The  formation  of  the  pollen-tube,  and  its  growth  down  through 
the  tissue  of  the  ])istil  to  the  ovule,  was  observed  by  Amici  ('23), 
Ikongniart  ('26),  and  Robert  Brown  ('31);  and  in  1833-34  Corda  was 
able  to  follow  its  tip  through  the  micropyle  into  the  ovule.'^  Stras- 
burger first  demonstrated  the  fact  that  the  generative  nucleus,  carried 
at  the  tip  of  the  pollen-tube,  enters  the  ovum  and  unites  with  the  egg- 
nucleus,  and  the  facts  have  been  since  carefully  studied  by  himself, 
by  Guignard,  Mottier,  Webber,  Ikeno,  Mirase,  and  a  number  of  others. 
In  the  cycads,  according  to  the  last-named  two  observers,  a  single 
spermatozoid  enters  the  egg,  its  nucleus  soon  fusing  with  that  of  the 

'  The  eight  cells  are  at  first  arranged  in  an  upper  and  a  lower  "  tetrad  "  of  four  cells  each, 
the  former  including  the  ovum,  two  synergida-,  and  an  "  upper  j)olar  cell,"  the  latter  a 
"lower  |X)lar  cell"  and  three  antipodal  cells  (Figs.  io6,  107);  cj.  p.  263. 

2  Cf.  p.  264. 

'  It  is  interesting  to  note  that  the  botanists  of  the  eighteenth  century  engaged  in  the  same 
fantastic  controversy  regarding  the  origin  of  the  enibryo  as  that  of  the  zoologists  of  the 
time.  Morcland  (1703),  followed  by  Etienne  Francois  Geoff roy,  Needham,  and  others, 
placed  himself  on  the  side  of  Leeuwenhoek  and  the  spermatists,  maintaining  that  the  pollen 
5,..  .,1.  .1  ji^p  embryo  which  entered  the  ovule  through  the  micropyle  (the  latter  had  been 
«!■  -  ;  '1  by  CIrew  in  1672)  ;  and  even  Schleiden  adopted  a  similar  viey'.  On  the  other 
hand,  Adanson  (1763)  and  others  maintained  that  the  ovule  contained  the  germ  which  was 
excited  to  development  by  an  aura  or  vapour  emanating  from  the  pollen  and  entering  through 
the  trachea  of  the  pistil. 


FERTILIZATION  IN  PLANTS 


219 


egg  (Fig.  108);  and  the  earlier  observers  of  the  angiosperms,  includ- 
ing Strasburger  ('84,  '88)  and  Guignard  ('91,  i),  likewise  found  that 
only  one  of  the  generative  nuclei  entered  the  embryo-sac.      Guignard 


//   • 


Fig.  107.  —  Fertilization  in  the  lily.     \^D  from  MOTTIEK,  the  others  from  GflGNARD.] 

A.  Embryo-sac,  ready  for  fertilization.  B.  Both  generative  nuclei  have  entered  the  embrvo- 
sac  ;  one  is  approaching  the  egg-nucleus,  the  other  uniting  with  the  upjier  polar  nucleus.  C'.  Union  of 
the  germ-nuclei ;  below,  union  of  the  second  generative  nucleus  and  the  two  polar  nuclei,  D.  'I"he 
fertilized  egg,  showing  fusion  of  the  germ-nuclei.  E.  The  fertilized  egg  dividing;  below,  division 
of  the  endosperm-nuclei,  a.  antipodal  cells ;  e.  endosperm-nuclei;  t'.  the  oosphere  or  ovum; 
/.  polar  nuclei ;  /.  t.  pollen-tube. 

and  Nawaschin  have,  however,  recently  made  the  remarkable  dis- 
covery that  in  Liliuni  and  Fntillaria  both  generative  nuclei  enter 
the  embryo-sac.     One  of  these  conjugates  with  the  egg-nucleus  and 


220 


FERTILIZATION  OF  THE   OVUM 


thus  effects  fertilization  (  Fig.  107).  The  other  coujugatcs  with  one  of 
the poiar  uuciei  {\is\\2l\\\  the  upper),  which  then  unites  with  the  other 
polar  nucleus  (</.  p.  264).  By  division  of  the  tertiHzed  egg  arises  the 
embryo  ;  while  hv  division  of  the  compound  nucleus  resulting  from  the 

fusion  of  the  polar  nuclei 
and  the  second  sperm  nu- 
cleus are  formed  the  endo- 
sperm-cells, which  serve 
for  the  nourishment  of  the 
embryo.  This  remarkable 
double  copulation  within 
the  embryo-sac  is  without 
a  parallel  and  is  of  wholly 
problematical  meaning,  but 
in  no  way  contradicts  the 
general  rule  regarding  the 
union  of  two  germ-nuclei 
to  produce  the  embryo.^ 

1  As  in  the  case  of  animals  (p. 
176),  the  germ-nuclei  of  phanero- 
gams also  show  marked  differ- 
ences in  structure  and  staining-reac- 
tion  before  their  union,  though  they 
ultimately  become  exactly  equiva- 
lent. Thus,  according  to  Rosen 
('92,  p.  443),  on  treatment  by 
fuchsin-methyl-blue  the  male  germ- 
nucleus  is  "  cyanophilous,"  the 
female  "  erythrophilous,''  as  de- 
scribed by  Auerbach  in  animals. 
Strasburger,  while  confirming  this 
observation  in  some  cases,  finds  the 
reaction  to  be  inconstant,  though 
the  germ-nuclei  usually  show  marked 
differences  in  their  staining-capac- 
ity.  These  are  ascril^cd  by  Stras- 
burger ('92,  '94)  to  differences  in 
the  conditions  of  nutrition  ;  by 
/acharias  and  Schwarz  to  corre- 
sponding differences  in  chemical 
composition,  the  male  nucleus  i)eing 
in  general  richer  in  nuclcin,and  the 
female  nucleus  poorer.  This  dis- 
tinction disappears  during  fertiliza- 
liun,  and  Strasburger  has  observed,  in  the  case  of  gymnosperms  (after  treatment  with  a 
mixture  of  fuchsin-iodine-green),  that  the  paternal  nucleus,  which  is  at  first  "  cyanophil- 
ous,"  .becomes  "  er)throphilous,"  like  the  egg-nucleus  before  the  pollen-tube  has  reached 
the  egg.  Within  the  egg  both  stain  exactly  alike.  These  facts  indicate,  as  Strasburger 
insists,  that  the  differences  between  the  germ-nuclei  of  plants  are,  as  in  animals,  of  a 
temporary-  and  non-essential  character. 


Fig.  108.  —  Fertilization  in  a  cycad,  /.anna.  [WEBBER.] 

A.  Spermatozoid.     B.  The  same  after  entrance  into 
the  egg.  showing  nucleus  («)  and  cilia-bearing  Ijand  (^r). 

C.  The  ovum  shortly  after  entrance  of  the  spermatozoid. 

D.  Union   of  the  germ-nuclei,  cilia-bearing   band   near 
periphery  (V). 


FERTILIZATION  IN  PLANTS  22  1 

The  nature  and  origin  of  the  achromatic  elements  involved  in  the 
fertilization  of  plants  is  still  almost  wholly  in  the  dark.  No  observer 
has  yet  succeeded  in  observing  either  centrosomes  or  asters  in  the 
fertilization  of  the  thallophytes,  despite  the  fact  that  in  some  of  these 
forms  mitosis  takes  place  with  both  these  structures  in  a  manner 
nearly  analogous  to  that  observed  in  animals.^  In  the  cycads  Zamia 
and  Cyras,  Webber  and  Ikeno  ('98)  agree  that  the  entire  spermato- 
zoid  enters,  but  only  the  nucleus  appears  to  be  concerned  in  fertiliza- 
tion. The  cilia-bearing  band  —  a  product  of  the  blepharoplast,  and, 
as  described  at  page  175,  probably  the  analogue  of  the  middle-piece 
of  the  animal  spermatozoon  —  remains  near  the  egg-peripherv,  gives 
rise  to  no  astral  or  other  fibrillar  formations,  and  apparentl)'  remains 
quite  passive  (Fig.  108). 

In  angiosperms,  too,  the  evidence  seems  to  show  that  no  centro- 
somes are  concerned  in  fertilization.  Guignard  ('91,  i ),  in  a  very 
detailed  and  clearly  illustrated  paper,  gave  an  account  of  the  centro- 
somes in  the  lily  agreeing  almost  exactly  with  the  "  quadrille  of 
centres"  as  described  by  Fol,^  paternal  and  maternal  centrosomes 
conjugating  two  by  two.  The  later  and  very  careful  studies  of  Mot- 
tier  and  others  have,  however,  entirely  failed  to  confirm  Guignard 's 
results,  the  germ-nuclei  fusing  without  the  participation  of  centro- 
somes or  astral  formations,  and  after  a  time  dividing,  without  centro- 
somes, in  the  manner  characteristic  of  the  higher  plants.'^  Neither 
in  the  cryptogams  has  any  one  thus  far  succeeded  in  finding  fertiliza- 
tion-centrosomes  or  asters  at  the  time  the  germ-nuclei  unite.  Stras- 
burger  contributes,  however,  the  interesting  observation  that  in  Fncns 
the  cleavage-centrosomes  afterward  appear  on  that  side  of  the 
cleavage-nucleus  derived  from  the  sperm-nucleus,  which  he  believes 
from  analogy  may  indicate  the  importation  of  a  "new  dynamic 
centre  "  into  the  Qgg  by  the  spermatozoid.'*  Combining  these  facts 
with  the  phenomena  involved  in  the  origin  of  the  spermatozoids, 
Strasburger  suggests  that  the  sperm-nucleus  may  import  into  tlie 
Qgg  either  a  formed  centrosome  (probably  thus  in  Finns)  or  a  cer- 
tain quantity  of  **  kinoplasm,"  which  incites  the  mitotic  phenomena 
in  the  absence  of  individualized  centrosomes.^  This  view  harmo- 
nizes with  that  suggested  at  pages  in  and  214,  and  we  may  perhaps 
here  in  the  end  find  a  reconciliation  between  the  various  types,  not 
only  of  fertilization  but  also  of  mitosis,  in  plants  and  animals. 

On  their  face  the  facts  of  fertilization  in  plants,  especially  in  the 
phanerogams,  seem  to  indicate  that  the  stimulus  to  develojiment 
is  given  by  the  paternal  germ-nucleus.  Nevertheless,  the  analogy  of 
animal  fertilization  would  lead  us  to  expect  that  the  fertilizing  sub- 

1  Cf.  p.  82.  3  cf.  p.  S2.  6  '97.  P-  420. 

2  Cf.  p.  210.  *  '97»  P-  418. 


223  FERTILIZATION   OF   THE    OVUM 

Stance  is  contained  not  in  the  nucleus  but  in  the  cytoplasm  —  more 
specifically,  in  the  case  of  spermatozoids,  in  the  cilia-bearing  body 
derived  from  the  blepharoplast,  which  in  its  development  so  strongly 
suggests  a  centrosome  (p.  172).  Webber's  and  I keno's  observations 
on  the  cycads  are  not  necessarily  fatal  to  this  view;  for,  as  I  have 
shown  (p.  188),  the  middle-piece  in  the  echinoderm  is  likewise  cast 
off  and  degenerates  near  the  periphery  of  the  ^^^,  and  the  ccntro- 
sonie  is  a  body  far  more  minute.  The  possibility  has  been  admitted 
that  this  centrosome  may  be  formed  dc  novo  under  the  influence  of 
the  middle-piece,  which  itself  ]:)erishes.  In  like  manner  it  may  also 
be  possible  that  the  primary  stimulus  in  Zai)iia  and  like  cases  is  given 
by  the  cilia-bearing  body,  even  though  this  body  itself  disappears  and 
the  mitotic  apparatus  is  not  formed  until  long  afterward. 


E.     Conjugation  in  Unicellular  Forms 

The  conjugation  of  unicellular  organisms  possesses  a  peculiar  inter- 
est, since  it  is  undoubtedly  a  prototype  of  the  union  of  germ-cells 
in  the  multicellular  forms.  Biitschli  and  Minot  long  ago  maintained 
that  cell-divisions  tend  to  run  in  cycles,  each  of  which  begins  and 
ends  with  an  act  of  conjugation.  In  the  higher  forms  the  cells  pro- 
duced in  each  cycle  cohere  to  form  the  multicellular  body  ;  in  the 
unicellular  forms  the  cells  separate  as  distinct  individuals,  but  those 
belonging  to  one  cycle  are  collectively  comparable  with  the  multi- 
cellular body.  The  validity  of  this  comparison,  in  a  morphological 
sense,  is  generally  admitted.^  No  process  of  conjugation,  it  is  true,  is 
known  to  occur  in  many  unicellular  and  in  some  multicellular  forms, 
and  the  cyclical  character  of  cell-division  still  remains  sub  jiidicc? 
It  is  none  the  less  certain  that  a  key  to  the  fertilization  of  higher 
forms  must  be  sought  in  the  conjugation  of  unicellular  organisms. 

The  difficulties  of  observation  are,  however,  so  great  that  we  are 
as  yet  acquainted  with  only  the  outlines  of  the  process,  and  have  still 
no  very  clear  idea  of  its  finer  details  or  its  physiological  meaning. 
The  phenomena  have  been  most  closely  followed  in  the  Infusoria  by 
Hutschli,  ICngclmann,  Maupas,  and  Richard  Hertwig,  though  many 
valuable  observations  on  the  -conjugation  of  unicellular  plants  have 
been  made  by  De  Hary,  Schmitz,  Klebahn,  and  Overton.  All  these 
obser\-ers  have  reached  the  same  general  result  as  that  attained 
through  study  of  the  fertilization  of  the  o.^^, ;  namely,  that  an  essen- 
tial phenomenon  of  conjugation  is  a  luiion  of  tJic  nuclei  of  the  conju- 
gating^ cells.  Among  the  unicellular  plants  both  the  cell-bodies  and 
the  nuclei  completely  fuse.     Among  animals  this  may  occur ;  but  in 

r/p.  58.  2  c/ p.  178. 


CONJUGATION  IN  UNICELLULAR  FORMS 


22^ 


many  of  the  Infusoria  union  of  the  cell-bodies  is  only  temporary,  and 
the  conjugation  consists  of  a  mutual  exchange  and  fusion  of  nuclei. 
It  is  impossible  within  the  limits  of  this  work  to  attempt  more  than  a 
sketch  of  the  process  in  a  few  forms. 

We    may   first  consider  the   conjugation  of   Infusoria.     Maupas's 
beautiful  observations  have  shown  that  in  this  group  the  life-history 


Second  fission. 


First  fission,  after  separation. 

Differentiation    of    iniirn-    nnd 
macronuclei. 


Separation  of  the  gametes. 


>  Division    of    the     cleavage-nu- 
cleus. 


—    Cleavage-nucleus. 

Exchange    and    fusion     of    the 
germ-nuclei. 

Germ-nuclei. 


Formation  of  the  polar  bodies. 


Union  of  the  gametes. 


Fig.  109.  —  Diagram  showing  the  history  of  the  micronuclei  during  the  conjugation  of  Para- 
moecium.     [Modified  from  Maupas.] 

♦  -V  and  F represent  the  opposed  macro-  and  micronuclei  in  the  two  respective  gametes;  circles 
represent  degenerating  nuclei ;  Vjlack  dots,  persisting  nuclei. 

of  the  species  runs  in  cycles,  a  long  period  of  multiplication  by  coil- 
division  being  succeeded  by  an  *' epidemic  of  conjugation."  which 
inaugurates  a  new  cycle,  and  is  obviously  comparable  in  its  physio- 
logical aspect  with  the  period  of  sexual  maturity  in  the  Metazoa.  If 
conjugation  does  not  occur,  the  race  rapidly  degenerates  and  dies  out ; 
and  Maupas  beheves  himself  justified  in  the  conclusion  that  conju- 


2^.  FERTILIZATIOX  OF  THE   OVUM 

o-ation   counteracts   the  tendency  to  senile   degeneration  and  causes 
rejuvenescence,  as  maintained  bv  Hiitschli  and  Minot.^ 

In  StvloHVi/iia  pustulata.  which  Miiupas  followed  continuously  from  the  end  of 
February  until  Juh",  the  first  conjugation  occurred  on  April  29th,  after  128  bi-parti- 
tii.ns;  and  the  epidemic  reached  its  iieight  three  weeks  later,  after  175  bi-partitions. 
'I'he  descendants  of  individuals  prevented  from  conjugation  died  out  through  '-senile 
iKueneracy."  after  316  bii)artitions.  Similar  facts  were  observed  in  many  other 
forms.  The  degeneracy  is  manifested  by  a  very  marked  reduction  in  size,  a  partial 
atrophy  of  the  cilia,  and  especially  by  a  more  or  less  complete  ihxradation  of  the 
nuiUar  apparatus.  I n  Stylonycliia  pustulata  and  Onyiliodromus ^i^randis  this  process 
especially  atTects  the  micronucleus,  which  atrophies,  and  finally  disappears,  though 
the  animals  still  actively  swim,  and  for  a  time  divide.  Later,  the  macronucleus 
becomes  irregular,  and  sometimes  breaks  up  into  smaller  bodies.  In  other  cases, 
the  degeneration  first  aflects  the  macronucleus,  which  may  lose  its  chromatin, 
undergo  fatty  degeneration,  and  may  finally  disappear  altogether  {StylonycJiia 
tnvtilus),  af'ter  which  the  micronucleus  soon  degenerates  more  or  less  completely, 
and  the  race  dies.  It  is  a  very  significant  fact  that  toward  the  end  of  the  cycle,  as 
the  nuclei  degenerate,  the  animals  become  incapable  of  taking  food  and  of  growth  ; 
and  it  is  probable,  as  Maupas  points  out.  that  the  degeneration  of  the  cytoplasmic 
organs  is  due  to  disturbances  in  nutrition  caused  by  the  degeneration  of  the  nucleus. 

The  more  essential  phenomena  occurring  during  conjugation  are 
as  follows.  The  Infusoria  possess  two  kinds  of  nuclei,  a  large 
macnmuclcHS  and  one  or  more  small  micronnclei.  During  conjuga- 
tion the  macronucleus  degenerates  and  disappears,  and  the  micronu- 
cleus alone  is  concerned  in  the  essential  part  of  the  process.  The 
latter  divides  several  times,  one  of  the  products,  the  gcrm-imclcus, 
conjugating  with  a  corresponding  germ-nucleus  from  the  other  indi- 
vidual, while  the  others  degenerate  as  "corpuscules  de  rebut."  The 
dual  nucleus  thus  formed,  which  corresponds  with  the  cleavage- 
nucleus  of  the  ovum,  then  gives  rise  by  division  to  both  macronuclei 
and  micronuclei  of  the  offspring  of  the  conjugating  animals  (Fig.  109). 

These  facts  may  be  illustrated  by  the  conjugation  of  Paramaxium 
caiidatiim,  which  possesses  a  single  macronucleus  and  micronucleus, 
and  in  which  conjugation  is  temporary  and  fertilization  mutual.  The 
two  animals  become  united  by  their  ventral  sides  and  the  macronu- 
cleus of  each  begins  to  degenerate,  while  the  micronucleus  divides 
twice  to  form  four  spindle-shaped  bodies  (Fig.  i  10,  A,  />).  Three  of 
these  degenerate,  forming-  the  "corpuscules  de  rebut,"  which  play 
no  further  part.  The  fourth  divides  into  two,  one  of  which,  the 
•'  female  pronucleus,"  remains  in  the  body,  while  the  other,  or  "male 
pronucleus,"  passes  into  the  other  animal  and  fuses  with  the  female 
pronucleus  (Fig.  1 10,  C-H ).  Each  animal  now  contains  a  cleavage- 
nucleus  equally  derived  from  both  the  conjugating  animals,  and  the 
latter  soon   separate.     The  cleavage-nucleus   in    each    divides   three 

1  C/p.  179. 


// 

Fig.  no.  —  Conjugation  of  Paramascium  caudatum.  [.-/-C,  after  R.  Hertwic. ;  D-K,  after 
Maupas.]     (The  macronuclei  dotted  in  all  the  figures.) 

A.  Micronuclei  preparing  for  their  first  division.  R.  Second  division.  C.  Third  division; 
three  polar  bodies  or  "  corpuscules  de  rebut,"  and  one  dividing  germ-nucleus  in  each  animal.  D. 
Exchange  of  the  germ-nuclei.  E.  The  same,  enlarged.  F.  Fusion  of  the  germ-nuclei.  G.  The 
same,  enlarged.  H.  Cleavage-nucleus,  {c)  preparing  for  the  first  division.  /.  The  cleavage- 
nucleus  has  divided  twice,  y.  After  three  divisions  of  the  cleavage-nucleus;  macronucleus 
breaking  up.     K.  Four  of  the  nuclei  enlarging  to  form  new  macronuclei.     The  first  fission  soon 


226 


FERTILIZATIOX  OF  THE   OVUM 


times  successively,  and  of  the  eight  resulting  bodies  four  become 
macronuclei  and  four  micronuclei  (Fig.  no,  H-K).  By  two  suc- 
ceeding fissions  the  four  macronuclei  are  then  distributed,  one  to  each 
of  the  four  resulting  individuals.  In  some  other  species  the  micro- 
nuclei  are  equally  distributed  in  like  manner,  but  in  J\  caudatnni  the 
j)rocess  is  more  complicated,  since  three  of  them  degenerate,  and 
the  fourth  divides  twice  to  produce  four  new  micronuclei.  In  cither 
case  at  the  close  of  the  process  each  of  the  conjugating  individuals 


B 


Fig.  III.  —  Conjugation  of  Vorticellids.     [MaL'TAS.] 
A.  Attachment   of  the  small  frec-swimining  microgamete  to   the   large  fixed   macrogamete ; 

micronucleus  dividing    in  each   (Carc/iesiuin).     B.  Microgamete   containing  eight   micronuclei; 

;  •  imete  four  (  I'orticella).     C.  All  but  one  of  the  micronuclei   have  degenerated  as  polar 

,    .  t>r"corpuscules  de  rebut."    D.  liach  of  the  micronuclei  of  the  last  stage  has  divided  into 

.  to  form  the  germ-nuclei ;  two  of  these,  one  from  each  gamete,  have  conjugated  to  form  the 

cleavage-nucleus  seen  at  the  left ;  the  other  two,  at  the  right,  are  degenerating. 

has  given  rise  to  four  descendants,  each  containing  a  macronucleus 
and  micronucleus  derived  from  the  cleavage-nucleus.  From  this  time 
forward  fission  follows  fission  in  the  usual  manner,  both  nuclei  divid- 
ing at  each  fission,  until,  after  many  generations,  conjugation  recurs. 
Fssentially  similar  facts  have  been  observed  by  Richard  Hertwig 
and  Maupas  in  a  large  number  of  forms.  In  cases  of  permanent 
conjugation,  as  in  l\)rticclla,  where  a  smaller  microgamete  unites  with 
a  larger  ^nacrogamcte,  the  process  is  essentially  the  same,  though  the 
details  are  still  more  complex.  Here  the  germ-nucleus  derived  from 
each  gamete  is  in  the  macrogamete  one-fourth  and  in  the  microgamete 


CONJUGATION  IN  UNICELLULAR  FORMS 


227 


one-eighth  of  the  original  micronucleus  (Fig.  i  1 1 ).  Each  germ- 
nucleus  divides  into  two,  as  usual,  but  one  of  the  products  of  each 
degenerates,  and  the  two  remaining  pronuclei  conjugate  to  form  a 
cleavage-nucleus. 

The  facts  just  described  show  a  very  close  parallel  to  those  observed 
in  the  maturation  and  fertilization  of  the  ^g^^.  In  both  cases  there 
is  a  union  of  two  similar  nuclei  to  form  a  cleavage-nucleus  or  its 
equivalent,  equally  derived  from  both  gametes,  and  this  is  the  pro- 
genitor of  all  the  nuclei  of  the  daughter-cells  arising  by  subsequent 
divisions.  In  both  cases,  moreover  (if  we  confine  the  comparison 
to  the  ^gg),  the  original  nucleus  does  not  conjugate  with  its  fellow 
until  it  has  by  division  produced  a  number  of  other  nuclei  all  but 
one    of  which   degenerate.     Maupas   does   not    hesitate    to   compare 


^  Be 

Fig.  112,  —  Conjugation  oi Noctiluca.     [Ishikawa.] 

A.  Union  of  the  gametes,  apposition  of  the  nuclei.  D.  Complete  fusion  of  the  gametes. 
Above  and  below  the  apposed  nuclei  are  the  centrosomes.  C.  Cleavage-spindle,  consisting  of 
two  separate  halves. 

these  degenerating  nuclei  or  "corpuscules  de  rebut"  with  the  polar 
bodies  (p.  181),  and  it  is  a  remarkable  coincidence  that  their  number, 
like  that  of  the  polar  bodies,  is  often  three,  though  this  is  not  always 
the  case. 

A  remarkable  peculiarity  in  the  conjugation  of  the  Infusoria 
is  the  fact  that  tJie  germ-nuclei  ujiite  ivJien  in  tJic  form  of  spimiies 
or  mitotic  figures.  These  spindles  consist  of  achromatic  fibres,  or 
*'archoplasm,"  and  chromosomes,  but  no  asters  or  undoubted  cen- 
trosomes have  been  thus  far  seen  in  them.  During  union  the 
spindles  join  side  by  side  (Fig.  lio,  G\  and  this  gives  good  reason 
to  believe  that  the  chromatin  of  the  two  gametes  is  equally  distrib- 
uted to  the  daughter-nuclei  as  in  Metazoa.  In  the  conjugation  of 
some  other  Protozoa  the  nuclei  unite  while  in  the  resting  state;  but 
very  little  is  known  of  the  process  save  in  the  cystoflagellate  Xocti- 
luca,  which  has  been  studied  with  some  care  by  Cienkowsky  and 
Ishikawa  (Fig.  112).  Here  the  conjugating  animals  completely  fuse, 
but  the  nuclei  are  merely  apposed  and  give  rise  each  to  one-half  of 


228 


FERriUZATION   OF   THE    OVUM 


the  mitotic  ^^wxo..     At  either  pole  of  the  spindle  is  a  centrosome,  the 
orif(in  of  which  remains  undetermined. 

It  is  an  interestini;  fact  that  in  Xoctilnca,  in  the  gregarines,  and 
probably  in  some  other  Protozoa,  conjugation  is  followed  by  a  very 
rapid  multiplication  of  the  nucleus  followed,  by  a  corresponding  divi- 
sion of  the  cell-body  to  form  "spores,"  which  remain  for  a  time 
closely  aggregated  before  their  liberation.     The  resemblance  of  this 


D 

Fig.  113.  — Conjugation  oi  Spirogyta,     [OVERTON.] 

A.  Union  of  the  conjugating  cells  {S.  communis).  B.  The  typical,  though  not  invariable, 
mode  of  fusion  in  S.  Weber i ;  the  chromatophore  of  the  "female"  cell  breaks  in  the  middle, 
while  that  of  the  "  male  "  cell  passes  into  the  interval.  C.  The  resulting  zygospore  filled  with 
pyrenoids.  before  union  of  the  nuclei.  D.  Zygospore  after  fusion  of  the  nuclei  and  formation 
of  the  membrane. 


process  to  the  fertilization  and  subsequent  cleavage  of  the  ovum  is 
particularly  striking. 

The  conjugation  of  unicellular  plants  shows  some  interesting 
features.  Here  the  conjugating  cells  completely  fuse  to  form  a 
"zygospore"  (Figs.  113,  140),  which  as  a  rule  becomes  surrounded 
by  a  thick  membrane,  and,  unlike  the  animal  conjugate,  may  long 
remain  in  a  quiescent  state  before  division.     Not  only  do  the  nuclei 


SUMMARY  AXD    COXCLUSION  220 

unite,  but  in  many  cases  the  plastids  also  (chromatophores).  In 
Spirogyra  some  interesting  variations  in  this  regard  have  been  ob- 
served. In  some  species  De  Bary  has  observed  that  the  long  band- 
shaped  chromatophores  unite  end  to  end  so  that  in  the  zvgote  the 
paternal  and  maternal  chromatophores  lie  at  opposite  ends.  In 
5.  Weberi,  on  the  other  hand,  Overton  has  found  that  the  single 
maternal  chromatophore  breaks  in  two  in  the  middle  and  the  paternal 
chromatophore  is  interpolated  between  the  two  halves,  so  as  to  lie 
in  the  middle  of  the  zygote  (Fig.  113).  It  follows  from  this,  as  De 
Vries  has  pointed  out,  that  the  origin  of  the  chromatophores  in  the 
daughter-cells  differs  in  the  two  species,  for  in  the  former  case  one 
receives  a  maternal,  the  other  a  paternal,  chromatophore,  while  in 
the  latter,  the  chromatophore  of  each  daughter-cell  is  equally  derived 
from  those  of  the  two  gametes.  The  final  result  is,  however,  the 
same;  for,  in  both  cases,  the  chromatophore  of  the  zygote  divides 
in  the  middle  at  each  ensuing  division.  In  the  first  case,  therefore, 
the  maternal  chromatophore  passes  into  one,  the  paternal  into  the 
other,  of  the  daughter-cells.  In  the  second  case  the  same  result  is 
effected  by  two  succeeding  divisions,  the  two  middle-cells  of  the  four- 
celled  band  receiving  paternal,  the  two  end-cells  maternal,  chro- 
matophores. In  the  case  of  a  Spirogyra  filament  having  a  single 
chromatophore  it  is  therefore  "wholly  immaterial  whether  the  indi- 
vidual cells  receive  the  chlorophyll-band  from  the  father  or  the 
mother"  (De  Vriesy 

F.     Summary  and  Conxlusion 

All  forms  of  fertilization  involve  a  conjugation  of  cells  bv  a 
process  that  is  the  exact  converse  of  cell-division.  In  the  lowest 
forms,  such  as  the  unicellular  algae,  the  conjugating  cells  are,  in  a 
morphological  sense,  precisely  equivalent,  and  conjugation  takes 
place  between  corresponding  elements,  nucleus  uniting  with  nucleus, 
cell-body  with  cell-body,  and  even,  in  some  cases,  plastid  with  plastid. 
Whether  this  is  true  of  the  centrosomes  is  not  known,  but  in  the 
Infusoria  there  is  a  conjugation  of  the  achromatic  spindles  which 
certainly  points  to  a  union  of  the  centrosomes  or  their  equivalents. 
As  we  rise  in  the  scale,  the  conjugating  cells  diverge  more  and  more, 
until  in  the  higher  plants  and  animals  they  differ  widely  not  only 
in  form  and  size,  but  also  in  their  internal  structure,  and  to  such  an 
extent  that  they  are  no  longer  equivalent  either  morphologically  (^r 
physiologically.     Both  in  animals  and  in   plants  the  paternal  gerni- 

1  De  Vries's  conclusion  is,  however,  not  entirely  certain;  for  it  is  impossible  to  deter- 
mine, save  by  analogy,  whether  the  chromatophores  maintain  their  indivitluality  in  the 
zygote. 


3^0  FERTJLIZATIOX  OF   THE    OVUM 

cell  loses  most  of  its  cytoplasm,  the  main  bulk  of  which,  and  hence 
the  main  body  of  the  embryo,  is  now  supi)lied  by  the  egg;  and 
in  the  higher  plants,  the  egg  alone  retains  the  plastids  which 
are  thus  supplied  by  the  mother  alone.  On  the  other  hand,  the 
paternal  germ-cell  is  the  carrier  of  something  which  incites  the  (t^z 
to  develoi)ment,  and  thus  constitutes  the  fertilizing  element  in  the 
narrower  sense.  There  is  strong  ground  for  the  conclusion  that  in 
the  animal  spermatozocin  this  element  is,  if  not  an  actual  centro- 
some,  a  body  or  a  substance  directly  derived  from  a  centrosome  of 
the  parent  body  and  contained  in  the  middle-piece.  Boveri's  theory, 
according'-  to  which  fertilization  consists  essentially  of  the  replace- 
ment of  a  missing  or  degenerating  egg-centrosomc  by  the  importation 
of  a  sperm-centrosome,  was  stated  in  too  simple  and  mechanical  a 
form  ;  for  the  facts  of  spermatogenesis  show  conclusively  that  the 
sjiermatid-centrosome  is  not  simply  handed  on  unmodified  by  the 
spermatozoon  to  the  Qgg.  and  the  theory  wholly  breaks  down  in 
the  case  of  the  higher  ])lants.  l^ut  although  the  theory  probably 
cannot  be  sustained  in  its  morphological  form,  it  may  still  contain 
a  large  element  of  truth  when  recast  in  physiological  terms.  Like 
mitosis,  fertilization  is  perhaps  at  bottom  a  chemical  process,  the 
stimulus  to  development  being  given  by  a  specific  chemical  substance 
carried  in  some  cases  by  an  individualized  centrosome  or  one  of  its 
morphological  products,  in  other  cases  by  less  definitely  formed 
material.  In  the  case  of  animals,  we  cannot  ignore  the  historical 
continuity  shown  in  the  origin  of  the  spermatid-centrosomes,  the 
formation  of  the  middle-piece,  and  the  origin  of  the  sperm-centro- 
somcs  and  sperm-amphiaster  in  the  (tgg,  even  though  we  do  not 
yet  know  whether  the  sperm-centrosome  is  as  such  imported  into 
the  egg.  And  this  chain  of  phenomena  suggests  that  even  in  the 
higher  plants,  where  no  centrosomes  seem  to  occur,  the  fertilizing 
substance,  even  if  brought  into  the  ^^^^  in  an  unformed  state,  may 
still  be  genetically  related  to  the  mitotic  apparatus  of  the  preceding 
division.^ 

Through  the  differentiation  between  the  paternal  and  germ-cells 
in  the  higher  forms  indicated  above,  their  original  morphological 
ecjuivalence  is  lost  and  only  the  nuclei  remain  of  exactly  the  same 
value.  This  is  shown  by  their  history  in  fertilization,  each  giving 
rise  to  the  same  number  of  chromosomes  exactly  similar  in  form, 
size,  and  staining-reactions,  equally  distributed  by  cleavage  to  the 
daughter-cells,  and  probably  to  all  the  cells  of  the  body.  Wc  tJius 
find  the  essential  fact  of  fertilization  and  sexual  reproduction  to  be  a 
union  of  equivalent  nuclei  ;  and  to  tJiis  all  ot/ier  processes  are  tributary. 

As  regards  the  most  highly  differentiated  type  of  fertilization  and 

1  Cf.  Strasburger's  view,  ]).  221. 


LITERATURE  231 

development  we  reach  therefore  the  following  conception  .  From  the 
mother  comes  in  the  main  the  cytoplasm  of  the  embryonic  body  which 
is  the  principal  substratum  of  growth  and  differentiation.  From  both 
parents  comes  the  hereditary  basis  or  chromatin  by  which  these  pro- 
cesses are  controlled  and  from  which  they  receive  the  specific  stamp  of 
the  race.  From  the  father  comes  the  stimulus  inducing  the  organiza- 
tion of  the  machinery  of  mitotic  division  by  which  the  ^^^^  splits  up 
into  the  elements  of  the  tissues,  and  by  which  each  of  the.se  elements 
receives  its  quota  of  the  common  heritage  of  chromatin.  Huxley  hit 
the  mark  two  score  years  ago  when  in  the  words  that  head  this  chap- 
ter he  compared  the  organism  to  a  web  of  which  the  warp  is  derived 
from  the  female  and  the  woof  from  the  male.  Our  principal  advance 
upon  this  view  is  the  knowledge  that  this  web  is  probably  to  be  sought 
in  the  chromatic  substance  of  the  nuclei;  and  perhaps  we  shall  not 
push  the  figure  too  far  if  we  compare  the  amphiaster  to  the  loom  on 
which  the  fabric  is  woven. 


LITERATURE.      IV 1 

Van  Beneden,  E.  —  Recherches  sur  la  maturation  de  roeuf,  la  fecondation  et  la  divi- 
sion cellulaire  :  Arch.  Biol.,  IV.      1883. 
Van  Beneden  and  Neyt. — Nouvelles  recherches  sur  la  fecondation  et  la  division 

mitosique  chez  TAscaride  megalocephale :  Bull.  Acad.  roy.  de  Belgique.  III.  14. 

No.  8.     1887. 
Boveri,  Th. — Uber  den  Anteil  des  Spermatozoon  an  der  Teilung  des  Eies  :  Sitz.- 

Ber.  d.  Ges.f.  Morph.  u.  PJiys.  in  Miincheiu  B.  III.,  Heft  3.     1887. 
Id.  — Zellenstudien,  II.     1888.  j 

Id. —  Befruchtung:  Merkel  intd  Bonnefs  Ergeb7tisse,l.     1891. 
Id.  —  Uber  das  Verhalten  der  Centrosomen  bei  der  Befruchtung  des  Seeigeleies.  etc. : 

Verhandl.  Phys.  Med.  Ges.  Wnrsburg,  XXIX.     1895. 
Biitschli,  0. —  Studien  Uber  die  ersten  Entwicklungsvorgange  der  Eizelle,  it.  s.  zc. : 

Abh.  Senckenb.  Ges.,  X.     1876. 
Coe,  W.  R.,  99.     The  Maturation  and  Fertilization  of  the  Egg  of  Cerebratulus  :  /^ool. 

Jahrb.,  XII. 
Fick,  R.  —  tjber  die  Reifung  and  Befruchtuns;  des  Axolotleies  :  Zcitsclir.  W'iss.Zodl., 

LVI.  4.     1893. 
Griffin,  B.  B.  —  Studies  on  the  Maturation,  Fertilization,  and  Cleavage  ot  Thalassema 

and  Zirphaea:  Journ.  Morph.,  XV.     1899. 
Guignard,  L.  —  Nouvelles   etudes   sur   la   fecondation:  Ann.  d.  Sciences   nat.  Bot., 

XIV.     1 89 1. 
Hartog,  M.  M.  —  Some    Problems    of  Reproduction,  etc.  :   Quart.  Journ.  Mu.  ^t/.. 

XXXIII.     1891. 
Hertwig,  0.  —  Beitrage   zur  Kenntniss   der  Bildung,   Befruchtuiii;  unci  Teilunij  iles 

tierischen  Eies,  I.  :  MorpJi.  JaJirb.,  I.      1875. 
Hertwig,  R. — Uber  die  Konjugation  der  Infusorien:  Abh.  d.  bayr.  Akad.  d.  ll'iss., 

II.  CI.  XVII.     1888-89. 
Id.  —  ijber  Befruchtung  und  Konjugation  :    I'erh.  deutsch.  Zo'dl.  Ges.  Berlin,  1892. 

1  See  also  Literature,  V..  p.  2S7. 


2}2 


FERTILIZATION   OF   THE    OVUM 


Kostanecki,  K.  v.,  and  Wierzejski,  A. — Uber  das   \'erhalten   der  sogen.  achromati- 

schen  Substanzcn  iiu  bctruchteten  Ei  :   Arch.  mik.  A//a/.,XL\'\l.  2.     1896. 
Mark,  E.  L.  —  Maturation.    Fecundation,  and   Segmentation  of  Umax  ca/Jipcstris : 

Hull.  Mm.  Com  p.  Zool.  Harvard  Collci^e.  L'ainhridi:^t\  Mass.,  \'l.     1881. 
Maupas.  —  Lc    rejeunissement    karyoganiique    chez    les  Cili(5s :  An/i.  d.   Zo'oL,  2"^« 

sorio.  \11.      18S9. 
Mead,  A.  D.      The  Origin  and  Heliaviour  of  the  Centrosomes  of  tlie  Annelid  Egg: 

Journ.  Morph.,  X I  \'.  2 .      1 898. 
Ruckert,  J.  —  Uber  das  Selbstiindigbleiben  der  vaterlichen  und  miittcrlichen  Kern- 

substanz  wiihrend  der  ersten  Entwickiung  des  befruchteten  Cyclops-Eies  :  Arch. 

mik.  .htat..  \L\'.  3.      1895. 
Strasburger,  E.  —  Neue    L'ntersuchungen    iibcr   den    Hefruchtungsvorgang   bei    den 

I'h.inerogamen,  als  Crundlage  t'iir  eine  Theorie  der  Zeugung.    Jena,  1884. 
Id.  — L'ber   Kern-  und    Zellteilung    im    Tflanzenreich,    nebst    einem    Anhang   liber 

Bcfnichtung.    Jena,  18S8.     (See  Literature  II.) 
Vejdovsky,    F.  —  EntwickeUmgsgeschichtliche    Untersuchungen,    Heft     i.    Reifung, 

liefruchtung  und  Furchung  des  Rhynchehnis-Eies.     Prag,  1888. 
Waldeyer,  W.  —  Befruchlung  und  \'ererbung  :    /  'crh.  Ges.  deutsch.  iVaturf.  u.  Aerzte^ 

\k\\.     1897. 
Wilson,  Edm.  B.  — Atlas  of  Fertilization  and  Karyokinesis.     A\^d'  York,  1895. 
Zoja,  R.  —  Stato  Attuale  degli  Studi  sulla  Fecondazione  :  Boll.  Scientif.  di  Pavia-, 

XVlil..  XIX.     1896-97. 


CHAPTER   V 

OOGENESIS  AND   SPERMATOGENESIS.     REDUCTION  OF  THE 

CHROMOSOMES 

"  Es  konimt  also  in  der  Generationenreihe  der  Keimzelle  irgendwo  zu  einer  Reduktion 
der  ursprunglich  vorhandenen  Chromosomenzahl  auf  die  Halfte,  und  diese  Zr?///^';;. reduk- 
tion ist  demnach  nicht  etwa  nur  ein  theoretisches  Postulat,  sondern  eine  Thatsache." 

BOVKKI.I 

Van  Beneden's  epoch-making  discovery  that  the  nuclei  of  the  con- 
jugating germ-cells  contain  each  one-half  the  number  of  chromosomes 
characteristic  of  the  body-cells  has  now  been  extended  to  so  many 
plants  and  animals  that  it  may  probably  be  regarded  as  a  universal 
law  of  development.  The  process  by  which  the  reduction  in  number 
is  effected,  forms  the  most  essential  part  of  the  phenomena  of  matura- 
tion by  which  the  germ-cells  are  prepared  for  their  union.  No  phe- 
nomena of  cell-life  possess  a  higher  theoretical  interest  than  these. 
For,  on  the  one  hand,  nowhere  in  the  history  of  the  cell  do  we  find  so 
unmistakable  and  striking  an  adaptation  of  means  to  ends  or  one  of 
so  marked  a  prophetic  character,  since  maturation  looks  not  to  the 
present  but  to  the  future  of  the  germ-cells.  On  the  other  hand,  the 
chromatin-reduction  suggests  questions  relating  to  the  morphological 
constitution  of  nucleus  and  chromatin,  which  have  an  important 
bearing  on  all  theories  of  the  ultimate  structure  of  living  matter  and 
now  stand  in  the  foreground  of  scientific  discussion  among  the  most 
debatable  and  interesting  of  biological  problems. 

Two  fundamentally  different  views  have  been  held  of  the  manner 
in  which  the  reduction  is  effected.  The  earlier  and  simpler  view, 
which  was  suggested  by  Van  Beneden  and  adopted  in  the  earlier 
works  of  Weismann,  Boveri,  and  others,  assumed  an  actual  degenera- 
tion or  casting  out  of  half  of  the  chromosomes  during  the  growth 
of  the  germ-cells  —  a  simple  and  easily  intelligible  process.  Later 
researches  conclusively  showed,  however,  that  this  view  cannot  be 
sustained,  and  that  reduction  is  effected  by  a  rearrauiienient  and  redis- 
tribution of  the  7iuciear  siibstance  without  loss  of  an\'  of  its  essential 
constituents.  It  is  true  that  a  lars^e  amount  of  chromatin  is  lost  dur- 
ing  the  growth  of  the  Qggr-  It  is  nevertheless  certain  that  this  loss  is 
not  directly  connected  with  the  process  of  reduction  ;  for,  as  Hertwig 


1  Zellenstiidien,  IH.,  p.  62.  '^  Cf.  Figs.  97,  116. 


234 


REDUCTION   OF   THE  CHROMOSOMES 


and  others  have  shown,  no  such  loss  occurs  during  spermatogenesis, 
and  even  in  the  oogenesis  the  evid^ence  is  clear  that  an  explanation 
must  be  sought  in  another  direction.  The  attempts  to  find  such  an 
explanation  have  led  to  some  of  the  most  interesting  researches  of 
modern  cytology  ;  and  though  only  partially  successful,  they  have 
raised  manv  new  questions  which  ])romise  to  give  in  the  end  a  deeper 
insight  into  some  of  the  fundamental  questions  of  cell-morphology. 
Kor  this  reason  they  deserve  careful  consideration,  despite  the  fact 
that  taken  as  a  whole  the  subject  still  remains  an  unsolved  riddle  in 
the  face  of  which  we  can  only  return  again  and  again  to  Boveri's 
remark  that  whatever  be  its  theoretical  interpretation  the  numerical 
reduction  of  the  chromosomes  is  itself  not  a  theory  but  a  fact. 


ABC 

Fig.  114. —  Kormation  of  the  polar  bodies  before  entrance  of  the  spermatozoon,  as  seen  in  the 
living  ovarian  egg  of  the  sea-urchin,  Toxopneustes  (X  365). 

A.  Preliminary  change  of  form  in  the  germinal  vesicle.  B.  The  first  polar  body  formed,  the 
second  forming,  C.  The  ripe  egg,  ready  for  fertilization,  after  formation  of  the  two  polar  bodies 
(/.  ^.  1.  2)  ;  c.  the  egg-nucleus.  In  this  animal  the  first  polar  body  fails  to  divide.  For  its  division 
see  Fig.  89. 


A.     General  Outline 


The  general  phenomena  of  maturation  fall  under  two  heads  :  viz. 
Oflgcficsis,  which  includes  the  formation  and  maturation  of  the  ovum, 
and  spcrmatoi:;cn€sis,  comprising  the  corresponding  phenomena  in  case 
of  the  spermatozoon.  Recent  research  has  shown  that  maturation 
conforms  to  the  same  type  in  both  sexes,  which  show  as  close  a  paral- 
lel in  this  regard  as  in  the  later  history  of  the  germ-nuclei.  Stated  in 
the  most  general  terms,  this  parallel  is  as  follows :  ^  In  both  sexes  the 
final  reduction  in  the  number  of  chromosomes  is  effected  in  the  course 
of  the  last  two  cell-divisions,  or  Diatiiration-divisiojis,  by  which  the 
definitive  germ-cells  arise,  each  of  the  four  cells  thus  formed  having 
but  half  the  usual  number  of  chromosomes.      In  the  female  but  one 

^  The  parallel  was  first  clearly  pointed  out  by  Plainer  in  1 889,  and  was  brilliantly  demon- 
strated by  Oscar  Hertwig  in  the  following  year. 


GENERAL    OUTLINE 


235 


of  the  four  cells  forms  the  "  ovum  "  proper,  while  the  other  three, 
known  as  the  polar  bodies,  are  minute,  rudimentary,  and  incapable  of 
development  (Figs.  89,  97,  114).  In  the  male,  on  the  other  hand,  all 
four  of  the  cells  become  functional  spermatozoa.  This  difference 
between  the  two  sexes  is  probably  due  to  the  physiological  division  of 
labour  between  the  germ-cells,  the  spermatozoa  being  motile  and  very 
small,  while  the  ^gg  contains  a  large  amount  of  protoplasm  and  yolk, 
out  of  which  the  main  mass  of  the  embryonic  body  is  formed.  In  the 
male,  therefore,  all  of  the  four  cells  may  become  functional ;  in  the 
female  the  functions  of  development  have  become  restricted  to  but  one 


Primordial  germ-cell. 


Oogonia. 


Primary  oocyte  or  ovarian  egg. 

Secondary  oocytes  (egg  and 

first  polar  body). 


Mature  egg  and  three  polar  bodies. 


■  Division-period  (the  number  of  divi- 
sions is  much  greater). 


Growth-period. 


.  Maturation-period. 


Fig.  115.  —  Diagram  showing  the  genesis  of  the  egg.     [After  BOVERI.] 


of  the  four,  while  the  others  have  become  rudimentary  (r/.  p.  124). 
The  polar  bodies  are  therefore  not  only  rudimentary  cells  (Giard,  '76), 
but  may  further  be  regarded  as  abortive  eggs  —  a  view  first  put  forward 
by  Mark  in  1881,  and  ultimately  adopted  by  nearly  all  investigators.^ 
The  evidence  is  steadily  accumulating  that  reduction  is  accomplished 
by  two  maturation-divisions  throughout  the  animal  kingdom,  even  in 
the  unicellular  forms ;  though  in  certain  Infusoria  an  additional  divi- 
sion occurs,  while  in  some  other  Protozoa  only  one  maturation-division 
has  thus  far  been  made  out.     Among  plants,  also,  two   maturation- 

1  A  beautiful  confirmation  of  this  view  is  given  by  Francottes's  ('97)  observations  on  a 
turbellarian,  Prosthecerceus.  The  first  polar  body  is  here  often  abnormally  larj^c,  all  grada- 
tions having  been  observed  from  the  normal  size  up  to  cells  nearly  as  large  as  the  egg  itself. 
Such  polar  bodies  are  occasionally  fertilized  ^nA  develop  into  small  gastrulas,  first  forming  a 
single  polar  body  like  the  second  polar  body  of  the  egg.  Here,  therefore,  two  of  the  four 
cells  are  exceptionally  capable  of  development.  It  may  be  added  that  Fol  long  ago  observed 
the  penetration  of  the  small  polar  bodies  by  spermatozoa  in  the  echinoderms;  and  this  has 
been  more  recently  observed  by  Kostanecki  in  mollusks. 


236  REDUCTIOy  OF   THE    CHROMOSOMES 

divisions  occur  in  all  the  hi^i^her  forms  (Muscineoe,  pteridophytes,  and 
phanerogams),  and  in  some,  at  least,  of  the  lower  ones.  Here,  how- 
ever, the  phenomena  are  complicated  by  the  fact  that  the  two  divi- 
sions do  not  as  a  rule  give  rise  directly  to  the  four  sexual  germ-cells, 
but  to  four  asexual  spores  which  undergo  additional  divisions  before 
the  detinitive  germ-cells  are  produced.  In  the  flowering  plants  there 
are  onlv  a  few  such  divisions,  which  give  rise  to  structures  within  the 
pollen-tube  or  embryo-sac.  In  the  archegoniate  cryptogams,  on  the 
other  hand,  each  spore  gives  rise,  by  repeated  divisions,  to  a  "  sexual 
generation"  (prothallium,  etc.)  that  intervenes  between  the  process 
of  reduction  and  that  of  fertilization.  The  following  account  deals 
primarily  with  reduction  in  animals,  the  plants  being  afterward  con- 
sidered. 

I .   Reduction  ill  tlic  Female.     Fonnation  of  the  Polar  Bodies 

As  described  in  Chapter  III.,  the  Q.gg  arises  by  the  division  of  cells 
descended  from  the  primordial  egg-cells  of  the  maternal  organism, 
and  these  may  be  differentiated  from  the  somatic  cells  at  a  very  early 
period,  sometimes  even  in  the  cleavage-stages.  As  development  pro- 
ceeds, each  primordial  cell  gives  rise,  by  division  of  the  usual  mitotic 
type,  to  a  number  of  descendants  known  as  oogonia  (Fig.  115),  which 
are  the  immediate  predecessors  of  the  ovarian  (tgg.  At  a  certain 
period  these  cease  to  divide.  Each  of  them  then  grows  to  form  an 
ovarian  egg,  its  nucleus  enlarging  to  form  the  germinal  vesicle,  its 
cytoplasm  becoming  more  or  less  laden  with  food-matters  (yolk  or 
deutoj)lasm),  while  egg-membranes  may  be  formed  around  it.  The 
ovum  may  now  be  termed  the  oocyte  (Boveri)  or  ovarian  (igg. 

In  this  condition  the  egg-cell  remains  until  near  the  time  of  fertili- 
zation, when  the  process  of  maturation  proper  —  i.e.  the  formation  of 
the  polar  bodies  —  takes  place.  In  some  cases,  e.o;.  in  the  sea-urchin, 
the  polar  bodies  are  formed  before  fertilization,  while  the  (igg  is  still 
in  the  ovary.  More  commonly,  as  in  annelids,  gasteropods,  nema- 
todes, they  are  not  formed  until  after  the  spermatozoon  has  made 
its  entrance  ;  while  in  a  few  cases  one  polar  body  may  be  formed 
before  fertilization  and  one  afterward,  as  in  the  lamprey-eel,  the  frog, 
and  Ainphioxiis.^  In  all  these  cases  the  essential  phenomena  are  the 
same.  Two  minute  cells  are  formed,  one  after  the  other,  near  the 
upper  or  animal  pole  of  the  ovum  (Figs.  97,  116);  and  in  many  cases 
the  first  of  these  divides  into  two  as  the  second  is  formed  (Fig.  89). 

A  group  of  four  cells  thus  arises,  namely,  the  mature  Q,gg,  w^hich 
gives  rise  to  the  embryo,  and  three  small  cells  or  polar  bodies  which 
take  no  part  in  the  further  development,  are  discarded,  and  soon  die 

1  Cf.  p.  189. 


GENERAL    OUTLINE 


2^7 


without  further  change.     The  egg-nucleus  is  now  ready  for   union 
with  the  sperm-nucleus. 


f  :y.: 


D 


— -p.v 


^^^1. 
'-?^/i^ 


E  H 

Fig.  ii6.  —  Diagrams  showing  the  essential  facts  in  the  maturation  of  the  egg.  The  somatic 
number  of  chromosomes  is  supposed  to  be  four. 

A.  Initial  phase;  two  tetrads  have  been  formed  in  the  germinal  vesicle.  />'.  The  two  tctr.ids 
have  been  drawn  up  about  the  spindle  to  form  the  equatorial  plate  of  the  first  polar  mitotic 
figure.  C.  The  mitotic  figure  has  rotated  into  position,  leaving  the  remains  of  the  germinal 
vesicle  at  g.v.  D.  P'ormation  of  the  first  polar  body ;  each  tetrad  divides  into  two  dyads. 
E.  First  polar  body  formed;  two  dyads  in  it  and  in  the  egg.  F.  lYeparation  for  the  second 
division.  G.  Second  polar  body  forming  and  the  first  dividing;  each  dyad  divides  into  two 
single  chromosomes.  H.  Final  result;  three  polar  bodies  and  the  egg-nucleus  (9).  each  con- 
taining two  single  chromosomes  (half  the  somatic  number)  ;  c.  the  egg-centrosome  which  now 
degenerates  and  is  lost. 


23S  K EDUCTION   OF   THE    CHROMOSOMES 

A  Study  of  the  nucleus  during;  these  changes  brings  out  the  follow- 
ing facts.  During  the  multijilication  of  the  oogonia  the  number  of 
chromosomes  is  the  same  as  that  occurring  in  the  division  of  the 
somatic  cells,  and  the  same  number  enters  into  the  formation  of  the 
chromatic  reticulum  of  the  germinal  vesicle.  During  the  formation 
of  the  polar  bodies  this  number  becomes  reduced  to  one-half,  the 
nucleus  of  each  polar  body  and  the  egg-nucleus  receiving  the  reduced 
number.  In  some  manner,  therefore,  the  formation  of  the  polar 
bodies  is  connected  with  the  process  by  which  the  reduction  is  ef- 
fected. The  precise  nature  of  this  process  is,  however,  a  matter 
which  has  been  certainly  determined  in  only  a  few  cases. 

We  need  not  here  consider  the  history  of  opinion  on  this  subject 
further  than  to  point  out  that  the  early  observers,  such  as  Purkinje, 
\'on  Haer,  Bischoff,  had  no  real  understanding  of  the  process  and 
believed  the  germinal  vesicle  to  disappear  at  the  time  of  fertilization. 
To  Hiitschli  ('76),  Hertwig,  and  Giard  ('76,  ^TT)  we  owe  the  discovery 
that  the  formation  of  the  polar  bodies  is  through  mitotic  division,  the 
chromosomes  of  the  equatorial  plate  being  derived  from  the  chro- 
matin of  the  germinal  vesicle.^  In  the  formation  of  the  first  polar 
body  the  group  of  chromosomes  splits  into  two  daughter-groups,  and 
this  process  is  immediately  repeated  in  the  formation  of  the  second 
icithout  ati  intcrccniiig  reticular  rcstiiig  stage.  The  egg-nucleus 
therefore  receives,  like  each  of  the  polar  bodies,  one-fourth  of  the 
mass  of  chromatin  derived  from  the  germinal  vesicle. 

But  alth(jugh  the  formation  of  the  polar  bodies  was  thus  shown  to 
be  a  process  of  true  cell-division,  the  history  of  the  chromosomes  was 
found  to  differ  in  some  very  important  particulars  from  that  of  the 
tissue-cells.  The  essential  facts,  which  were  first  carefully  studied 
in  Ascaris  by  Van  Beneden  ('83,  '87),  and  especially  by  Boveri  ('87,  i), 
are  in  a  typical  case  as  follows  (Figs.  116,  117):  As  the  &gg  prepares 
for  the  formation  of  the  first  polar  body,  the  chromatin  of  the  ger- 
minal vesicle  groups  itself  in  a  number  of  masses,  each  of  which 
splits  up  into  a  group  of  four  bodies  united  by  linin-threads  to  form  a 
"quadruple  group"  or  tetrad  (Vierergruppe).  T/ie  number  of  tetrads 
is  ahuays  oie-half  the  usual  nundier  of  chromosomes.  Thus  in  Ascaris 
{megalocephalay  bivalens)  \.\\c.gQrm\v\-a.\  vesicle  gives  rise  to  two  tetrads, 
the  normal  number  of  chromosomes  in  the  earlier  divisions  being 
four ;  in  the  mole-cricket  there  are  six  tetrads,  the  somatic  number 
of  chromosomes  being  twelve  ;  in  Cyclops  the  respectiv^e  numbers  are 
twelve  and  twenty-four  (one  of  the  most  frequent  cases);  while  in 
Artemia  there    are  eighty-four  tetrads  and  one  hundred  and   sixty- 

'  The  early  accounts  asserting  the  disappearance  of  the  germinal  vesicle  were  based  on 
the  fact  that  in  many  cases  only  a  small  fraction  of  the  chromatic  network  gives  rise  to 
chromosomes,  the  remainder  disintegrating  and  being  scattered  through  the  yolk. 


GENERAL   OUTLIXE 


eight  somatic  chromosomes  —  the  h,Vh»  .         , 

counted.     As  the  first  polar  body  foSs  LT'^V'"^  '^'  ^<^^"-tely 

to  form  two  double  groups,  or  4X  2e  "th'  ^'1^ ^^''^  '^  ,'^^'-d 


remains  in  the  Q<rg 


...  J-.  •} 


V 


S  :7'a--Vv-  ^^ 


/^ 


// 


/ 


F  _  ^  A' 

Shaped  'e.rad%^o„,ylV£7;ror„rL^rrb«  "  f  S"«  ^""^'"^'  vesicle  con.aiL  ,  ^  '  d- 
been  four.  B.  The  ,e,rads  seen  in  profile  CnJ°'  ^'>™"'°f°'"'--»  i"  earlier  divisions  I^>1 
C   rb  'T  'k""''  '"<'  «^™>'-'  -slej.   ^'Krrr":'r"''™"'-   Z*-  F'™  spindle  fonninf 

Jir.-':L^:s;rd!io-'7"¥iyj^^ 

3.n.le  chro„oso,„es,  co„,p,e,in,  .h/;eL^'iot''^,^:r'frr-s4s'?etl^r9o.)"^  ''^^''^''  """  '"° 


240 


HE  DUCT!  ox   OF   THE    CHROMOSOMES 


while  the  other  passes  into  the  polar  body.  Both  the  (i^g  and  the 
first  polar  body  therefore  receive  each  a  number  of  dyads  equal  to 
one-half  the  usual  number  of  chromosomes.  The  e^*;  now  proceeds 
at  once  to  the  formation  of  the  second  polar  body  without  previous 
reconstruction  of  the  nucleus.  Tlach  dyad  is  halved  to  form  two 
single  chromosomes,  one  of  which,  again,  remains  in  the  egg  while 
its  sister  pas.ses  into  the  polar  body.  Both  the  <igg  and  the  second 
polar  bodv  accordingly  receive  two  single  chromosomes  (one-half  the 
usual  number),  each  of  which  is  one-fourth  of  an  original  tetrad 
group.  From  the  two  remaining  in  the  (t^)^  a  reticular  nucleus,  much 
smaller  than  the  original  germinal  vesicle,  is  now  formed. ^ 

Primordial  sjerm-cell. 


Spermatogonia. 


Primary  spermatocyte. 


Secondary  spermatocytes. 


Division-period  (the  number  of  divi- 
sions is  much  greater). 


Growth-period. 


M  aturation-period. 


Spermatids. 
Spermatozoa. 
Fig.  ii8.  —  Diagram  showing  the  genesis  of  the  spermatozoon.     [After  BOVF.RI.] 


Essentially  similar  facts  have  now  been  determined  in  a  consider- 
able number  of  animals,  though,  as  we  shall  presently  see,  tetrad- 
formation  is  not  of  universal  occurrence,  nor  is  it  always  of  the  same 
type.  For  the  moment  we  need  only  point  out  that  the  numerical 
reduction  of  chromatin-w^ri-jTi"  takes  place  before  the  polar  bodies 
are  actually  formed,  through  processes  which  determine  the  number 
of  tetrads  within  the  germinal  vesicle.  The  numerical  reduction  is 
therefore  determined  in  the  grandmother-cell  of  the  Q,^g.  The  actual 
divisions  by  which  the  polar  bodies  are  formed  merely  distribute  the 
elements  of  the  tetrads. 


^  It  is  nearly  certain  that  the  division  of  the  first  polar  body  (which,  however,  may  be 
omitted)  is  analogous  to  that  by  which  the  second  is  formed,  i.e.  each  of  the  dyads  is 
similarly  halved.      Cf.  Griffin,  '99. 


GENERAL    OUTLIXE  24 1 

2.  Reduction  in  the  Male.     Spermatogenesis 

The  researches  of  Platner  ('89),  Boveri,  and  especially  of  Oscar 
Hertwig  ('90,  i)  have  demonstrated  that  reduction  takes  place  in  the 
male  in  a  manner  almost  precisely  parallel  to  that  occiirrin^;  in  the 
female.  Platner  first  suggested  ('89)  that  the  formation  of  the  polar 
bodies  is  directly  comparable  to  the  last  two  divisions  of  the  sperm 
mother-cells  (spermatocytes).  In  the  following  year  Boveri  reached 
the  same  result  in  Ascaris,  stating  his  conclusion  that  reduction  in 
the  male  must  take  place  in  the  '*  grandmother-cell  of  the  sperma- 
tozoon, just  as  in  the  female  it  takes  place  in  the  grandmother-cell 
of  the  Qgg,''  and  that  the  egg-formation  and  sperm-formation  reallv 
agree  down  to  the  smallest  detail  ('90,  p.  64).  Later  in  the  same 
year  appeared  Oscar  Hertwig's  splendid  work  on  the  spermato- 
genesis of  Ascaris,  which  established  this  conclusion  in  the  most 
striking  manner.  Like  the  ova,  the  spermatozoa  are  descended  from 
primordial  germ-cells  which  by  mitotic  division  give  rise  to  the 
spermatogonia  from  which  the  spermatozoa  are  ultimatelv  formed 
(Fig.  118).  Like  the  oogonia,  the  spermatogonia  continue  for  a  time 
to  divide  with  the  usual  (somatic)  number  of  chromosomes,  i.e.  four 
in  Ascaris  vicgalocephala  bivalens.  Ceasing  for  a  time  to  divide,  they 
now  enlarge  considerably  to  form  spermatocytes,  each  of  which  is 
morphologically  equivalent  to  an  unripe  ovarian  ovum,  or  oocyte. 
Each  spermatocyte  finally  divides  twice  in  rapid  succession,  giving 
rise  first  to  two  daughter-spermatocytes  and  then  to  four  spermatids, 
each  of  which  is  directly  converted  into  a  single  spermatozoon.  The 
history  of  the  cJiromatin  in  these  two  divisions  is  exactly  parallel  to 
that  in  the  formation  of  the  polar  bodies  (Figs.  119,  120).  From  the 
chromatin  of  the  spermatocyte  are  formed  a  number  of  tetrads  equal 
to  one-half  the  usual  number  of  chromosomes.  Each  tetrad  is  halved 
at  the  first  division  to  form  two  dyads  which  pass  into  the  respective 
daughter-spermatocytes.  At  the  ensuing  division,  which  occurs  with- 
out the  previous  formation  of  a  resting  reticular  nucleus,  each  dyad 
is  halved  to  form  two  single  chromosomes  which  enter  the  resj^ec- 
tive  spermatids  (ultimately  spermatozoa).  From  each  spermatocyte,, 
therefore,  arise  four  spermatozoa,  and  each  sperm-nucleus  receives 
half  the  usual  number  of  single  chromosomes.  The  parallel  with  the 
egg-reduction  is  complete. 

These  facts  leave  no  doubt  that  the  spermatocyte  is  the  morpho- 
logical equivalent  of  the  oocyte  or  immature  ovarian  Q.gg,  and  that 
the  group  of  four  spermatozoa  to  which  it  gives  rise  is  equivalent 
to  the  ripe  ^g%  plus  the  three  polar  bodies.  Hertwig  was  thus  led  to 
the  following  beautifully  clear  and  simple  conclusion  :  '*  The  polar 
bodies    are  abortive   eggs  which   are   formed  by   a   final  process   of 

R 


242 


KEDUCTIOX  OF   THE    CHROMOSOMES 


division  from  the  egg-mother-cell  (oocyte)  in  the  same  manner  as  the 
spermatozoa  are  formed  from  the  sperm-mother-cell  (spermatocyte). 
Hut  while  in  the  latter  case  the  products  of  the  division  are  all  used 
as  functional  spermatozoa,   in  the  former  case  one  of   the  products 


Pig.  119.  —  Diapranis  showing  the  essential  facts  of  reduction  in  the  male.  The  somatic  num- 
ber of  thromosomes  is  supposed  to  be  four. 

.  /.  /?.  Division  of  one  of  the  spermatogonia,  showing  the  full  number  (four)  of  chromosomes. 
C.  Primary  spermatocyte  preparing  for'division  ;  the  chromatin  forms  two  tetrads.  D.  It.  F.  First 
division  to  form  two  secondary  spermatocytes  each  of  which  receives  two  dyads.  G.  H.  Division 
of  the  two  .secondary  spermatocytes  to  form  four  spermatids.  Each  of  the  latter  receives  two 
single  chromosomes  and  a  centrosome  which  passes  into  the  middle-piece  of  the  spermatozoon. 


of  the  egg-mother-cell  becomes  the  ^%g,  appropriating  to  itself  the 
entire  mass  of  the  yolk  at  the  cost  of  the  others  which  persist  in 
rudimentary  form  as  the  polar  bodies."  ^ 

1  '90,  I,  p.  126. 


GENERAL    OUTLINE  243 

3.     Weismauji  s  Interpretation  of  Reduction 

Up  to  this  point  the  facts  are  clear  and  intelligible.  Before  com- 
ing to  closer  quarters  with  them  it  will  be  useful  to  make  a  digression 
in  order  to  consider  some  of  the  theoretical  aspects  of  reduction  ; 
though  the  reader  must  be  warned  that  this  will  lead  us  into  very 
uncertain  ground  traversed  by  a  labyrinth  of  conflicting  hypotheses 
from  which  no  exit  has  yet  been  discovered. 

The  process  of  reduction  is  very  obviously  a  provision  to  hold  con- 
stant the  number  of  chromosomes  characteristic  of  the  species ;  for 
if  it  did  not  occur,  the  number  would  be  doubled  in  each  succeeding 
generation  through  union  of  the  germ-cells.^  A  number  of  writers 
have  contented  themselves  with  this  simple  interpretation,  Oscar 
Hertwig,  for  example,  regarding  reduction  as  ''  merely  a  process  to 
prevent  a  summation  through  fertilization  of  the  nuclear  mass  and  of 
the  chromatic  elements."  ^  A  moment's  reflection  reveals  the  entire 
inadequacy  of  such  an  explanation.  As  far  as  the  chromatin-mass  is 
concerned,  it  does  not  agree  with  the  facts ;  for  in  reduction  with 
tetrad-formation  the  chromatin-mass  is  reduced  not  to  one-half,  but  to 
one-fourth.  That  reduction  must  mean  more  than  mere  mass-reduc- 
tion is  moreover  proved  by  the  fact  that  the  bulk  of  the  nucleus  mav 
enormously  increase  or  decrease  at  different  periods  in  the  same  cell, 
irrespective  of  the  number  of  chromosomes.  The  real  problem  is 
why  the  number   of  chromosomes   should    be    held    constant.     The 

1  Of  the  many  earlier  attempts  to  interpret  the  meaning  of  the  polar  bodies,  we  need  only 
consider  at  this  point  the  very  interesting  suggestion  of  Minot  ('77),  afterward  adopted  by 
Van  Beneden  ('83),  that  the  ordinary  cell  is  hermaphrodite,  and  that  maturation  is  for  the 
purpose  of  producing  a  unisexual  germ-cell  by  dividing  the  mother-cell  into  its  sexual  con- 
stituents, or  "genoblasts."  Thus,  the  male  element  is  removed  from  the  egg  in  the  polar 
bodies,  leaving  the  mature  egg  a  female.  In  like  manner  he  believed  the  female  element  to 
be  cast  out  during  spermatogenesis  (in  the  "  Sertoli  cells  "),  thus  rendering  the  spermatozoa 
male.  By  the  union  of  the  germ-cells  in  fertilization,  the  male  and  female  elements  are 
brought  together  so  that  the  fertilized  egg  or  oosperm  is  again  hermaphrodite  or  neuter. 
This  ingenious  view  was  independently  advocated  by  Van  Beneden  in  his  great  work  on 
Ascaris  <y"^'^.  A  fatal  objection  to  it,  on  which  both  Strasburger  and  Weismann  have 
insisted,  lies  in  the  fact  that  male  as  well  as  female  qualities  are  transmitted  by  the  egg-cell, 
while  the  sperm-cell  also  transmits  female  qualities.  The  germ-cells  are  therefore  non-sexual. 
The  researches  of  many  observers  show,  moreover,  that  all  of  the  four  spermatids  derived 
from  a  spermatocyte  become  functional  spermatozoa.  Minot's  hypothesis  must,  therefore,  in 
my  opinion,  be  abandoned. 

Balfour  doubtless  approximated  more  nearly  to  the  truth  when  he  said.  "  In  the  formation 
of  the  polar  cells  part  of  the  constituents  of  the  germinal  vesicle,  which  are  requisite  for  its 
functions  as  a  complete  and  independent  nucleus,  is  removed  to  make  room  for  the  supply 
of  the  necessary  parts  to  it  again  by  the  spermatic  nucleus"  ('So,  p.  62).  He  fell,  however, 
into  the  same  error  as  Minot  and  Van  Beneden  in  characterizing  the  germ-nuclei  as  "  male  " 
and  "female";  and,  as  shown  at  pages  194.  353.  it  has  been  found  that  a  single  germ- 
nucleus  is  able  to  carry  out  development  of  an  embryo  without  union  with  another. 

2  '90,  I,  p.  112.      Cf.  Hartog,  '91,  p.  57. 


244 


REDUCTIOX   OF   THE    CHROMOSOMES 


deeper  meanin^^  of  the  phenomena  was  first  seriously  considered  by 
Weismann  in  his  essays  of  1S83  and  1887  ;  and,  although  his  conclu- 
sions were  of  a  highlv'speculative  character,  they  nevertheless  gave  so 


l 


^ 


^j 


\ 


K 


E 


B 


.r.O. 


D 


-<?C«>n 


F 


/ 


/ 


\ 


® 


\ 


I 


I 


© 


l\ 


® 


\ 


:)^ 


AT 


\ 


/ 


X" 


II 


L 


Fig.  120.  —  Reduction  in  the  spermatogenesis  of  Ascaris  mcgalocephala,v7iX.  bivalens.   [Brauer.]  1 

.^l-G.  Successive  stages  in  tlie  division  of  the  primary  spermatocyte.  The  original  reticulum 
undergoes  a  ver)'  early  division  of  the  chromatin-granules  which  then  form  a  doubly  split  spireme- 
tir'ad.Zf.  This  shortens  (6),  and  breaks  in  two  to  form  the  two  tetrads  (Z)  in  profile,  .fi"  viewed 
endwise).  F.  G.H.  Urst  division  to  form  two  secondary  spermatocytes,  each  receiving  two  dyads. 
/.  Secondary  sp<Tmatocyte.  J.  K.  The  same  dividing.  Z.  Two  resulting  spermatids,  each  with 
two  single  chromosomes  and  a  centrosome. 

great  a  stimulus  to  the  study  of  the  entire  problem  that  his  views 
deserve  special  attention.  Weismann's  interpretation  was  based  on  a 
remarkable  paper  published  by  Wilhelm  Roux  in  1883,^  in  whicTi  arej! 

1  For  division  of  the  spermatogonia  see  Fig.  55;   for  the  corresponding  phenomena  in  var. 
univalens  see  Fig,  148. 

'^  Uber  die  Bedeiitiing  der  KerntheilungsfigHren. 


GENERAL    OUTLINE  24  ^ 

developed  certain  ideas  which  afterward  formed  the  foundation  of 
Weismann's  whole  theory  of  inheritance  and  development.  Roux 
argued  that  the  facts  of  mitosis  are  only  explicable  under  the  assump- 
tion that  chromatin  is  not  a  uniform  and  homogeneous  substance,  but 
differs  qualitatively  in  different  regions  of  the  nucleus ;  that  the  col- 
lection of  the  chromatin  into  a  thread  and  its  accurate  division  into 
two  halves  is  meaningless  unless  the  chromatin  in  different  regions  of 
the  thread  represents  different  qtcalities  which  are  to  be  divided  and 
distributed  to  the  daughter-cells  according  to  some  definite  law.  He 
urged  that  if  the  chromatin  were  qualitatively  the  same  throughout 
the  nucleus,  direct  division  would  be  as  efficacious  as  indirect,  and  the 
complicated  apparatus  of  mitosis  would  be  superfluous.  Roux  and 
Weismann,  each  in  his  own  way,  subsequently  elaborated  this  con- 
ception to  a  complete  theory  of  inheritance  and  development,  but  at 
this  point  we  may  confine  our  attention  to  the  views  of  Weismann. 
The  starting-point  of  his  theory  is  the  hypothesis  of  De  Vries  that  the 
chromatin  is  a  congeries  or  colony  of  invisible  self-propagating  vital 
units  or  biopJiores  somewhat  like  Darwin's  **  gemmules  "  (p.  12),  each 
of  which  has  the  power  of  determining  the  development  of  a  particu- 
lar quality.  Weismann  conceives  these  units  as  aggregated  to  form 
units  of  a  higher  order  known  as  ''determinants,"  which  in  turn  are 
grouped  to  form  **  ids,"  each  of  which,  for  reasons  that  need  not  here 
be  specified,^  is  assumed  to  possess  the  complete  architecture  of  the 
germ-plasm  characteristic  of  the  species.  The  "ids"  finally,  which 
are  identified  with  the  visible  chromatin-granules,  are  arranged  in 
linear  series  to  form  "  idants  "  or  chromosomes.  It  is  assumed  further 
that  the  '*  ids  "  differ  slightly  in  a  manner  corresponding  with  the  indi- 
vidual variations  of  the  species,  each  chromosome  therefore  being  a 
particular  group  of  shghtly  different  germ-plasms  and  differing  quali- 
tatively from  all  the  others. 

We  come  now  to  the  essence  of  W^eismann's  interpretation.  The 
end  of  fertilization  is  to  produce  new  combinations  of  variations  by 
the  mixture  of  different  ids.  Since,  however,  their  number,  like  that 
of  the  chromosomes  which  they  form,  is  doubled  by  the  union  of  two 
germ-nuclei,  an  infinite  complexity  of  the  chromatin  would  soon  arise 
did  not  a  periodic  reduction  occur.  Assuming,  then,  that  the  "  ances- 
tral germ-plasms"  (ids)  are  arranged  in  a  linear  series  in  the  spireme- 
thread  or  the  chromosomes  derived  from  it,  Weismann  ventured  the 
prediction  {'%j)  that  two  kinds  of  mitosis  would  be  found  to  occur. 
The  first  of  these  is  characterized  by  a  longitudinal  sj)litting  of  the 
thread,  as  in  ordinary  cell-division,  "  by  means  of  which  all  the  ances- 
tral germ-plasms  are  equally  distributed  in  each  of  the  daughter-nuclei 
after  having  been  divided  into  halves."     This  form  of  division,  which 

1  Cf.  the  Germ-plasm,  p.  60. 


246  REDUCTIOX   OF   THE    CHROMOSOMES 

he  called  eqnal  division  ( Aequationsthcilung),  was  then  a  known  fact. 
The  second  form,  at  that  time  a  ])urely  theoretical  postulate,  he  as- 
sumed to  be  of  such  a  character  that  each  dau*;hter-nucleus  should 
receive  onlv  half  the  number  of  ancestral  germ-plasms  possessed 
bv  the  mother-nucleus.  This  he  termed  a  reducing  division  ( Re- 
duktionstheilung),  and  suggested  that  this  might  be  effected  either 
by  a  tninsvtrsf  division  of  the  chromosomes,  or  by  the  elimination  of 
entire  chromosomes  without  division.^  By  either  method  the  number 
of  **  ids  "  would  be  reduced  ;  and  W'eismann  argued  that  such  reduc- 
ing divisions  must  be  involved  in  the  formation  of  the  polar  bodies, 
and  in  the  parallel  phenomena  of  spermatogenesis. 

The  fulfilment  of  Weismann's  prediction  is  one  of  the  most  inter- 
estiniT  results  of  recent  cvtological  research.  It  has  been  demon- 
strated,  in  a  manner  which  seems  to  be  incontrovertible,  that  the 
reducing  divisions  postulated  by  Weismann  actually  occur,  though 
not  precisely  in  the  manner  conceived  by  him.  Unfortunately  for 
the  general  theory,  however,  transverse  divisions  have  been  cer- 
tainlv  determined  in  only  a  few  types,  while  in  others,  of  which 
Ascaris  is  the  best-known  example,  the  facts  thus  far  known  seem 
clearly  opposed  to  the  assumption.  On  the  whole,  the  evidence  of 
reducing  divisions,  i.e.  such  as  involve  a  transverse  and  not  a  longi- 
tudinal division  of  the  chromatin-thread,  has  steadily  increased  ;  but 
it  remains  quite  an  open  question  whether  they  have  the  significance 
attributed  to  them  by  Weismann. 

B.     Origin  of  the  Tetrads 

I.    General  Sketch 

In  considering  the  origin  of  the  tetrads  or  their  equivalents,  it 
should  be  borne  in  mind  that  true  tetrad-formation,  as  described 
above,  has  only  been  certainly  observed  in  a  few  groups  (most 
clearly  in  the  nematodes  and  arthropods).  But  even  in  cases  where 
the  chromatin  does  not  condense  into  actual  tetrads  these  bodies 
are  represented  by  chromosomes  in  the  form  of  rings,  crosses,  and 
the  like,  which  arc  clo.sely  similar,  and  doubtless  equivalent,  to  those 
from  which  actual  tetrads  .arise,  and  present  us  with  the  same  prob- 
lems. With  a  few  apparent  exceptions,  described  hereafter,  the 
tetrads  of  their  equivalents  alwaws  arise  by  a  double  division  of  a 
single  primary  chromatin-rod  or  mass.  Nearly  all  observers  agree 
further  that  the  number  of  primary  rods  at  their  first  appearance  in 
the  germinal  vesicle  or  in  the  spermatocyte-nucleus  is  one-half  the 
usual  number  of  chromosomes,  and  that  this  numerical  reduction  is 
due  to  the  fact  that  the  spireme-thread  segments  into  one-half  the 

1  Essay  VI.,  p.  375. 


OA'TGIN   OF   THE    TETRADS 


247 


A      B       C      D 


i;  T 


a 


} 
4 

5 
6 

7 
8 


a 


a 


o 


usual  number  of  pieces.     Apparently,  however,  there  are  two  radi- 
cally different  types  of  tetrad-formation  as  follows. 

In  the  first  type  the  tetrad  arises  by  one  longitudinal  and  one  trans- 
verse division  of  each  primary  cJironiatin-rod,  the  latter  effecting  the 
reduction  demanded  by 
Weismann's  hypothesis(Fig. 
121,  I).  To  give  the  usual 
graphic  representation,  let 
us,  for  the  sake  of  discus- 
sion, assume  the  somatic 
number  of  chromosomes  to 
be  four,  designating  the 
spireme-thread  as  a  b  c  d,  £ 
each  letter  representing  a 
chromosome,  each  of  which 
we  may  in  turn  assume  to 
consist  of  a  series  of  four 
granules  or  ** ids  "(Fig.  121).  4  <^       S^^8 

In  ordinary  mitosis  the  spi- 
reme would  segment  into 
a  —  b  —  c  —  d,  which  then 
would  divide  lengthwise  to 
form  pairs  of  identical  sister  ^ 

1                        a      b      c      d 
chromosomes 

abed 
To  form  the  tetrad,  on  the 
other  hand,  the  spireme  first 
segments  into  two  rods  ab 
and  cd,  each  of  which,  in 
view  of   its  subsequent  his-  ^^      s^ks 

tory,  may  be  regarded  as 
bivalent,  representing  two 
chromosomes  united  end  to 
end  (Vom  Rath,  Riickert, 
Hacker).       Each    of 


II 


3 

4 


7 
8 


a 


\ 


a 


ab    ab 


z 

I 


ab   ab 


8 


S 


Fig.    121. — Diagrams   of  tetrad-formation;    I.  with 

one  transverse  and  one  longitudinal  division  (copepod 

type) ;  II,  witii  two  longitudinal  divisions  {Ascaris  type). 

A-D,  successive   stages;    chromatin-granules    num- 

these    bered  from  i  to  8.     The  two  types  diverge  at  C.     In  I> 

divides    once    longitudinally,     thegranulesof  each  constituent  of  the  tetrad  fuse  to  form 

i^         ■  -J  ^    a.  homogeneous  sphere. 

giving  the  identical  pairs  or 

dyads   —  —  £^,  and  once  transversely,  giving  the  tetrads  —  — -' 

ab      cd  ''    '^      ^    " 

Inspection  of  Fig.  121,  I,  shows  that  through  the  second  or  transverse 
division,  each  member  of  the  tetrad  receives  only  half  the  number  of 
ids  contained  in  the  original  segment.  This  number,  four,  is  the  same 
as  that  assumed  for  a  single  chromosome  ;  and,  since  each  of  the  two 
tetrads  contributes  one  chromosome  to  the  germ-cell,  the  latter  receives 


248  REDUCTJOX  OF   THE    CHROMOSOMES 

but  half  the  usual  number  both  of  chromosomes  and  of  ids.  This  mode 
of  tetrad-formation  has  been  most  clearly  demonstrated  in  insects 
and  copepods.  and  an  equivalent  ])rocess  occurs  also  in  mollusks, 
annelids,  turbellarians,  and  some  other  animals,  as  described  beyond. 
In  the  second  type,  illustrated  especially  by  Ascaris,  the  tetrad  is 
apparently  formed  by  t:^>o  longitudinal  divisions  of  each  primary 
chromatin-rod,  and  no  reducing  division  occurs.  If,  therefore,  we 
adopt  the  same  terminology  as  before,  we  have  first  ab  and  cd.  then 

it  _  12/,  and  hnall V  — !—  -  —  — ,  bv  two  longitudinal  divisions.  In 
ab      id  '   ab  ! ab      cd ^  cd 

this  case,  according  to  Brauer's  careful  studies,  each  chromatin-granule 
(**  id")  divides  at  each  longitudinal  division  of  the  primary  rod.  The 
four  chromosomes  of  the  tetrad  are  therefore  exactly  equivalent,  being 
derived  from  the  same  region  of  the  spireme-thread,  and  containing 
the  undiminished  number  of  "  ids  "  (Fig.  121,  II). 

The  contradiction  may  be  stated  in  a  different  way.  In  the  first 
type  of  tetrad  formation,  the  number  both  of  granules  and  of  chro- 
mosomes is  first  doubled  {i.e.  in  the  assumed  case,  through  the  forma- 
tion of  two  tetrads,  each  consisting  of  four  chromosomes,  or  eight  in 
all),  and  then  reduced  to  half  that  number  by  the  two  successive  matu- 
ration-divisions. In  the  second  type,  on  the  other  hand,  the  number 
of  chromosomes  is  likewise  doubled,  but  that  of  the  granules  is  quad- 
rupled, so  that,  although  in  both  types  the  two  maturation-divisions 
reduce  the  number  of  elirouibsonies  to  one-half,  only  in  the  first  type 
do  they  reduce  the  number  of  granules  or  **ids,"  as  Weismann's 
hvpothesis  demands.  We  must  therefore  distinguish  sharply  between 
the  reduction  of  the  chromosomes  and  that  of  the  *'ids."  The  former 
is  primarilv  effected  b\'  the  segmentation  of  the  jDrimary  s])ireme- 
thread,  or  the  resolution  of  the  nuclear  reticulum,  into  one-half  the 
usual  number  of  segments  (?>.  the  "pseudo-reduction"  of  Riickert); 
and  here  the  real  secret  of  the  reduction  of  the  chromosomes  lies.  The 
reduction  of  the  "ids,"  if  they  have  any  real  existence,  is  a  distinct, 
and  as  yet  unsolved,  question. 

2.      Detailed  Evidence 

We  may  now  consider  some  of  the  phenomena  in  detail,  though  the 
limits  of  this  work  will  only  allow  the  consideration  of  a  few  typical 
cases. 

{a)  Tetrad for)nation  ivith  one  Longitudinal  and  one  Trajisverse 
Division.  —  In  many  of  the  cases  of  this  type  the  tetrads  arise  from 
ring-shaped  bodies  which  are  analogous  to  the  ring-shaped  chromo- 
somes occurring  in  heterotypical  mitosis  (p.  %6).  First  observed  by 
Henking('9i)  in  /^j';r//^rf7;7i-,  tetrad-origin  of  this  type  has  since  been 
found  in  other  insects  by  Vom  Rath,  Toyama,  Paulmier,  and  others, 


ORIGIX  OF   THE    TETRADS 


249 


in  copepods  by  Riickert,  Hacker,  and  Vom  Rath,  in  pteridophytcs  by 
Calkins  and  Osterhout,  in  the  onion,  Alliiiui,  by  Ishikawa,  and  in 
various  other  forms  where  their  history  has  been  less  clearly  made 
out.  The  genesis  of  the  ring  was  first  determined  by  \k)w\  Rath  in 
the  mole  cr'ickQt  {Gryllotalpa,  '92),  and  has  been  thoroughly  elucidated 
by  the  later  work  of  Riickert  ('94),  Hacker  ('95,  i),  and  Paulmier 
('99).  All  these  observers  have  reached  the  same  conclusion; 
namely,  that  the  ring  arises  by  the  longitudinal  splitting  of  a  primary 
chromatin-rod,  the  two  halves  remaining  united  by  their  ends,  and 
opening  out  to  form  a  ring.     The  ring-formation  is,  in  fact,  a  form  of 


A 


D  E  F 

Fig.  122.  — Origin  of  the  tetrads  by  ring-formation  in  the  spermatogenesis  of  the  mole-cricket 
Gryllotalpa.     [\'(m  RATH.] 

A.  Primary  spermatocyte,  containing  six  double  rods,  each  of  which  represents  two  chromo- 
somes united' end  to  end  and  longitudinally  split  except  at  the  free  ends.  B.  C.  Opening  out  of 
the  double  rods  to  form  rings.  D.  Concentration  of  the  rings.  E.  The  rings  broken  up  into 
tetrads.     F.  First  division-figure  established. 

heterotypical  mitosis  (p.  S6).  The  breaking  of  the  ring  into  four 
parts  involves,  first,  the  separation  of  these  two  halves  (corresponding 
with  the  original  longitudinal  split),  and  second,  the  tniusrnsc  division 
of  each  half,  the  latter  being  the  reducing  division  of  W'cismann. 
The  number  of  primary  rods,  from  which  the  rings  arise,  is  one-half 
the  somatic  number.  Hence  each  of  them  is  conceived  by  Vom  Rath, 
Hacker,  and  Riickert  as  bivalent  or  double  ;  i.e.  as  representing  two 
chromosomes  united  end  to  end.  This  appears  with  the  greatest 
clearness  in  the  spermatogenesis  of  G)-yUotalpa   (Fig.    122).     Here 


2;0 


REDVCTIOiY  OF   THE    CHROMOSOMES 


the  spireme-thread  splits  lengthwise  before  its  segmentation  into  rods. 
It  then  divides  transversely  to  form  six  double  rods  (half  the  usual 
number  of  chromosomes),  which  open  out  to  form  six  closed  rings. 
These  become  small  and  thick,  break  each  into  four  parts,  and  thus 


<m'  //iKX  \^° 

'  7 


I  I 

I  I 
I 


^, 


^  \r?. 


o"oo 

C'OOo 


o;?po 


1*f     ^ 


oc^co' 


JOOf, 


0  i*  0  S  0  oof-oo  o°^o^ 


n'^0  o  o 


0, 


'00  0 
^OqO 


A'<p 


D 


p  op,  ,9 
0  o  o3 

qCqO/ 
0^° 


0. 


%8 

J^O   0  0 

ooOOO 


(The 


Fig.  123.  —  I-ormation  of  the  tetrads  and  polar  bodies  in  Cyclops,  slightly  schematic, 
full  number  of  tetrads  is  not  shown.)      [RUCKKRT.] 

A.  Germinal  vesicle  containing  eight  longitudinally  split  chromatin-rods  (half  the  somatic 
number).  li.  Shortening  of  the  rods;  transverse  division  (to  form  the  tetrads)  in  progress. 
C.  Position  of  the  tetrads  in  the  first  polar  spindle,  the  longitudinal  split  horizontal.  D.  Ana- 
phase; longitudinal  divisions  of  the  tetrads.  E.  The  first  polar  body  formed;  second  polar 
spindle  with  the  eight  dyads  in  position  for  the  ensuing  division,  which  will  be  a  transverse  or 
reducinfT  division. 


ive  rise  to  six  typical  tetrads.  An  essentially  similar  account  of  the 
ring-formation  is  given  by  Vom  Rath  in  EucJicBta  and  Calanus,  and 
by  Riickert  in  Hctcrocopc  and  Diaptomns. 

That  the  foregoing  interpretation  of  the  rings  is  correct,  is  beauti- 
fully demonstrated  by  the  observations  of  Hacker,  and  especially  of 


ORIGIN   OF  THE   TETRADS 


251 


Ruckert,  on  a  number  of  other  copepods  {Cyclops,  Ca7ithocamptiis\ 
in  which  rings  are  not  formed,  since  the  splitting  of  the  primary 
chromatin-rods  is  complete.  The  origin  of  the  tetrads  has  here  been 
traced  with  especial  care  in  Cyclops  strciiims,  by  Ruckert  ('94),  whose 
observations,  confirmed  by  Hacker,  are  quite  as  convincing  as  those 


a 


Fig.  124.  —  Diagrams  of  various  modes  of  lelrad-formation.     [Hacker.] 

a.  Common  starting-point,  a  double  spireme-thread  in  the  germinal  vesicle;  d.  common  re- 
sult, the  typical  tetrads ;  b.  c.  intermediate  stages :  at  the  left  the  ring-formation  (as  in  Diaptomiis, 
Gryllotalpa,  Heterocope)  ;  middle  series,  complete  splitting  of  the  rods  (as  in  Cyclops  accordini:  to 
Ruckert,  and  in  Canthocaniptus)  ;  at  the  right  by  breaking  of  the  V-shaped  rods  (as  in   Cy.i  ; 
strenuus,  according  to  Hacker. 


of   Brauer  on  Ascains,  though  they  led  to  a  diametrically  opposite 
result. 

The  normal  number  of  chromosomes  is  here  twenty-two.  In  the 
germinal  vesicle  arise  eleven  threads,  which  split  lengthwise  (Fig.  123), 
and  finally  shorten  to  form  double  rods,  manifestly  equivalent  to  the 
closed  rings  of  Diaptomus.     Each  of  these  now  segments  transversely 


2''s2 


KEDLCTIOX   OF   THE    CHROMOSOMES 


to  form  a  tetrad  group,  and  the  eleven  tetrads  then  place  themselves 
in  the  equator  of  the  spindle  for  the  first  polar  body  (Fig.  123,  C\  in 
such  a  manner  that  the  longitudnial  split  is  transverse  to  the  axis  of 
the  spindle.  As  the  polar  body  is  formed,  the  longitudinal  halves 
of  the  tetrad  separate,  and  the  formation  of  the  first  polar  body  is 
thus  demonstrated  to  be  an  "equal  division"  in  Weismann's  sense. 
The  eleven  dyads  remaining  in  the  eggs  now  rotate  (as  in  Ascaris), 


/•//v^'.-- 

■•^^m;; 


rG->!©^y* 


Fig.  125.  —  Germinal  vesicles  of  various  eggs,  showing  chromosomes,  tetrads,  and  nucleoli. 
A.     .\  copepod  {Heterocope)  showing  eight  of  the  sixteen  ring-shaped  tetrads  and  the  nucleo- 
lus.   [krcKKki.] 

/?.   I^ter  stage  of  the  same,  condensation  and  segmentation  of  the  rings.     [RUCKERT.] 

C.  "  Cyclops  strenuus,"  illustrating   Hacker's  account  of  the  tetrad-formation  from  elongate 
double  rods;  a  group  of  "  accessory  nucleoli."     [Hackkr.] 

D.  Germinal  vesicle  of  an  annelid  {^Op/nyotrocha) -^how-xng  nucleolus  and  four  chromosomes. 
[KORSCHELT.] 

SO  that  the  transverse  division  lies  in  the  equatorial  plane,  and  are 
halved  during  the  formation  of  the  second  polar  body.  The  division 
is  accordingly  a  "reducing  division,"  which  leaves  eleven  single  chro- 
mosomes in  the  egg.  Paulmier's  work  on  Auasa  and  other  Hemip- 
tera  ('99)  gives  the  same  result  as  the  above  in  regard  to  the  origin 
of  the  tetrads  (Figs.  126,  127).  The  process  is,  however,  slightly 
complicated  by  the  fact  that  no  continuous  spireme-thread  is  formed, 
while  the  rings  are  often  bent  or  twisted  and  never  open  out  to  a 


ORIGIN   OF   THE    TETRADS  253 

circular  form.  They  finally  condense  into  true  tetrads  which  are 
successively  divided  into  dyads  and  monads  by  the  two  divisions; 
but  it  is  an  interesting  fact  that  the  order  of  division  occurring  in  the 
copepods  appears  here  to  be  reversed,  the  first  division  being  the 
transverse  and  the  second  the  longitudinal  one  — a  result  agreeinc^ 
with  Henking's  earlier  conclusion  in  the  case  of  Pyrrochoris.  '^Oster'- 
hout  ('97)  and  Calkins  ('97)  independently  discovered  tetrads  in  the 
vascular  cryptogams  (Equisetum,  Ptcris),  and  the  last-named  observer 
finds  that  in  Ptej'is  they  may  arise  either  from  rings,  as  in  (iryllotalpa 
or  Heterocope,  or  from  double  rods  as  in  Cyclops,  the  halves  in  the 
latter  case  being  either  parallel  or  forming  a  cross.  This  longitu- 
dinal spHt,  occurring  in  the  spireme,  is  followed  by  a  trans\x'rse 
division  by  which  the  tetrad  is  formed.  Tetrads  having  an  essentially 
similar  mode  of  origin  are  also  described  by  Atkinson  ('99)  in  Ari- 
scEma,  and  tetrad-formation  is  nearly  approached  in  Allium  according 
to  Ishikawa  ('99).^     These  cases  are  considered  at  page  263. 

Resume.  In  all  the  foregoing  cases  the  tetrads  arise  from  a  spi- 
reme which  splits  lengthwise,  segments  into  one-half  the  somatic 
number  of  rods  (each  longitudinally  divided)  and  each  of  the  latter 
divides  transversely  to  form  the  tetrad.  When  the  ends  of  the 
daughter-chromosomes  resulting  from  the  longitudinal  split  remain 
united  (as  in  insects)  ring-forms  result,  and  the  earlier  phases  of  tetrad- 
formation  are  thus  identical  with  those  of  heterotypical  mitosis. 
When  the  split  is  complete,  so  that  the  ends  remain  free,  double 
rods  result;  while,  if  the  daughter-chromosomes  remain  temporarily 
united  at  the  middle  or  at  the  end,  X-,  Y-,  and  V-shaped  figures  may 
arise.  In  all  these  forms  tetrad-formation  is  completed  by  the  com- 
plete separation  of  the  daughter-rods,  the  transverse  division  of 
each  in  the  middle,  and  the  condensation  of  the  four  resulting  bodies 
into  a  quadruple  mass.  As  will  be  shown  in  Section  C  (p.  258) 
the  transverse  division  is  in  many  forms  delayed  until  after  sepa- 
ration of  the  longitudinal  halves.  In  such  cases  no  actual  tetrads 
are  formed,  though  the  result  is  the  same. 

{I))  Second  Type.  Tetrad-formation  ivith  two  Longitudinal  Divi- 
sions. —  The  only  accurately  known  case  of  this  type  is  Ascaris,  the 
object  in  which  tetrads  were  first  discovered  by  Van  Beneden  in 
1883.  Carnoy  ('S6,  2)  reached  the  conclusion  that  the  tetrads  in 
some  other  nematodes  {Ophiostomum,  Ascaris  clavata,  .i.  lumbricoidcs) 
arose  by  a  double  longitudinal  splitting  of  the  primary  chromatin-rods. 

1  Vom  Rath  ('93,  '59)  has  endeavoured  to  show  that  a  process  involving  the  formation  of 
true  tetrads  occurs  in  the  salamander  and  the  frog,  but  the  later  and  more  accurate  studies 
of  Meves  ('96)  seem  to  leave  little  doubt  that  this  was  an  error,  and  that  the  tetrads  observed 
in  these  forms  are  not  of  normal  occurrence,  as  Flemming  ('87)  had  earlier  concluded. 
Cf.  p.  259. 


234 


REDUCTION   OF   THE    CHROMOSOMES 


In  the  first  of  his  classical  cell-studies  Boveri  ('87,  i)  reached  the 
same  result  through  a  careful  study  of  Ascaris  mcgaloccphala,  showing 
that  each  tetrad  appears  in  the  germinal  vesicle  in  the  form  of  four 
j)arallel  rods,  each  consisting  of  a  row  of  chromatin-granules  (Fig.  1 17, 
A-C ).  He  believed  these  rods  to  arise  by  the  double  longitudinal 
splitting  of  a  single  primary  chromatin-rod,  each  cleavage  being  a 


Fig.  126.  —  Tetrad-formation  in  an  insect,  Anasa.     [PauLMIER.] 

A.  Resting  spermatogonium  with  single  plasmosome  and  two  chromatin-nucleoli.  B.  Equa- 
torial plate  of  dividing  spermatogonium  ;  twenty  large  and  two  small  chromosomes,  6'.  Final 
spcrmatogonium-division.  D-I.  Prophases  of  first  maturation-division.  D.  E.  Synapsis,  with 
single  chromatin-nucleolus.  /'.  Segmented  split  spireme.  G.  H.  Formation  of  the  tetrad-rings. 
//.  /.  Concentration  of  the  rings  to  form  tetrads. 

preparation  for  one  of  the  polar  bodies.  In  his  opinion,  therefore, 
the  formation  of  the  polar  bodies  differs  from  ordinary  mitosis  only 
in  the  fact  that  the  chromosomes  split  very  early,  and  not  once,  but 
twice,  in  preparation  for  two  rapidly  succeeding  divisions  without  an 
intervening  resting  period.  He  supported  this  view  by  further  obser- 
vations in  1890  on  the  polar  bodies  of  Sagitta  and  several  gastero- 
pods,  in  which  he  again  determined,  as  he  believed,  that  the  tetrads 


ORIGIN  OF   THE    TETRADS 


-'55 


arose  by  double  longitudinal  splitting.  An  essentially  similar  view 
of  the  tetrads  was  taken  by  Hertwig  in  1890,  in  the  spermatogenesis 
of  Ascaris,  though  he  could  not  support  this  conclusion  by  very  con- 
vincing evidence.  In  1893,  finally,  Brauer  made  a  most  thorough 
and  apparently  exhaustive  study  of  their  origin  in  the  spermatogene- 
sis of  Ascaris,  which  seemed  to  leave  no  doubt  of  the  correctness  of 
Boveri's  result.  Every  step  in  the  origin  of  the  tetrads  from  the  retic- 
ulum of  the  resting  spermatocytes  was  traced  with  the  most  pains- 
taking care.  In  the  early  prophases  of  the  first  division  the  nuclear 
reticulum   breaks   up  more  or   less  completely  into  granules,  which 


^^'m'> 


Fig.  127.  —  Maturation-divisions  in  an  insect,  Afiasa.     [Paui.MIER.] 

A.  Primary  spermatocyte  in  metaphase.  B.  Equatorial  plate,  showing  ten  large  tetrads  and 
one  small  one;  "odd  chromosome"  at  o.  C.  Separation  of  the  dyads.  D.  Telophase,  which  is 
also  a  prophase  of  the  second  division.  E.  Secondary  spermatocyte ;  division  of  the  dyads ; 
small  dyad  shown  undivided.  F.  Final  anaphase ;  small  dyad  near  the  lower  chromosome-group. 
(The  figures  are  numbered  from  left  to  right.     For  later  states,  see  ¥\g.  82.) 

become  in  part  aggregated  in  a  mass  at  one  side  of  the  nucleus 
("synapsis,"  p.  276),  from  which  delicate  threads  e.xtend  througii  the 
remaining  nuclear  space  (Fig.  120,  A).  Even  at  this  period  the 
granules  of  the  threads  are  divided  into  four  parts.  As  the  process 
proceeds  the  chromatin  resolves  itself  into  a  single  spircme-thread, 
consisting  of  four  parallel  rows  of  granules,  which  break  in  two  to 
form  the  two  tetrads  (var.  bivalcns\  or  is  directly  converted  into  a 
single  tetrad  (var.  luiivalcns)  (Fig.  120).  From  these  observations 
Brauer  concludes  that  each  tetrad  arises  from  a  rod,  doubly  split 
lengthwise  by  a  process  initiated  at  a  very  early  period  through  the 


256 


K EDUCTION  OF  THE    CHROMOSOMES 


double  fission  of  the  chromatin-f^ranulcs.  U  this  be  correct,  there 
can  be  no  reduction  in  Weismann's  sense  ;  tor  the  four  products  of 
each  primary  chromati^-^^■ranule  are  equally  distributed  among  the 
four  daughter-cells.  A  similar  conclusion,  based  on  much  more 
incomi)lete  evidence,  was  reached  by  Hrauer  (92)  in   tlic   phyllopod 

Branchipus. 

Hrauer's  evidently  conscientious  figures  very  strongly  sustain  his 
conclusion,  which,  reinforced  by  the  earlier  work  of  Ilertwig  and 
Boveri,  has  until  now  seemed  to  rest  upon  an  unassailable  basis. 
The  recent  work  of  Sabaschnikoff  ('97)  nevertheless  raises  the  possi- 
bilitv  of  a  different  interpretation.  l^rauer  himself  justly  urges  that 
the  essence  of  the  process  lies  in  the  double  fission  of  the  chromatin- 
granules  to  which  the  formation  of  chromosomes  is  secondary.^ 
Kvervthing,  therefore,  turns  on  the  manner  in  which  the  quadruple 
granules  arise  ;  and  Sabaschnikoff's  work  gives  some  ground  for  the 
view  that  they  may  arise,  not  by  a  double  fission,  but  in  some  other 
way. 

Accordin«r  to  this  author  there  is  a  period  (in  the  oiigcnesis)  at  which  the  nuclear 
threads   wholly   di.sappear,   the   entire   chromatin    Ijeing    broken    up   into  granules. 
From  this  state  the  granules  emerge  in  quadruple  form  to  arrange  themselves  in  the 
doubly  split  spireme  exactly  as  Brauer  describes  :    and  a  few  observations  are  given 
(regarding  the  size  and  arrangement  of  the  granules)  which  suggest  the  possibility 
that  the  quadruple  granules  may  arise  by  the  conjugation  either  of  four  separate 
granules  or  of  two  pairs  of  double  granules.     Since  there  is  ground  for  the  view  that 
tetrads  may  ari.se  by  the  conjugation  of  chromosomes  (see  following  section),  there 
is  no  a  priori  objection  to  such  a  conclusion.    Could  it  be  sustained,  the  maturation- 
divisions  Q>{  Ascaris  would  in  fact  involve  a  true  reduction  in  Weismann  s  sense; 
for  despite  the  fact  that  the  chromosomes  are  only  longitudinally  divided,  the  four 
longitudinal  constituents  of  each  tetrad  would  not  be  equivalent  with  respect  to  the 
granules,  and  it  is  the  reduction  of  the  latter  ('•  ids'")  that  forms  the  essence  of  Weis- 
mann's hypothesis  (p.  245).     Another  consideration,  suggested  to  mc  by  Professor 
T.  11.  Morgan,  opens  still  another  possibility,  which  .seems  well  worthy  of  test  by 
further  research.     As  already  stated  (p.  88),  the  long  chromosomes  oi  Ascaris  are 
plurivalent,  since  in  all  but  the  germ-cells  each  breaks  up  into  a  much  larger  number 
of  smaller  chromosomes  (Fig.  73.  p.  U^)-       If-  therefore,  the  latter  correspond  to 
the  chromosomes  of  other  forms  in  which  tetrads  occur  (i-.;'-.  Cyclops  or  Arteniia), 
the  .so-called   "  tetrad "  of  Ascaris  is  a  compound  body  :    and  the  true  process  of 
reduction  mu.st  be  .sought  in  the  origin  of  the  smaller  elements  of  which  it  is  com- 
posed, which   are,   perhaps,  directly  comparable  with    Sabaschnikoff's   "granules." 
Until  the  questions  thus  opened  have  been  further  studied,  the  case  iox  Ascaris  must 
remain  open  :   and  it  is  perhaps  worth  suggesting  that  a  new  point  of  view  may  here 
be  found  for  further  study  al.so  of  reduction  in  the  vertebrates. - 

1  Cf,  p.  113. 

-  Bodies  closely  resembling  tetrads  are  sometimes  formed  in  mitosis,  where  no  reduction 
should  occur.  Thus,  R.  Hertwig  ('95)  has  observed  tetrads  in  the  first  cleavage-spindle  of 
echinuderm-eggs  after  treatment  with  dilute  poisons  (p.  306).  Klinckowstrom  figures  them 
in  the  .f.v^;/</ polar  spindle  of  Prostheicrwus  eggs,  while  Moore  ('95)  describes  in  the  elasmo- 
branchs  small  ring-shaped  chromosomes,  not  only  in  the  first  but  also  in  the  secotid  sperma- 
tocyte-divisions,  concluding  that  no  reduction  occurs  in  either  division. 


ORIGIN  OF   THE   TETRADS  257 

{c)  The  Forviation  of  Tetrads  by  Conjugation.  —  A  considerable 
number  of  observers  have  maintained  that  reduction  may  be  effected 
by  the  union  or  conjugation  of  chromosomes  that  were  previously 
separate.  This  view  agrees  in  princii)le  with  that  of  Ruckert, 
Hacker,  and  Vom  Rath  ;  for  the  bivalent  chromosomes  assumed  by 
these  authors  may  be  conceived  as  two  conjugated  chromosomes,  ft 
seems  to  be  confirmed  by  the  observations  of  Born  and  Pick  on 
Amphibia  and  those  of  Ruckert  on  selachians  {Pristiunis)\  for  in  all 
these  cases  the  number  of  chromatin-masses  at  the  time  the  first 
polar  body  is  formed  is  but  half  the  number  obser\^ed  in  younger 
stages  of  the  germinal  vesicle.  In  Pristinrus  there  are  at  first  thirty- 
six  double  segments  in  the  germinal  vesicle.  At  a  later  period  these 
give  rise  to  a  close  spireme,  which  then  becomes  more  open,  and  is 
found  to  form  a  double  thread  segmented  into  eighteen  double  seg- 
ments ;  i.e.  the  reduced  number.  In  this  case,  therefore,  the  prelimi- 
nary pseudo-reduction  is  almost  certainly  effected  by  the  union  of 
the  original  thirty-six  double  chromosomes,  two  by  two.  The  most 
specific  accounts  of  such  a  mode  of  origin  have,  however,  been  given 
by  Calkins  (earthworm)  and  Wilcox  (grasshopper).  The  latter 
author  asserts  ('95)  that  in  Caloptenus  the  spireme  of  the  first  sj^erma- 
tocyte  gives  rise  without  longitudinal  division  to  twenty-four  chromo- 
somes (double  the  somatic  number).  These  then  become  associated 
in  pairs,  and  still  later  the  twelve  pairs  conjugate  two  and  two  to  form 
six  tetrads.  There  is,  therefore,  no  longitudinal  splitting  of  the  chro- 
mosomes. The  a  priori  improbability  of  such  a  conclusion  is  in- 
creased by  the  studies  of  Paulmier  on  the  Hemiptera,  which  demon- 
strate the  occurrence  of  a  longitudinal  division  in  a  number  of  these 
forms  and  confirm  the  original  studies  of  Vom  Rath  on  Cryllotalpa} 

The  second  case,  which  is  perhaps  better  founded,  is  that  of  the 

earthworm  {Linnbriciis  terrestris),  as  described  by  Calkins  ('95,   2), 

whose  w^ork  was  done  under  my  own  direction.      Calkins  finds  that 

the    spireme    splits     longitudinally    and    then    divides    transverselv 

into  32  double  segments.     These   then   unite,  two  by  two,  to  form 

16   tetrads.     The   32   primary  double   segments   therefore   represent 

chromosomes  of  the  normal  number   that  have  split  huigitudinally, 

a      b  1     ,       r  ^      r  ,   .     a  b         a\x       ^,     , 

I.e. -,   etc.,  and  the  formula  for  a  tetrad  is      I ,    or  — i—      Such 

a      b  a\b         a  x 

a  tetrad,  therefore,  agrees   as  to  its  composition  with  the  formulas 

of  Hacker,  Vom  Rath,  and   Ruckert,  and  agrees  in   mode  of  origin 

with  the   process  described   by   Ruckert   in   the  eggs   of  Pristinnis. 

While  these  observations  are  not  absolutely  conclusive,  they  never- 

^  Montgomery,  who  has  denied  the  occurrence  of  a  longitudinal  division  in  roitotoma 
('98,  i),  has  subsequently  found  such  a  division  in  the  nearly  related  if  not  identical  genus 
Euchistis  ('99). 


258  REDUCTIOX  OF   THE    CHROMOSOMES 

theless  rest  on  strong  evidence,  and  they  do  not  stand  in  actual  con- 
tradiction of  what  is  known  in  the  copepods  and  vertebrates.  The 
possibility  of  such  a  mode  of  origin  in  other  forms  must,  I   think,  be 

held  open. 

Under  the  same  category  must  be  placed  Korschelt's  unique 
results  in  the  egg-reduction  of  the  annelid  Opliryotrocha  {'<^^\  which 
are  verv  ditficult  to  reconcile  with  anything  known  in  other  forms. 
The  typical  somatic  number  of  chromosomes  is  here  four.  The  same 
number  oi  chromosomes  a})pear  in  the  germinal  vesicle  (Fig.  125,  D). 
Thev  are  at  first  single,  then  double  by  a  longitudinal  split,  but  after- 
ward single  again  by  a  reunion  of  the  halves.  The  four  chromo- 
somes group  themselves  in  a  single  tetrad,  two  passing  into  the  first 
polar  body,  while  two  remain  in  the  ^^g,  but  meanwhile  each  of  them 
again  splits  into  two.  Of  the  four  chromosomes  thus  left  in  the  ^gg 
two  are  passed  out  into  the  second  polar  body,  while  the  two  remain- 
ing in  the  ^'g^^  give  rise  to  the  germ-nucleus.  From  this  it  follows 
that  the  formation  of  the  first  polar  body  is  a  reducing  division  —  a 
result  which  agrees  with  the  earlier  conclusions  of  Henking  on 
Pyrroclioris,  and  with  those  of  Paulmier  on  the  Hemiptera. 


C.   Reduction  without  Tetrad-formation 

As  already  stated  (p.  246),  the  formation  of  actual  tetrads  is  of 
relatively  rare  occurrence,  being  thus  far  certainly  known  only  in  the 
arthropods,  nematodes,  and  some  annelids.  In  the  greater  number  of 
cases  the  two  divisions  of  the  primary  chromatin-masses  {i.e.  of  the 
primary  oocyte  or  spermatocyte)  are  separated  by  a  considerable  inter- 
val, during  which  the  first  maturation  cell-division  takes  place  or  is  ini- 
tiated, and  hence  no  actual  tetrads  are  formed.  This  obviously  differs 
onlv  in  degree  from  tetrad-formation,  the  latter  occurring  only  when 
the  two  divisions  are  simultaneous  or  occur  in  rapid  succession. 

In  the  cases  now  to  be  considered  the  length  of  the  pause  between 
the  maturation-divisions  varies  considerably,  and  in  some  forms  (verte- 
brates, flowering  plants)  it  is  so  prolonged  that  the  nucleus  is  partially 
reconstructed.  In  all,  or  nearly  all,  these  cases  the  first  maturation- 
division  is  of  the  heterotypieal form,  the  chromosomes  having  the  form 
of  rings  and  arising  by  a  process  that  agrees  in  most  of  its  features 
with  that  leading  to  tetrad-formation.  There  is  here,  however,  exactly 
the  same  contradiction  of  results  as  in  the  case  of  tetrad-formation 
described  at  page  247,  and  a  bewildering  confusion  of  the  subject  still 
exists.  In  brief,  it  may  be  stated  that  most  observers  of  reduction  of 
this  type  in  the  lower  animals  (flat-worms,  annelids,  mollusks)  have 
found  one  transverse  and  one   longitudinal   division  ;  most  of  those 


RED  UC  TION   WI THO  UT   TE  TRA  D- FORM  A  TION 


259 


who  have  studied  the  vertebrates   find   two   longitudinal    divisions ; 
while  opinion  regarding  the  plants  is  still  divided. 

{a)  Animals.  —  In  the  gephyrean  Tlialasscvia  and  the  mollusk 
Z/;///^<^  (Figs.  128-130)  Griffin  ('99)  finds  that  the  rings,  arising  as 
described  above,  place  themselves  in  the  equator  of  the  spindle  with 
the  longitudinal  division  in  the  equatorial  plane.  They  are  then 
drawn  out  toward  the  spindle-poles  from  the  middle  point,  first 
assuming  the  form  of  a  double  cross,  then  of  elongated  ellipses,  and 
finally  break  into  two  daughter-U's  or  -Vs.  The  first  division  is 
therefore  longitudinal.  During  the  late  anaphase  the  V's  break  at 
the  apex,  the  two  limbs  come  close  together,  so  as  to  give  the  decep- 


A 


B 


Fig.  128.  —  Diagrams  of  reduction  in  the  types  represented  by  Thalassema  (.-/)  and  SaU- 
mandra  {B).  In  both  the  first  division  is  heterotypical.  The  second  division  (6)  is  transverse  in 
the  first  and  longitudinal  in  the  second. 

tive  appearance  of  a  longitudinal  split,  and  are  separated  by  the 
second  division  (following  immediately  upon  the  first  without  inter- 
vening resting  stage).  The  latter  is  therefore  a  transverse  division 
(Fig.  130).  An  essentially  similar  result,  though  less  comi)letely 
worked  out,  is  independently  reached  by  Holies  Lee  (97)  in  Hf/ix  .- 
by  Klinckowstrom  ('97)  in  the  turbellarian  ProstJuxcnrus :  and  by 
Francotte  ('97)  and  Van  der  Stricht  ('98,  i )  in  Thysanzoon.  Klinckow- 
strom shows  that  there  is  much  variation  in  the  way  in  which  the 
rings  open  out  and  break  apart,  though  the  result  is  the  same  in  all. 
In  case  of  the  vertebrates,  Flemming  {'%7)  long  since  described 
and  figured  typical  tetrads  in  the  salamander,  but  regarded  them  as 
"anomalies."     Vom  Rath's  later  conclusion  (93,    95)  that  they  are 


26o  KEDVCriOX  OF   THE    CHROMOSOMES 

normal  tetrads  has  not  been  sustained  by  the  still  more  recent  work 
of  Meves  ('96).  whose  careful  studies,  together  with  those  of  Moore, 
Lenhossck,  and  others,  thus  far  give  no  evidence  of  tetrad-formation, 
and  seem  oi)posed  to  the  occurrence  of  reducing  divisions  in  the 
vertebrates.  Meves's  work  in  the  main  confirms  the  earlier  results 
of  Flemming,  except  that  he  shows  that,  as  in  so  many  other  animals, 
onlv  two  generations  of  spermatocytes  exist.  At  the  first  division 
the'  nuclear  reticulum  resolves  itself  into  twelve  (the  reduced  num- 
ber) segments,  which  split  lengthwise,  the  halves  remaining  united  to 
form    elongated  rings  (Fi©-^-   -7'    ^:>7 )■     These  do  not,  however,  con- 


«  • ' 


•    •  •  ••  . 


B 


yi 


'•  .  • 


•  ^ 


•    •••     .•./•. 


c 


••  •  •     ^  •  •••- 


•  ,•  •••• 


'  • 

•  •    . 

•   • 

•^: 

••• 

•    ••  • 

^i 

•  •• 

•     •        - 

•••• 

••  •  • 

••••  • 
•     •  • .  • 

• 

•  .  •• 

Fig.  129— Maturation  and  fertilization  in  an  annelid  (armed  gephyrean)  Thalassema. 
[(IklKIIN.j 

A.  .\  few  moments  after  entrance  of  the  spermatozoon,  showing  accessory  asters;  tetrads 
forming.  />'.  T^-irly  prophase  of  first  polar  mitosis  with  centrosomes.  C.  In-pushing  of  nuclear 
w;i!l.  D.  Central  spindle  established;  elimination  of  nucleolus  and  nuclear  reticulum.  E.  Slightly 
:  ,t.r  stage  viewed  from  above.  t\  First  polar  spindle  established,  cross-shaped  tetrads,  crossing 
of  astral  rays;  sperm-head  at  j. 

dense  into  tetrads,  but  break  apart  during  the  first  division  at  the 
points  corresponding  with  the  ends  of  the  united  halves.  The  first 
division  is  therefore  an  equation-division.  As  the  V-shaped  halves  sep- 
arate they  again  split  lengthwise  ( Fig.  131),  each  of  the  secondary  sper- 
matocytes receiving  twelve  double  V's  or  dyads.  In  the  telophases 
and  ensuing  resting  stage,  however,  all  traces  of  this  splitting  are 
lost,  the  nuclei  partially  returning  to  the  resting  stage,  but  retaining 
traces  of  a  spireme-like  arrangement  (Fig.  131).  In  the  second 
division  twelve  double  V's  reappear,  showing  a  longitudinal  division 
which   Flemming  and   Meves  believe  to  be   directly  related  to  that 


REDUCTION   WITHOUT    TETRAD-FORMATION 


261 


seen  during  the  foregoing  anaphases.  There  is  therefore  no  evi- 
dence of  a  transverse  division.  McGregor  ('99)  describes  a  nearly 
sniiilar  process  in  Amphiuma,  where  the  longitudinal  division  of  the 


A 


X 


\ 


!0 


c 


4"<**  + 


r 


D 


'  I  > 


-/^■^ 


\ 


■'f^?^-^ 


^Mil^ 


i».- 


yi>    ■■nf 


!    Z' 


J 


V>v^ 


J»s^n«* 


J 


■^ 


G 


// 


y 


y 


Fig.  130.  —  Maturation  in  the  lamellibranch  Zirp/uca  and  in  I/taiasst'/nii.     [GRIFFIN.] 

A-E,  Zirp/icea;    F-/,    Thalassema. 

A.  Unfertilized  ^gg,  ring-shaped  and  cross-shaped  chromosomes.  D.  Prophase  of  first  polar 
mitosis.  C.  First  polar  spindle  ;  double  crosses.  D.  Slightly  later  stage.  E.  The  double  crosses 
have  broken  apart  (equation-division).  G.  Ensuing  stage;  daughter-V's  broken  apart  at  ihe 
apex.  H.  Telophase  of  first,  early  prophase  of  second,  division  ;  limbs  of  tlie  V's  separate  but 
closely  opposed.     F.  Later  prophase  of  second  division.     /.  Second  polar  spindle  in  mctaphase. 


daughter-V's   is    seen    with    the    greatest   clearness    throughout    the 
anaphases. 

The  weak  point  in  both  the  foregoing  cases  is  the  fact  that  all 
traces  of  the  second  longitudinal  division  are  lost  during  the  ensuing 


262 


REDUCTIOX  OF  THE    CHROMOSOMES 


resting  period  ;  and  I  do  not  think  that  even  the  observations  of 
Flemming  (97),  who  has  pubHshed  the  fullest  evidence  in  the  case, 
completely  establish  the  occurrence  of  a  subsequent  longitudinal  divi- 


Fig.  131.  —  (Compare  Fig.  27).  Maturation-divisions  in  Salanjatidra.  [E  from  FlemMING, 
the  others  from  Meves.] 

A.  First  division  in  mctaphase,  showing  heterotype  rings.  B.  Anaphase;  longitudinal  split- 
ting of  the  daughter-loops.  C.  Telophase.  D.  Ensuing  pause.  E.  Early  prophase  of  second 
division  with  longitudinally  divided  segmented  spireme.  F.  Later  prophase.  G.  Metaphase  of 
second  division. 


sion  of  the  chromosomes  in  the  second  mitosis.  In  DesmognatJins, 
however,  where  the  resting  stage  is  less  complete,  Kingsbury  ('99) 
finds  the   longitudinal  split  in    the   persistent    chromosomes    of    the 


REDUCTION   WITHOUT    TETRAD-FORMATION  263 

pause  following  the  first  division;  and  he  believes  this  to  be  the 
same  division  as  that  seen  during  the  anaphase.  Carnoy  and  Le  Hrun 
('99)  reach  the  same  result  in  the  formation  of  the  polar  bodies  in 
Triton,  though  their  general  account  of  the  heterotypical  mitosis 
differs  very  considerably  from  that  of  other  authors,  the  rings  being 
stated  to  arise  by  a  double  instead  of  a  single  longitudinal  split 
These  observers  describe  the  rings  of  the  early  anaphase  as  having 
almost  exactly  the  same  double  cross-form  as  those  in  Thahisscma  ox 
Ztrphcea  (Griffin,  '99),  but  believe  them  to  arise  in  a  manner  nearly 
in  accordance  with  Strasburger's  abandoned  view  of  1895,1  and  with 
Guignard's  ('98,  2)  and  Gregoire's  ('99)  latest  results  on  the  flowering 
plants,  the  ring  being  stated  to  arise  by  a  double  longitudinal  split""- 
ting,  as  explained  at  page  265. 

In  the  elasmobranch  Scy Ilium  Moore  ('95)  finds  twelve  (the  re- 
duced number)  ring-shaped  chromosomes  at  the  first  division.  These 
closely  resemble  tetrads ;  but  a  resting  stage  follows,  and  the  second 
division  is  likewise  stated  to  be  of  the  heterotypical  form,  l^oth  divi- 
sions are  stated  to  be  equation-divisions  —  a  conclusion  well  sup- 
ported in  case  of  the  first,  but  so  far  from  clear  in  the  .second  that  a 
careful  reexamination  of  the  matter  is  highly  desirable. 

In  mammals  the  first  division  is  of  the  heterotypical  form  (Her- 
mann, '89,  Lenhossek,  '98),  though  the  rings  are  much  smaller  than 
ni  the  salamander,  recalling  those  seen  in  arthropods.  No  true 
tetrads  are,  however,  formed,  and  the  two  divisions  are  separated  by 
a  resting  period.  The  character  of  the  second  division  is  undeter- 
mined, though  Lenhossek  believes  it  to  be  heterotypical,  like  the  first. 

(b)  Plants.  —  It  is  in  the  flowering  plants,  where  reduction  likewise 
occurs,  as  a  rule,  without  true  tetrad-formation,  that  the  contradiction 
of  results  reaches  its  cHmax ;  and  it  must  be  said  that  until  further 
research  clears  up  the  present  confusion  no  definite  result  can  be 
stated.  The  earlier  work  of  Strasburger  and  Guignard  indicated  that 
no  reducing  division  occurred,  the  numerical  reduction  being  directly 
effected  by  a  segmentation  of  the  spireme-thread  into  half  the  somatic 
number  of  chromosomes.  Thus  these  observers  found  in  the  male 
that  the  chromosomes  suddenly  appeared  in  the  reduced  number 
(twelve  in  the  lily,  eight  in  the  onion)  at  the  first  division  of  the 
pollen-mother-cell,  and  in  the  female  at  the  first  division  of  the 
mother-cell  of  the  embryo-sac.  The  subsequent  ])henomena  differ 
in  a  very  interesting  way  from  those  in  animals,  owing  to  the  fact 
that  the  two  maturation-divisions  are  followed  in  the  female  by  one 
and  in  the  male  by  two  or  more  additional  divisions,  in  both  of  which 
the  reduced  number  of  chromosomes  persists.  In  the  male  the  two 
maturation-divisions  give  rise  to  four  pollen-grains,  in  the  female  to 

1  Cf.  p.  269. 


264 


KEDUC710X   OF   THE    CHROMOSOMES 


the  four  primary  cells  of  the  embryo-sac  (Fig.  132);  and  these  two 
divisions  undoubtedly  correspond  to  the  two  maturation-divisions  in 
animals.  In  the  female,  as  in  the  animals,  only  one  of  the  four 
resulting  cells  gives  rise  to  the  (i<^^,  the  other  three  corresponding  to 
the  polar  bodies  in  the  animal  (t'^^^j:,,  though  they  here  continue  to 
divide,  and   thus    form   a   rudimentary    prothalHum.^     The   first-men- 


\\    ■ 


7 


A 


^c- 


C 


'   iV^:- 


m^^ 


0        ...    IV 


^ 


"4.  ^  -   •  ■ . 


-V 


b-,.--'/ 


F 


E 


Fig.  132.  —  General  view  of  the  maturation-divisions  in  flowering  plants.     [MOITIER.] 
A-C,  in  the  male;  D-F,  in  the  female.      A.  The  two  secondary  spermatocytes  (pollen-mother- 
cells)  just  after  the  first  division  (Liliurn).     B.  Final  anaphase  of  second  division  {Podophyllum^. 

C.  Resulting  telophase,  which  by  division  of  the  cytoplasmic  mass  produces  four  pollen-grains. 

D.  Embryo-sac  after  completion  of  the  first  nuclear  division  {Liliuni).  E.  The  same  after  the 
second  division.  F.  The  upper  four  cells  resultin.;  from  the  third  division  {c/.  Fig.  io6)  :  o,  ovum; 
/,  upper  polar  cell ;  s,  synergida;.     (For  further  details,  see  Figs.  133,  134.) 


1  Of  these  three  cells  one  divides  to  form  the  "synergidre,"  the  other  two  divide  to  form 
thriee  "antipodal  cells"  (which  like  the  synergida:;  finally  degenerate)  and  a  "lower  polar 
cell."  The  latter  sooner  or  later  conjugates  with  the  "  upper  polar  cell "  (the  sister-cell  of 
the  egg)  to  form  the  "  secondary  embryo-sac-nucleus,"  by  the  division  of  which  the  endo- 
sperm-cells arise.     Of  the  whole  group  of  eight  cells  thus  arising  only  the  egg  contributes 


REDUCTION    WITHOUT   TETRAD-FORMATION  265 

tioned  cell,  however,  does  not  directly  become  the  e^^g,  but  divides 
once,  one  of  the  products  being  the  ^gg  and  the  other  the '' upper 
polar  cell"  (Fig.  132,  F),  which  contributes  to  the  endosperm-forma- 
tion (see  footnote,  and  compare  page  218). 

In  the  male  the  two  maturation-divisions  are  in  the  angiosperms 
followed  by  two  others,  one  of  which  separates  a  "vegetative"  trom 
a  "generative"  cell,  while  the  second  divides  the  generative  nucleus 
into  two  definite  germ-nuclei.  In  the  gymnosperms  more  than  two 
such  additional  divisions  take  place.  In  these  later  divisions,  both  in 
the  male  and  in  the  female  (with  the  exception  noted  in  the  footnote 
below),  the  reduced  number  persists,  and  the  principal  interest 
centres  in  the  first  two  or  maturation-divisions.  Strasburger  and 
Guignard  found  in  Lilium  that  while  both  these  divisions  differed  in 
many  respects  from  the  mitosis  of  ordinary  vegetative  cells,  neither 
involved  a  transverse  or  reducing  division,  the  chromosomes  under- 
going a  longitudinal  splitting  for  each  of  the  maturation-divisions. 
Further  investigations  by  Farmer  ('93),  Belajeff  ('94),  Dixon  (96), 
Sargant  ('96,  '97),  and  others,  showed  that  the  first  division  is  often 
of  the  heterotypical  form,  the  daughter-chromosomes  in  the  late-meta- 
phase  having  the  form  of  two  V's  united  by  their  bases  (<>). 
Despite  the  complication  of  these  figures,  due  to  torsion  and  other 
modifications,  their  resemblance  to  the  ring-shaped  bodies  observed 
in  the  first  maturation-division  of  so  many  animals  is  unmistakable, 
as  was  first  clearly  pointed  out  by  Farmer  and  Moore  ('95). 

Botanists  have  differed,  and  still  differ,  widely  in  their  interpreta- 
tion both  of  the  origin  and  subsequent  history  of  these  bodies  upon 
which  the  question  of  reduction  turns.  According  to  Strasburger's 
('95)  first  account  their  origin  has  nothing  in  common  with  that  of 
the  tetrad-rings,  since  they  were  described  as  arising  by  a  double  \o\\- 
gitudinal  splitting  of  a  primary  rod,  the  halves  then  separating  first 
from  one  end  along  one  of  the  division-planes,  and  then  from  the 
other  end  along  the  other  plane,  meanwhile  opening  out  to  form  a 
ring  such  as  is  shown  in  Fig.  133.  (This  process,  somewhat  difficult 
to  understand  from  a  description,  will  be  understood  from  the  dia- 
gram. Fig.  135,  E~I^  The  four  elements  of  the  ring  are  then  distrib- 
uted without  further  division  by  the  two  ensuing  maturation-divisions  ; 
and  the  process,  except  for  the  peculiar  opening  out  of  the  ring,  is 

to  the  morphological  formation  of  the  embryo.  It  is  a  highly  interesting  fact  that  the  nuni- 
ber  of  chromosomes  shown  in  the  division  of  the  lower  of  the  two  nuclei  {i.e.  the  mother- 
nucleus  of  the  antipodal  cells  and  lower  polar-cell)  formed  at  the  first  division  of  the 
embryo-sac-nucleus  is  inconstant,  varying  in  the  lily  from  12,  16,  20,  to  24  (Cluignard,  '91,  l), 
in  which  respect  they  contrast  with  the  descendants  (egg,  synergidx)  of  the  upper  nucleus, 
which  always  show  the  reduced  number  (Mottier,  '97,  i),  i.e.  in  l.ilium  twelve.  This 
exception  only  emphasizes  the  rule  of  the  constancy  of  the  chromosome-number  in  general; 
for  these  cells  are  destined  to  speedy  degeneration. 


266 


REDUCTIOX   OF  THE    CHROMOSOMES 


essentially  in  agreement  with  the  facts  described  in  Ascaris,  and 
involves  no  reduction-division.  Essentially  the  same  result  is  reached 
by  Guignard  (98)  in  his  latest  paper  on  Naias,  and  by  Gregoire  ('99) 

in  the  Liliacere. 

Strasburger  twice  shifted  ground  in  rapid  succession.     First  (97,  2), 
with  M()tticr('97,  i ).  he  somewhat  doubtfully  adopted  a  view  agreeing 


,  AA'V^S'IA^. 


\ 


r?v^ 


\ 


X- 


/ 


A 


B 
E 


/TV 


^ 


Fig.  133.  —  The  first  maturation-division  in  flowering  plants.  [/>",  STRASBURGER  and  MOT- 
TlKk  ,  the  others  from  MorilER.] 

.  /.  Mother-cell  of  the  embrvo-sac  in  Lilium  ;  early  prophase  of  first  division  :  chromatin- 
threads  already  longitudinally  divided.  B.  Slightly  later  stage  (split  spireme)  in  the  nucleus  of 
the  poUen-mo'ther-cell.  C.  h  slightly  later  prophase  (pollen-mother-cell.  Podophyllum)  with 
twisted  split  spireme.  D.  Earlier  prophase  (ZL/V/ww,  female) ;  split  twisted  chromosomes.  E. 
Equatorial  plate  {Lilium,  male).  F..  First  maturation-spindle  {Fritillaria,  male).  G.  Diver- 
gence of  the  daughter-chromosomes  {Lilium,  male). 

essentially  with  the  interpretation  of  Vom  Rath,  Riickert,  etc.  (p.  247). 
The  primary  rods  split  once,  and  bend  into  a  V,  the  branches  of 
which  often  come  close  together,  and  may  be  twisted  on  themselves, 
thus  giving  the  appearance  of  the  second  longitudinal  split  described 
in  Strasburger's  paper  of  1895.  The  two  halves  of  the  split  U  then 
separate,  opening  out  from  the  apex,  to  form  the  0 -figure.     In  the 


REDUCTION   WITHOUT    TETRAD-FORMATION  267 

second  division  the  limbs  of  the  daughter-V's  again  come  close 
together,  remaining,  however,  united  at  one  end,  where  they  were 
believed  finally  to  break  apart  during  the  second  division.  The  latter 
was,  therefore,  regarded  as  a  true  reduction-division,  the  apparent 
longitudinal  split  being  merely  the  plane  along  which  the  halves  of 
the  V  come  into  contact  (Fig.  134,  C,  D). 

The  two  accounts  just  given  represent  two  extremes,  the  first 
agreeing  essentially  with  Ascaris,  the  second  with  the  copepods  or 
insects.  When  we  compare  them  with  others,  we  encounter  a  truly 
bewildering  confusion.  Strasburger  and  Mottier  ('97)  themselves 
soon  abandoned  their  acceptance  of  the  reducing  division,  returning 
to  the  conclusion  that  in  both  sexes  {Liliuin,  Podopliylhnn)  both  divi- 
sions involve  a  longitudinal  splitting  of  the  chromosomes  (Figs.  133, 
134).  In  the  first  division  the  longitudinally  split  spireme  segments 
into  twelve  double  rods,  which  bend  at  the  middle  to  form  double  V's, 
with  closely  approximated  halves.  Becoming  attached  to  the  spindle 
by  the  apex,  the  limbs  of  each  separate  to  form  a  o -figure.  At 
telophase  the  daughter-V's  shorten,  thicken,  and  join  together  to  form 
a  daughter-spireme  consisting  of  a  single  contorted  thread.  This 
splits  lengthwise  tJirougJioiit  its  whole  extent^  and  then  segments  into 
double  chromosomes,  the  halves  of  which  separate  at  the  second 
division  (Fig.  135,  L-M).  The  latter,  therefore,  like  the  first, 
involves  no  reducing  division.  This  result  agrees  in  substance  with 
the  slightly  earlier  work  of  Dixon  ('96)  and  of  Miss  Sargant  (96, 
'97),  whose  account  of  the  origin  of  the  0  -figure  of  the  first  division 
differs,  however,  in  some  interesting  details.  It  is  also  in  harmony 
with  the  general  results  of  Farmer  and  Moore  ('95),  of  Gregoire  ('99), 
and  of  Guignard  ('98),  who,  however,  describes  the  first  division  nearly 
in  accordance  with  Strasburger's  account  of  1895,  as  stated  above. 
On  the  other  hand,  Ishikawa  (pollen-mother-cells  of  Allium,  'gy)  and 
especially  Belajeff  (pollen-mother-cells  of  Iris,  98)  conclude  that  the 
second  division  is  a  true  transverse  or  reducing  division. ^  Ishikawa 
described  the  first  division  as  being  nearly  similar  to  the  ring-forma- 
tion in  copepods,  the  four  elements  of  the  ring  being  often  so 
condensed  as -nearly  to  resemble  an  actual  tetrad.  In  the  early  ana- 
phases the  daughter-V's  break  at  the  apex ;  and,  although  in  the  later 
anaphases  the  limbs  reunite,  Ishikawa  is  inclined  to  regard  the  trans- 
verse division  as  being  a  preparation  for  the  second  mitosis.  Bela- 
jeff's  earlier  work  ('94)  on  Liliuin  gave  an  indecisive  result,  though 
one  on  the  whole  favourable  to  a  reducing  division.  In  his  latest 
paper,  however  ('98,  i),  Belajeff  takes  more  positive  ground,  stating 
that  after  the  examination  of  a  large  number  of  forms  he  has  found 

1  Schaffner  ('97,  2)  reaches  exactly  the  reverse  result  in  Lilium  philaJelphicum,  i.e.  the 
first  division  is  transverse,  the  second  longitudinal. 


268 


REDUCTIOX   OF   THE    CHROMOSOMES 


in  the  pollen-mother-cells  of  Iris  a  nuich  more  favourable  object  of 
investi<;ation  than  Liliu))i,  I'^i til/aria,  and  the  other  forms  on  which 
most  of  the  work  thus  far  has  been  done,  and  one  in  which  the  sec- 
ond division  takes  place  with  "admirable  clearness";  he  also  gives 
interesting^  additional  details  ot  the  first  division  in  this  and  other 
forms.  In  the  first  division  the  spireme  splits  lengthwise,  and  then 
breaks  into  chromosomes,  which  assume  the  shape  of  a  V,  Y,  or  X 
(Fig.  135,  X-Q).     The  two  limbs  of  these  bodies  do  not,  as  might  be 


A 


■>-'^^ 


V    B 


C 


G    \^ 


Fig;.  134.  —  Tlie  seconrl  maturation-division  in  flowering  plants.  [B.  SXRASBURGER  and 
Mullii.K;    the  others  from  Moi  iiKR.] 

A.  Nucleus  of  secondary  spermatocyte  {Podophyllum).  D.  Prophase  of  second  division 
{Ltliutn,  male)  with  longitudinally  divided  chromatin-threads.  E.  Corresponding  stage  in  the 
female.  E.  Metaphase  of  second  division  {Podophyllnm,  male).  G.  Initial  anaphase  {Lilmvi, 
female).  CD.  illustrate  Mottier's  earlier  conclusions.  ('.  Second  division  {Lilium,  male),  with 
chromosomes  bent  together  so  as  to  simulate  a  sjilit.  A  Slightly  later  stage  [Eritillaria,  male), 
showing  stage  supposed  to  result  from  breaking  apart  of  the  limbs  of  the  U  at  point  of  flexure. 


supposed,  represent  sister-chromosomes  (resulting  from  the  longitu- 
dinal division  of  the  spireme)  attached  by  one  end  or  at  the  middle, 
since  each  X,  Y,  or  V  is  double,  consisting  of  two  similar  superim- 
posed halves.  Belajeff,  therefore,  regards  these  figures  as  longitu- 
dinally divided  bivalent  chromosomes,  having  the  value  of  tetrads, 
each  limb  being  a  longitudinally  split  single  chromosome.  The 
double  V's,  Y's,  and  X's  take  up  a  position  with  the  apex  (or  one  end 
of  the  X)  attached  to  the  spindle,  and  the  longitudinal  division  in  the 
equatorial  plane.      The  halves  then  progressively  diverge  from  the 


REDUCTION    WITHOUT  TETRAD-FORMATION  269 

point  of  attachment,  thus  giving  rise  to  0 -shaped,  ^>- -shaped,  or 
XX -shaped  figures,  all  of  which  in  the  end  assume  the  0 -shape. 
This  part  of  the  process  is  in  the  main  similar  to  that  described  by 
Strasburger  and  Mottier,  and  the  daughter-V's  diverge  in  the  same 
way  as  these  authors  describe.  The  second  division,  however,  differs 
radically  from  their  account,  since  no  splitting  of  the  spireme-thread 
occurs.  The  chromosomes  reappear  in  the  V-,  Y-,  and  X-forms,  but  are 
ufidivided,  and  only  half  as  thick  as  in  the  first  division.  Passing  to 
the  equator  of  the  spindle,  the  V-  and  Y-forms  break  apart  at  the 
apex,  while  the  X-forms  separate  into  the  two  branches  of  the  X,  the 
daughter-chromosomes  having  the  form  of  rods  slightly  bent  at  the 
outer  end  to  form  a  J-figure  (Fig.  135,  R-T).  This  division  is, 
accordingly,  a  transverse  or  reducing  one,  which  "  corresponds  com- 
pletely to  the  reduction-division  in  the  animal  organism"  ('98,2, 
p.  33.)  Atkinson  ('99)  reaches  the  same  general  result  in  Tnlliuni, 
stating  very  positively  that  no  longitudinal  division  occurs  in  the 
second  mitosis,  and  believing  that  the  daughter-V's  of  the  first  (hete- 
rotypical)  mitosis  retain  their  individuality  throughout  the  ensuing 
pause,  and  break  apart  at  the  apex  (reducing  division)  in  the  second 
mitosis.  This  observer  finds  further  that  in  Arisc^ma  the  heterotvpi- 
cal  rings  of  the  first  mitosis  condense  into  true  tetrads,  by  one  longi- 
tudinal and  one  transverse  division,  but  believes  that  in  this  case  it 
is  th.Qjirst  division  that  effects  the  reduction,  as  in  the  insects. 

Such  confusion  in  the  results  of  the  most  competent  observers  of 
reduction  in  the  flowering  plants  is  itself  a  sufficient  commentary  on 
the  very  great  difficulty  and  uncertainty  of  the  subject;  and  it  would 
be  obviously  premature  to  draw  any  positive  conclusions  until  further 
research  shall  have  cleared  up  the  matter.^ 

1  Strasburger's  new  book,  entitled  Uber  Redtiktionsiheiliing,  Spindelbildung,  Cfutroso- 
men  nnd  Cilienhildner  im  PJlanzenreich  (Jena,  1900),  is  received  while  this  work  is  in 
press,  too  late  for  analysis  in  the  text.  In  this  treatise  the  author  gives  an  exhaustive  review 
of  the  entire  subject,  contributing  also  many  new  and  important  observations  on  Liliuni, 
Iris,  Podophylliwi,  Tradescantia,  Allium,  larix,  and  several  other  forms.  The  general 
result  of  these  renewed  researches  leads  Strasburger  to  return,  in  tlie  main,  to  his  conclu- 
sions of  1895,  ^^'ith  which  agree,  as  stated  above,  the  results  of  (luignard  and  Cregoire;  and. 
in  a  careful  critique  of  Belajeff's  work,  he  shows  how  the  results  of  this  observer  may  be 
reconciled  with  his  own.  The  essence  of  Strasburger's  interpretation  is  as  follows.  In  the 
prophases  of  the  first  division  the  chromosomes  first  undergo  a  longitudinal  division,  shorten 
to  f9rm  double  rods,  and  then  again  split  lengthwise  in  a  plane  at  right  angles  to  the  lirst. 
The  following  stages  vary  even  in  the  same  species  {Uliiini)  \  and  here  lies  the  explanation 
of  much  of  the  divergence  between  the  accounts  of  different  observers,  (i)  In  the  typical 
case,  the  chromosomes  are  placed  radially,  with  one  end  next  the  spindle;  ami,  during  the 
metaphase,  they  open  apart  along  the  first  division-jilane,  from  the  spindle  outwards,  to  form 
h-  -shaped  figures.  These  figures  meanwhile  open  apart  from  the  free  end  inwards  along  the 
second  division-plane.  Thus  arise  the  characteristic  o -shaped  figures,  the  daughter-V\s 
having  separated  along  the  first  (equatorial)  division-plane,  while  the  two  hmbs  ut  each  V 
have  resulted,  not  through  bending,  but   from  a  secoml  (axial)  split  (I'ig.  135,  A-//').      The 


^ 


R  S  T 

Fig.  135.  —  Diagrams  illustrating  different  accounts  of  reduction  in  the  flowering  plants. 

A-D.  Vegetative  mitoses  (heterotypical  form)  in  Picea.    [BelajEFF.] 

E-I.  Illustrate  Strasburger's  earlier  account  (95)  and  the  later  one  of  Guignard,  of  the  first 
maturation-division.  E.  Doubly  split  rod.  F.  Metaphase,  in  profile.  G.  The  same  en  face, 
showing  the  heterotype  ring.     H.  I.     Opening  out  and  breaking  apart  of  the  ring. 

J-M.  Later  account  of  Strasburger  and  Mottier  {cf.  Figs.  133,  134).  J.  Longitudinally  split, 
V-shaped  chromosome  of  first  division.  K.  Opening  out  of  the  ring.  L.  Prophase  of  second 
division,  showing  longitudinally  split  segmented  spireme.     M.  Initial  anaphase  of  second  division. 

N-Q.  First  division.  [BELAJEFF.]  A'.  Longitudinally  split  chromosomes,  viewed  in  the  equa- 
torial plane.  O.  The  same  viewed  in  the  axis  of  the  spindle.  P.  Separation  of  the  daughter- 
chromosomes.     Q.  Anaphase,  all  the  chromosomes  assuming  the  V-form. 

/?- 7:  Second  division  in /Wj.  [Belajeff.]  R.  Equatorial  plate,  limbs  of  X's  and  V's  break- 
ing apart  (reducing  division).  6'.  Slightly  later  stage,  with  daughter-chromosomes  still  united  at 
one  end.     T.     Anaphase. 

270 


PECULIARITIES   OF  REDUCTION  IN   THE  INSECTS  2'Jl 

Resume.  In  reduction  without  tetrad-formation  the  spireme  seg- 
ments into  half  the  somatic  number  of  chromosomes,  which  split 
lengthwise  and  open  out  to  form  rings  for  the  first  (heterotypical) 
mitosis.  According  to  one  set  of  observers,  including  Flernming, 
Meves,  McGregor,  Kingsbury,  Moore,  Klinckowstrom,  Van  der  Stricht', 
Francotte,  Grifhn,  Belajeff,  Farmer,  Dixon,  Strasburger,  Sarganti 
Mottier,  Ishikawa,  and  Atkinson,  the  ring  arises  by  a  single  longi- 
tudinal division.  According  to  another  group,  including  Carnoy, 
Le  Brun,  Guignard,  and  Gregoire,  the  ring  arises  through  a  double 
longitudinal  division,  one  representing  the  axial  and  the  other  the 
equatorial  plane  of  the  <>  -figure.  The  second  group  of  observers 
regard  both  maturation-divisions  as  longitudinal.  Among  the  first 
group,  Flemming,  Meves,  McGregor,  Kingsbury,  Moore,  Farmer, 
Dixon,  Strasburger,  Sargant,  and  Mottier  likewise  believe  both  divi- 
sions to  be  longitudinal,  the  daughter-V's  or  their  products  again 
splitting  lengthwise  for  the  second  division  ;  while  Klinckowstrom, 
Van  der  Stricht,  Francotte,  Griffin,  Belajeff,  Ishikawa,  and  Atkinson 
beHeve  one  of  them  to  be  transverse,  the  daughter-V's  breaking  apart 
at  the  apex,  and  thus  giving  the  reducing  division  of  Weismann.^ 


D.    Some  Peculiarities  of  Reduction  in  the  Insects 

We  may  here  briefly  consider  some  interesting  observations  which  show  that  in 
some  cases  the  nuclear  substance  may  be  unequally  distributed  to  the  germ-nuclei. 
Henking  (''90)  discovered  that  in  the  second  spermatocyte-division  of  Pyrrocho- 
r is  one  of  the  *' chromosomes  ^'  passes  undivided  into  one  of  the  daughter-cells 
(spermatids)  which  receives  twelve  chromatin-elements  while  its  sister  receives  but 
eleven.  (The  number  of  chromosomes  in  the  spermatogonia,  and  of  rings  in  the 
first  spermatocyte-division  is  twenty-four).  This  anomalous  process  is  conrirmed 
with  interesting  additional  details  by  Paulmier  ('99)  in  Anasa,  and  obviously  rt-lated 
phenomena  are  described  by  Montgomery  ('99,  i )  in  Pentatonia.,  and  by  McClung  ("99) 
in  XipJiidin7n. 

breaking  apart  of  the  V's  at  the  apex,  as  described  by  Belajeff,  is,  therefore,  not  a  transverse 
division,  but  merely  the  completion  of  the  second  longitudinal  division.  (2)  In  a  second 
and  exceptional  type,  the  chromosomes  are  placed  tnngcniially  to  the  spindle,  and  the 
halves  separate  from  the  middle,  again  producing  <>  -shaped  tigures.  These,  however,  arc 
not  of  the  same  nature  as  those  arising  in  the  first  case,  since  they  are  formed  by  a  bending 
out  of  each  daughter-chromosome  at  the  middle  to  form  the  V,  and  not  by  the  secoml  longi- 
tudinal split.  The  effect  of  the  latter  is  in  this  case  to  render  each  daughter-V  in  itself 
double,  precisely  as  in  the  salamander.  The  difference  between  the  two  types  results 
merely  from  the  difference  of  position  of  the  chromosome  with  respect  to  the  spindle,  and 
the  final  result  is  the  same  in  both,  i.e.  two  longitudinal  divisions  and  no  reducing  one. 

This  highly  important  work  brings  very  strong  evidence  against  the  occurrence  of  trans- 
verse or  reducing  divisions  in  the  higher  plants,  and  seems  to  explain  satisfactorily  most  of 
the  differences  of  interpretation  given  by  other  observers.  It  will  be  interesting::  to  see 
whether  a  similar  interpretation  is  possible  in  the  case  of  mollusks,  anneliiis.  and  arthropods, 
where  the  early  stages,  in  many  cases,  so  strikingly  resemble  those  occurring  in  the  plants. 

^  Cf.  footnote  on  page  269. 


2-2  REDUCriOX  Of    THE    CI/EOMOSOMES 

In  Pcntatoma  the  number  of  chromosomes  in  the  spermatocyte  is  fourteen. 
During  the  final  anaphases  of  the  hist  division,  one  of  the  fourteen  daughter-chronio- 
somes\ssumcs  a  ilitTerent  stainin.i;-cai)acity  from  the  others,  and  becomes  a  "  chro- 
matin-nucleohis  "  which  fra.iimcnts  into  several  smaller  bodies  during  the  ensuing 
rcsting-stage.  During  each  of  the  succeeding  spermatocyte-divisions  appear  seven 
chronrosomes  and  a  single  small  chromatin-nucleolus,  and  both  of  these  kinds  t)f 
bodies  are  halved  at  each  division,  so  that  each  spermatid  receives  seven  chromo- 
somes and  a  single  chromatin-nucleolus.'  In  Xipliidittin  a  body  called  by  McClung 
the  -accessory  chromosome,"  and  believed  by  him  to  correspond  to  the  "chromatin- 
nucleolus '*  of  VVz/A/Av//./,  appears  in  the  early  prophases  of  the  last  spermatogoniuni- 
division  while  the  remaining  chromatin  still  forms  a  reticulum.  In  the  equatorial 
plate  this  lies  outside  the  ring  of  chromosomes,  but  divides  like  the  latter.  The 
same  bodv  appears  in  the  ensuing  resting-stage,  and  during  both  of  the  spermatocyte- 
divisions.'  In  these  it  lies,  as  before,  outside  the  chromosome-ring,  and  ditiers 
markedly  from  the  other  chromosomes,  but  divides  like  the  latter,  each  of  the  halves 
passing  into  one  of  the  spermatids,  where  it  appears  to  form  an  important  part  of  the 
sperm-nucleus. 

Despite  the  peculiarities  described  above,  tlie  chromatin,  as  a  whole,  seems  to  be 
equally  distrii)uted  in  both  Pcntatoma  and  Xiphidimn.  In  Anasa,  however,  Paul- 
niier's  studies  (98,  '99),  made  in  my  laboratory,  give  a  result  agreeing  with  that  of 
Henking.  and  suggest  some  very  interesting  further  questions.  The  spermatogonia- 
nuclei  contain  two  nucleolus-like  bodies,  and  give  rise  to  twenty-two  chromosomes, 
of  which  two  are  smaller  than  the  others  (Fig.  126).  In  the  first  spermatocyte-divi- 
sion  appear  eleven  tetrads.  Ten  of  these  arise  from  rings  like  those  of  uryllotalpa. 
etc.  The  eleventh,  which  is  much  smaller  than  the  others,  seems  to  arise  from  a 
single  nucleolus-like  body  of  the  spermatocyte-nucleus.  and  by  a  process  diftering 
coirsiderably  from  the  others.  All  of  these  bodies  are  halved  to  form  dyads  at  the 
first  division.  In  the  second  spermatocyte-division  (Fig.  127)  the  larger  dyads 
divide  to  form  single  chromosomes  in  the  usual  manner.  The  stnall  dyad,  Jwwevcr, 
fails  to  divide,  passitii^ over  bodily  into  one  of  the  spermatids.  In  tliis  case,  there- 
fore, half  of  the  spermatids  receive  ten  single  chromosomes,  while  the  remainder 
receive  in  addition  a  small  dyad. 

A  comparison  of  the  foregoing  results  indicates  that  the  small  tetrad  (dyad)  corre- 
sponds to  the  e.xtra  chromosome  observed  by  Henking  in  Pyrrochoris,  and  perhaps 
also  to  the  "accessory  chromosome"'  oi Xiphidiuni.  Whether  it  corresponds  to  the 
'•chromatin-nucleolus''  oi  Pentatonia  is  not  yet  clear.  The  most  remarkable  of 
these  strange  phenomena  is  the  formation  of  the  small  tetrad,  which  seems  to  be  a 
non-essential  element,  since  it  does  not  contribute  to  all  the  spermatozoa.  Paulmier 
is  inclined  to  ascribe  to  it  a  vestigial  significance,  regarding  it  as  a  "degenerating" 
chromosome  which  has  lost  its  functional  value,  though  still  undergoing  in  some 
measure  its  original  morphological  transformation  :  in  this  connection  it  should  be 
pointed  out  that  the  spermatocyte-nucleolus,  from  which  it  seems  to  be  derived,  is 
represented  in  the  spermatogonia  by  two  such  nucleoli,  just  as  the  single  small  tetrad 
is  represented  by  two  small  chromosomes  in  the  spermatogonia-mitoses.  The  real 
meaning  of  the  phenomenon  is,  however,  wholly  conjectural. 


E.     The  Early  History  of  the  Germ-nuclei 

There   are   many  peculiarities   in   the   early  history  of   the   germ- 
nuclei,  both  in  plants  and  animals,  that  have  a  special  interest  in  con- 

^  On  this  latter  point  Montgomery's  observations  do  not  seem  quite  decisive. 


EARLY  HISTORY   OF    THE    GERM-NUCUil 


273 


nection  with  the  reduction-problem  ;  and  some  of  these  have  raised 
some  remarkable  questions  regarding  the  origin  of  reduction.  A 
large  number  of  observers  are  now  agreed  that  during  the  growth- 
period  preceding  the  maturation-division  (p.  216),  in  both  sexes,  the 
nucleus  of  the  mother-cell  (spermatogonium,  oogonium),  both  in 
plants  and  in  animals,  passes  through  some  of  the  changes  prepara- 
tory to  reduction  at  a  very  early  period.  Thus,  in  the  (ty:^^  the  pri- 
mary chromatin-rods  are  often  present  in  the  very  young  ovarian 
eggs,  and  from  their  first  appearance  are  already  split  longitudinally.' 
Hacker  ('92,  2)  made  the  interesting  discovery  that  in  some  of  the 
copepods  {CantJiocamptiis,  Cyclops)  these  double  rods  could  be  traced 


OZ'."- 


Fig.  136.  —  Longitudinal  section  through  the  ovary  of  the  copepod  Canthocamptus.        [Hacker,] 

og.   The  youngest  germ-cells  or  oogonia  (dividing  at  og.'^) ;   a.  upper  part  of  the  growth-zone; 
oc.  oocyte,  or  growing  ovarian  egg;  ov.  fully  formed  egg,  with  double  ciiromatin-rods. 


back  continuously  to  a  double  spireme-thread,  following  immediately 
upon  the  division  of  the  last  generation  of  oogonia,  and  that  at  no 
period  is  a  true  retictibnn  formed  in  tJie  gerniiual  vesicle  (Fig.  136). 
In  the  following  year  Ruckert('93,  2)  made  a  precisely  similar  discm-- 
ery  in  the  case  of  selachians.  After  division  of  the  last  generation 
of  oosconia  the  daufrhtcr-chromosomes  do  not  give  rise  to  a  reticu- 
lum,  but  spHt  lengthwise,  and  persist  in  this  condition  throughout 
the  entire  growth-period  of  the  ^^g.  Riickert  therefore  concluded 
that  the  germinal  vesicle  of  the  selachians  is  to  be  regarded  as  a 
"  daughter-spireme  of  the  oogonium  {Vr-ei)  grown  to  enormous 
dimensions,  the  chromosomes  of  which  are  doubled  and  arranged  in 

1  Hacker,  Vom  Rath,  Riickert,  in  copepods;    Ruckert  in  selachians;    Rom  and   Fick  in 
Amphibia;    HoU  in  the  chick;    Ruckert  in  the  rabbit. 
T 


-V4 


REDUCTIOX   OF   THE    CHROMOSOMES 


pairs."  ^  In  this  case  their  number  seems  to  be  at  first  the  somatic 
number  (thirty-six),  which  is  afterward  halved  by  conjugation  of  the 
elements  tw(^  and  two  (  Riickert ),  as  in  /.//w/t/V//^- (Calkins).  It  is, 
however,  certain  that  in  many  cases  (insects,  copepods)  the  double 
rods  first  appear  in  the  reduced  number,  and  the  observations  of  Vom 
Rath  ('93)  and  Hacker  ('95,  3 )  K*^'^  some  reason  to  believe  that  the 
reduced  number  mav  in  some  forms  be  present  in  the  earlier  proi^eni- 
tors  of  the  germ-cells,  the  former  author  having  found  but  half  the 
normal  number  in  some  of  the  embryonic  cells  of  the  salamander,  while 
Hacker  ('95,  3)  finds  that  in  Cyclops  brcviconiis  the  reduced  number 
of  chromosomes  (twelve)  appears  in  the  primordial  germ-cells  which 
arc  differentiated  in  the  blastula-stage  (Fig.  74).  He  adds  the  inter- 
esting discovery  that  in  this  form  the  somatic  nuclei  of  the  cleavage- 
staL^es  show  the  same  number,  and  hence  concludes  that  all  the 
chromosomes  of  these  stages  are  bivalent.  As  development  proceeds, 
the  germ-cells  retain  this  character,  while  the  somatic  cells  acquire 
the  usual  number  (twenty-four)  —  a  process  which,  if  the  conception 
of  bivalent  chromosomes  be  valid,  must  consist  in  the  division  of  each 
bivalent  rod  into  its  two  elements.  We  have  here  a  wholly  new  light 
on  the  historical  origin  of  reduction  ;  for  the  pseudo-reduction  of  the 
germ-nuclei  seems  to  be  in  this  case  a  persistence  of  the  embryonic 
condition,  and  we  may  therefore  hope  for  a  future  explanation  of  the 
process  by  which  it  has  in  other  cases  been  deferred  until  the  penul- 
timate cell-generation,  as  is  certainly  the  fact  in  Ascaris? 

This  leads  to  the  consideration  of  some  very  interesting  recent  dis- 
coveries regarding  the  relation  of  reduction  to  the  alternation  of  gen- 
erations in  the  higher  plants.  As  already  stated  (p.  263),  Strasburger, 
Guignard,  and  other  observers  have  found  that  in  the  angiosperms 
the  two  maturation-divisions  are  in  both  sexes  followed  by  one  or 
more  divisions  in  which  the  reduced  number  persists.  The  cells  thus 
formed  are  generally  recognized  as  belonging  to  the  ve.stiges  of  the 
sexual  generation  (prothallium)  of  the  higher  cryptogams,  the  pollen- 
grains  (or  their  analogues  in  the  female)  corresponding  to  the  asexual 
spores  of  the  archegoniatc  cryptogams.  We  should,  therefore,  expect 
to  find  reduction  in  the  latter  forms  occurring  in  the  two  correspond- 
ing divisions,  by  which  the  "tetrad  "  of  spores  is  formed  (as  was  first 
pointed  out  by  Hartog,  '91).  Botanists  were  thus  led  to  the  surmise, 
first  expressed  by  Overton  in  1892,  that  the  reduced  number  would 
be  found  to  occur  in  the  ]-)rothallium-cclls  derived  from  those  spores. 


1  '92,  2,  p.  51. 


-  It  may  be  recalled  that  in  Ascai'is  Boveri  proved  that  the  ]irimordial  germ-cells  have 
the  full  number  of  chromosomes,  and  Hertwig  clearly  showed  that  this  number  is  retained 
up  to  the  last  division  of  the  spermatogonia.  Ishikawa  ('97)  finds  that  in  Alliiivi  the 
reduced  number  (eight)  appears  in  the  mitosis  of  the  "  UrpoUenzellen "  preceding  the 
pollen-mother-cells.     This  is,  however,  contradicted  by  Mottier  ('97,  2). 


EARLY  HISTORY  OF   THE    GERM-NUCLEI  2/5 

This  surmise  quickly  became  a  certainty.  Overton  himself  dis- 
covered ('93 )  that  the  cells  of  the  endosperm  in  the  i^ymnosperm 
Ccratozamia  divide  with  the  reduced  number,  namely  eight  ;  and 
Dixon  observed  the  same  fact  in  Pinus  at  the  same  time.  In  the 
following  year  Strasburger  brought  the  matter  to  a  definite  conclusion 
in  the  case  of  a  fern  {Osmitnda),  showing  that  a//  the  cells  of  the 
prothallinni,  from  the  original  spore-mother-cell  ouzuanls  to  the  for- 
mation of  the  germ-cells,  have  one-half  the  number  of  chromosomes 
found  in  the  asexual  generation,  namely  twelve  instead  of  twentv- 
four ;  in  other  words,  the  reduction  takes  place  in  the  formation  of 
the  spore  from  which  the  sexual  generation  arises,  many  cell-genera- 
tions before  the  germ-cells  are  formed,  indeed  before  the  formation  of 
the  body  from  which  these  cells  arise.  Similar  facts  were  determined 
by  Farmer  in  Pallavicinia,  one  of  the  Hepaticae,  where  all  of  the 
nuclei  of  the  asexual  generation  (sporogonium)  show  eight  chromo- 
somes during  division,  those  of  the  sexual  generation  (thallus)  four. 
It  now^  seems  highly  probable  that  this  will  be  found  a  general  rule. 

The  striking  point  in  these,  as  in  Hacker's  observations,  is  that  the 
numerical  reduction  takes  place  so  long  before  the  fertilization  for 
which  it  is  the  obvious  preparation.  Speculating  on  the  meaning  of 
this  remarkable  fact,  Strasburger  advances  the  hypothesis  that  the 
reduced  number  is  the  ancestral  number  inherited  from  the  ancestral 
type.  The  normal,  i.e.  somatic,  number  arose  through  conjugation 
by  which  the  chromosomes  of  two  germ-cells  were  brought  together. 
Strasburger  does  not  hesitate  to  apply  the  same  conception  to  ani- 
mals, and  suggests  that  the  four  cells  arising  by  the  division  of  the 
oogonium  {o-gg  plus  three  polar  bodies)  represent  the  remains  of  a 
separate  generation,  now  a  mere  remnant  included  in  the  body  in 
somewhat  the  same  manner  that  the  rudimentary  prothallium  of  angi- 
osperms  is  included  in  the  embryo-sac.  This  may  seem  a  highly 
improbable  conclusion,  but  it  must  not  be  forgotten  that  so  able  a 
zoologist  as  Whitman  expressed  a  nearly  related  thought,  as  long  ago 
as  1878  :  ''  I  interpret  the  formation  of  polar  globules  as  a  relic  of  the 
primitive  mode  of  asexual  reproduction.'^  ^  Strasburger's  \iew  is 
exactly  the  reverse  of  this  in  identifying  the  polar  bodies  as  the 
remains  of  a  sexual  generation;  and  as  Hacker  has  pointed  out  ('98, 
p.  102),  it  is  difficult  to  reconcile  with  the  fact  that  true  reduction 
appears  to  occur  already  in  the  unicellular  organisms  (p.  ijy).  The 
hypothesis  is  nevertheless  highly  suggestive  and  one  which  suggests 
a  quite  new  point  of  view  for  the  study  not  only  of  maturation  but 
also  of  the  whole  problem  of  sexuality. 

We  may  now  return  to  the  consideration  of  some  details.  In  a 
considerable  number  of  forms,  though  not  in  all,  the  early  prophase  is 

1  '78,  p.  262. 


2/6 


REDUCTION  OF  THE   CHROMOSOMES 


characterized,  especially  in  the  male,  by  a  more  or  less  complete 
concentration  of  the  chromatin-substance  at  one  side  of  the  nucleus. 
This  sta<;e,  to  which  Moore  has  given  the  name  synapsis  {Y\g.  120,  A\ 
sometimes  occurs  when  the  spireme  thread  is  already  split  {Ascaris, 
Lilium),  sometimes  before  the  division  is  visible  (insects).  In  cither 
case  till-  cluv}natin-sig)iu)its  ijfii?-i^c  from  the  synapsis  stage  longitndi- 
nallv  divided  and  in  the  redneed  nnniber,  a  fact  which  gives  ground 
for  the  conclusion  that  the  synapsis  is  in  some  way  concerned  with 
the  rearrangement  of  the  chromatin-substance  involved  in  the  numer- 
ical reduction.  During  the  synapsis  the  nucleolus  remains  cjuite 
distinct  from  the  chromatin,  and  in  many  cases  it  afterward  persists 
beside  the  tetrads,  in  the  formation  of  which  it  takes  no  j^art,  to  be 
cast  out  into  the  cytoplasm  (Fig.  124)  or  to  degenerate  ///  situ  during 
the  hrst  maturation-division. 

A  suggestive  phenomena,  described  by  several  observers,^  is  the 
casting  out  of  a  large  part  of  the  nuclear  reticulum  of  the  germinal 


ifc»-«y  *•' 


A 


io-;yji.iJ-r.- 


B 


C 


Fig.  137.  —  Types  of  maturation-spindles  in  the  female. 

./.  I'irst  polar  spindle  with  tetrads,  in  Hetetocope.  [HaCKER].  B.  Second  polar  spindle 
in   Triton.     [Caknoy  and  LeBkun.]     C.   First  polar  spindle  of  ^jrjr/^.     [FiJRST.] 

vesicle  at  the  time  the  polar  bodies  are  formed  (Figs.  97,  128).  In 
these  cases  {Asterias,  PolyeJio^nis,  Tlialassenia,  Nereis)  only  a  small 
fraction  of  the  chromatin-substance  is  preserved  to  form  the  chromo- 
somes, the  remainder  degenerating  in  the  cytoplasm.- 

As  a  final  point  we  must  briefly  consider  the  varying  accounts  of 
the  achromatic  maturation-figures  in  the  female  already  briefly  referred 
to  at  page  85.  In  many  forms  {^e.g.  in  turbellarians,  nemertines,  anne- 
lids, mollusks,  echinoderms)  the  polar  am]:)hiasters  are  of  quite  ty])ical 
form,  with  large  asters  and  distinct  centrosomes  nearly  similar  to  those 
of  the  cleavage-figures.  In  others,  however  (nematodes,  arthropods, 
tunicates,  vertebrates),  the  polar  spindles  differ  markedly  from  those 
of  the  cleavage-figures,  being  described  by  many  authors  as  entirely 
devoid  of  asters  and  even  in  some  cases  of  centrosomes  (Fig.   137). 

1  Cf.  Mathews  (Wilson  and  Mathews,  '95),  Gardiner  ('98),  Griffin  ('99). 

2  Cf.  the  enormous  reduction  of  the  chromatin-substance  in  the  elasmobranch  egg,  p.  338. 


REDUCTION  IN   UNICELLULAR   lOA'J/S  277 

There  can  be  no  doubt  that  these  polar  spindles  differ  from  the  usual 
type,  and  that  they  approach  those  recently  described  in  the  mitosis  of 
the  higher  plants,  but  it  is  doubtful  whether  the  apparent  absence  of 
asters  and  centrosomes  is  normal.  In  Ascaris,  the  first  polar  spindle 
arising  by  a  direct  transformation  of  the  germinal  vesicle  (Fig.  i  ij) 
has  a  barrel-shape,  with  no  trace  of  asters.  At  the  poles  of  the 
spindle,  however,  are  one  or  two  deeply  staining  granules  (Fig.  137), 
w^hich  have  been  identified  as  centrosomes  by  Hacker  (94)  and 
Erlanger  ('97,  4),  but  by  Fijrst  ('98)  are  regarded  as  central  granules, 
the  whole  spindle  being  conceived  as  an  enlarged  centrosome.*  For 
the  reasons  stated  at  page  314,  I  believe  the  former  to  be  the  correct 
interpretation. 2  Spindles  without  centrosomes  have  been  described  in 
the  eggs  of  tunicates  (Julin,  Hill,  Crampton),  in  Ajup/iioxits  CSohoiin), 
in  some  species  of  copepods  (Hacker),  and  in  some  vertebrates  {Dic- 
myctyhis,  Jordan  ;  mouse,  Sobotta).  In  Aviphioxus  (Sobotta)  and 
Triton  (Carnoy  and  LeBrun)  complete  asters  are  not  formed,  but 
fibrillas  apparently  corresponding  to  astral  rays  and  converging  to 
the  spindle-poles  are  found  outside  the  limits  of  the  spindle  (Fig.  137). 
In  the  guinea-pig,  according  to  Montgomery  ('98),  centrosomes  and 
asters  are  present  in  the  first  polar  spindle,  but  absent  in  the  second. 
The  evidence  is  on  the  whole  rather  strong  that  the  achromatic  figure 
in  these  cases  approaches  in  form  that  seen  in  the  higher  plants  ; 
but  it  is  an  open  question  whether  the  appearances  described  may  not 
be  a  result  of  imperfect  fixation. 

F.    Reduction  in  Unicellular  Forms 

Although  the  one-celled  and  other  lower  forms  have  not  yet  been 
sufficiently  investigated,  we  have  already  good  ground  for  the  conclu- 
sion that  a  process  analogous  to  the  reduction  of  higher  types  regularly 
recurs  in  them.  In  the  conjugation  of  Infusoria,  as  already  described 
(p.  223),  the  original  nucleus  divides  several  times  before  union,  and 
only  one  of  the  resulting  nuclei  becomes  the  conjugating  germ-nucleus, 
while  the  others  perish,  Hke  the  polar  bodies.  The  numerical  corre- 
spondence between  the  rejected  nuclei  or  "  corpuscules  de  rebut  "  has 
already  been  pointed  out  (p.  227).  Hertwig  could  not  count  the  chro- 
mosomes with  absolute  certainty,  yet  he  states  ('89)  that  in  Piircivuv- 
cinm  caudatum,  during  the  final  division,  the  number  of  spindle-fibres 
and  of  the  corresponding  chromatic  elements  is  but  4-6,  while  in  the 

1  QC  p.  312. 

2  Sala  ('94)  and  Fiirst  have  shown  that  occasionally  the  polar  spindles  of  Ascaris  are 
provided  with  large  typical  asters,  and  thus  resemble  those  of  annelids  or  mollusks.  Sala 
believed  this  to  be  an  effect  of  lowered  temperature,  but  Fiirst 's  observations  are  unfavour- 
able to  this  conclusion. 


2/8 


REDUCTIOX  OF   THE    CHROMOSOMES 


earlier  divisions  the  number  is  aj^proxiniately  double  this  (8-9).  This 
observation  makes  it  nearly  certain  that  a  numerical  reduction  of 
chromosomes  occurs  in  the  Protozoa  in  a  manner  similar  to  that  of 
the  higher  forms  ;  but  the  reduction  here  aj)])ears  to  be  deferred  until 


J 


\ 


^\^J    v.^/  o>' 


A 


C 


Fig.  138.  —  Conjugation  and  formation  of  the  polar  bodies  in  Actinophrys.     [SCHAUDINN.] 

A.    Union  of  the  gametes;  first  polar  spindle.     /?,    Fusion  of  the  cell-bodies;  a  single  polar 
body  near  the  periphery  of  each.     C.    Fusion  of  the  nuclei. 


the  final  division.  In  the  gregarines  Wolters  ('91)  has  observed  the 
formation  of  an  actual  polar  body  as  a  small  cell  segmented  off  from 
each  of  the  two  conjugating  animals  soon  after  their  union  ;  but  the 

number  of  chromo- 
somes was  not  deter- 
mined. Schaudinn 
('96,  2)  has  observed  a 
like  process  in  Acti- 
iwpJuys,    each    of    the 


gametes 


segmentmg 


A 


C 


B 


D 


off  a  single  polar  body, 
after  which  the  germ- 
nuclei  fuse  (Fig.  138). 
It  is  ])ossible,  as  R. 
Hertwig  ('98)  points 
out,  that  in  both  these 
forms  a  second  ])olar 
body  may  have  been 
overlooked,  owing  per- 
haps   to    its  rapid  dis- 


Fig.   139.  —  Formation   of  polar  bodies  and  conjugation  in  HltCgration.      \\\.  ActlllO- 

AcunosphcBrtum.    [R.  hertwig.]  spha^riuvi,  accordiug  to 

A.   Two  gametes  ("secondary  cysts"),   resulting   from   the  p      tt  •       /'    o  \      fU 

division  of  a  "primary  cyst";    second    maturation-spindle   in  rierLWlg    ^  9*^  A     '-^^ 

each;  first  polar  bodv  shown  in  the  right  gamete,  at/.     B.    Both  nuclcUS  of  Cach  gamete 

polarbodies(/»i;>2)   formed  in  the  right  gamete    the  second  |-    -^         ^     •  •  -^ 

one  formmir  m  the  left  gamete.       C.   Subsequent  fusion  of  the  ^ 


gametes;  nuclei  uniting,  two  polar  bodies  (probably  the  second.     SUCCeSSion  tO  form  tWO 
the  first  having  been  absorbed)  at/.    D.  The  young  AcfinospAcF- 
num  escaping   from  the   cyst-wall;    the  cleavage-nucleus   has 


the  first  having  been  absorbed)  at/.    D.  The  yonng  Actinosphce-    t-)q]ot-      bodies     (^nnrlei^ 


divided. 


which  degenerate,  after 


REDUCTION  IN  UNICELLULAR  FORMS  279 

Which  the  germ-nuclei  unite  (Fig.  139).     Whether  a  reduction  in  the 
number  of  chromosomes  occurs  in  these  cases  was  not  determined  • 


B 


f 


D 


H 

Fig.  140.  —  Conjugation  of  Closteriutti.     [Klehahn,] 

dkt^'  f'^^'n    ^J^^'  ""'^"'   ^°"'   chromatophores.     B.   Chromatophores   reduced   to   two.   nuclei 
distinct,     a   Fusion  of  the  nuclei.     D.    First  cleavage  of  tiie  zygote.     /.-.    Resulting  2-cell  stage 
/-.   becond  cleavage.     G.    Resulting  stage,  each   cell   bi-nucleate.     //.   Separation   of  tl,e  cells- 
one  01  the  nuclei  in  each  enlarging  to  form  the  permanent  nucleus,  the  other  (probably  reprd 
senting  a  polar  body)  degenerating.  ' 

1  Achnosphceriufu  forms  one  of  the  most  extreme  known  cases  of  in-iireeding;  for  the 
gametes  are  sister-cells  which  immediately  reunite  after  forming  the  polar  bodies.  The 
general  facts  are  as  follows :  The  mother  animal,  containing  verv  numerous  nuclei,  l>ecomes 
encysted,  and  a  very  large  number  of  the  nuclei  degenerate.     TJie  bodv  then  segments  into 


280  REDUCTIOX   OF   THE    CHROMOSOMES 

Adclea  (one  of  the  Coccidire)  is  a  \cry  interesting  case,  for  accord- 
ing to  Sicdlecki  ('99)  polar  bodies  or  their  analogues  are  formed  in 
both  sexes.  The  gametes  are  here  of  very  unequal  size.  Upon  their 
union  the  smaller  male  cell  divides  twice  to  form  apparently  equiva- 
lent spermatozoids,  of  which,  however,  only  one  enters  the  ovum,  while 
three  degenerate  as  polar  bodies.  These  two  divisions  are  of  different 
tvpe  ;  the  first  resembles  true  mitosis,  while  the  second  is  of  sim]-)ler 
character  and  is  belie\ed  bv  Siedlecki  to  effect  a  reduction  in  the 
number  of  chromosomes.  In  the  meantime  the  nucleus  of  the  macro- 
gamete  moves  to  the  surface  and  there  expels  a  portion  of  its  chro- 
matin, after  which  union  of  the  nuclei  takes  place.  Interesting  facts 
have  been  observed  in  unicellular  plants  which  indicate  that  the 
reduction  may  here  occur  either  before  (diatoms)  or  after  (desmids) 
fusion  of  the  conjugating  nuclei.  In  the  ioxxwitx {Rliopalodina)  Klebahn 
('96)  finds  that  each  nucleus  divides  twice,  as  in  many  Infusoria,  giving 
rise  to  two  large  and  two  small  nuclei.  Each  of  the  conjugates  then 
divides,  each  daughter-cell  receiving  one  large  and  one  small  nucleus. 
The  four  resulting  individuals  then  conjugate,  two  and  two,  the  large 
nuclei  fusing  while  the  small  (polar  bodies)  degenerate.  The  com- 
])arison  of  this  case  with  that  of  the  Infusoria  is  highly  interesting.  In 
the  desmids  on  the  other  hand  (^'/^^•/mV/;;/  and  Cosmariuin,  Fig.  140), 
according  to  Klebahn  ('92),  the  nuclei  first  unite  to  form  a  cleavage- 
nucleus,  after  which  the  zygote  divides  into  two.  Each  of  the  new 
nuclei  now  divides,  one  of  the  products  persisting  as  the  perma- 
nent nucleus,  while  the  other  degenerates  and  disappears.  Chmie- 
lewski  asserts  that  a  similar  process  occurs  in  Spirogyra.  Although 
the  numerical  relations  of  the  chromosomes  have  not  been  determined 
in  these  cases,  it  appears  probable  that  the  elimination  of  a  nucleus  in 
each  cell  is  a  process  of  reduction  occurring  after  fertilization. 


G.     M.\TUR.\TinN  OF  Parthexogenetic  Eggs 

The  maturation  of  eggs  that  develop  without  fertilization  is  a  sub- 
ject of  special  interest,  partly  because  of  its  bearing  on  the  general 
theory  of  fertilization,  partly  because  it  is  here,  as  I  believe,  that  one 
of  the  strongest  supports  is  found  for  the  hypothesis  of  the  individ- 
uality of  chromosomes.      In  an  early  article  by   Minot  {'yj)  on  the 

a  number  (Tive  lo  twelve)  of  "  jirimary  cysts."  each  containing  one  of  the  remaining  nuclei. 
Kach  primary  cyst  divides  by  mitosis  to  form  two  gametes  ("secondary  cysts  "),  whicii,  after 
forming  the  polar  bodies,  reunite,  their  nuclei  fusing  to  form  a  single  one.  The  resulting 
cell  soon  creeps  out  of  the  cyst-wall  and  assumes  the  active  life,  its  nucleus  meanwhile  mul- 
tiplying to  produce  the  multinuclear  condition  characteristic  of  the  adult  animal.  What  is 
here  the  physiological  motive  for  the  formation  of  the  polar  bodies,  and  how  shall  it  be 
explained  under  the  Weismann  hypothesis? 


MATURATION  OF  PARTHENOGEXETIC  EGGS  28  I 

theoretical  meaning  of  maturation,  the  suggestion  is  made  that 
parthenogenesis  may  be  due  to  failure  on  the  part  of  the  Kt^^y:^  to 
form  the  polar  bodies,  the  egg-nucleus  thus  remaining  hermaphrodite, 
and  hence  capable  of  development  without  fertiHzation.  This  sug- 
gestion forms  the  germ  of  all  later  theories  of  parthenogenesis.  lial- 
four  ('80)  suggested  that  the  function  of  forming  polar  cells  has  been 
acquired  by  the  ovum  for  the  express  purpose  of  preventing  parthe- 
nogenesis, and  a  nearly  similar  view  was  afterward  maintained  by 
Van  Beneden.i  These  authors  assumed  accordingly  that  in  par- 
thenogenetic  eggs  no  polar  bodies  are  formed.  Weismann  ('86)  soon 
discovered,  however,  that  the  parthenogenetic  eggs  of  Polyphemus 
(one  of  the  Daphnidae)  produce  a  single  polar  body.  This  observa- 
tion was  quickly  followed  by  the  still  more  significant  discovery  by 
Blochmann  i^^^)  that  in  Aphis  the  partJienogcnetic  eggs  produce  a  single 
polar  body,  zvhile  the  fertilized  eggs  produce  tzvo.  Weismann  was  al^le 
to  determine  the  same  fact  in  ostracodes  and  Rotifera,  and  was  thus 
led  to  the  view^  which  later  researches  have  entirely  confirmed,  that 
it  is  the  second  polar  body  that  is  of  special  significance  in  partheno- 
genesis. Blochmann  observed  that  in  insects  the  polar  bodies  were 
not  actually  thrown  out  of  the  ^gg,  but  remained  embedded  in  its 
substance  near  the  periphery.  At  the  same  time  Boveri  {'Sy,  i)  dis- 
covered that  in  Ascaris  the  second  polar  body  might  in  exceptional 
cases  remain  in  the  egg  and  there  give  rise  to  a  resting-nucleus  indis- 
tinguishable from  the  egg-nucleus  or  sperm-nucleus.  He  was  thus 
led  to  the  interesting  suggestion  that  parthenogenesis  might  be  due 
to  the  retention  of  the  second  polar  body  in  the  Qgg  and  its  union 
with  the  egg-nucleus.  "  The  second  polar  body  would  thus,  in  a 
certain  sense,  assume  the  role  of  the  spermatozoon,  and  it  might  not 
without  reason  be  said  :  ^^  PartJienogenesis  is  the  result  of  fertilizaiiou 
by  the  second  polar  body''  ^ 

This  conclusion  received  a  brilliant  confirmation  through  the  obser- 
vations of  Brauer  ('93)  on  the  parthenogenetic  egg  of  Arteviia, 
though  it  appeared  that  Boveri  arrived  at  only  a  part  of  the  truth. 
Blochmann  ('88-89)  had  found  that  in  the  parthenogenetic  eggs 
of  the  honey-bee  tzvo  polar  bodies  are  formed,  and  Platner  discov- 
ered the  same  fact  in  the  butterfly  Liparis  ('89)  —  a  fact  which 
seemed  to  contradict  Boveri's  hypothesis.  Brauer's  beautiful  re- 
searches resolved  the  contradiction  by  showing  that  there  are  txvo 
types  oi partJienogenesis  \M\i\Q\\  may  occur  in  the  same  animal.  In  the 
one  case  Boveri's  conception  is  exactly  realized,  while  the  other  is 
easily  brought  into  relation  with  it. 

{a)  In  both  modes  typical  tetrads  are  formed  in  the  germ-nucleus 
to  the  number  of  eighty-four.      In  the  first  and  more  frequent  case 

i  'ZZ,  p.  622.  2  Essay  VI.,  p.  359-  '  l-c-,  P-  73- 


282 


REDUCTION  OF   THE    CHROMOSOMES 


{Y'v^.  141 )  but  one  polar  body  is  formed,  which  removes  eighty-four 
dyads,  leaving  eighty-four  in  the  ^g^2,.  There  may  be  an  abortive 
attempt  to  form  a  second  polar  spindle,  but  no  division  results,  and 
the  eighty-four  dyads  give  rise  to  a  reticular  cleavage-nucleus.     From 


uo-^-^^%0 


(yo, 


B 


V-3-" 


T 


■^/m 


. '  li. 


Fig.  141.  —  First  type  of  maturation  in  the  parthenogenetic  egg  of  Artemia.     [BraUER.] 

A,  The  first  polar  spindle;  the  equatorial  plate  contains  84  tetrads.  D.  C.  Formation  of  the 
first  polar  body ;  84  dyads  remain  in  the  egg,  and  these  give  rise  to  the  egg-nucleus,  shown  in  D. 
F.  Appearance  of  the  egg-centrosome  and  aster.  E.  G.  Division  of  the  aster  and  formation 
of  the  cleavage-figure ;  the  equatorial  plate  consists  of  84  apparently  single  but  in  reality  bivalent 
chromosomes. 


this  arise  eighty-four  thread-like  chromosomes,  and  t/ic  same  juimbcr 
appears  in  later  cleavage-stages. 

{b)  It  is  the  second  and  rarer  mode  that  realizes  Boveri's  concep- 
tion (Fig.  142).  Both  polar  bodies  are  formed,  the  first  removing 
eighty-four  dyads  and  leaving  the  same  number  in  the  (tgg.  In  the 
formation  of  the  second,  the  eighty-four  dyads  are  halved  to  form 


MATURATION   OF  PARTIIENOGENETIC  EGGS 


28 


two  daughter-groups,  each  containing  eighty-four  single  chromosomes. 
Both  these  groups  reviain  in  the  egg,  and  eaeh  gives  rise  to  a  singli 
i^eticular  ujccleiis,  as  described  by  Boveri  in  Ascaris.  These  tzvo  niiclei 
place  themselves  side  by  side  in  the  cleavage-figure,  and  give  rise  each 
to  eighty-four  cJironiosomes,  precisely  like  two  germ-nuclei  in  ordinary 
fertilization.      The  one  hundred  and  sixty-eight  chromosomes  split 


A 


B 


D  E 

Fig.  142.  —  Second  type  of  maturation  in  the  parthenogenetic  egg  of  Arh-mia.     [BraL'ER.] 
A.   Formation  of  second  polar  body.     D.    Return  of  the  second  polar  nucleus  ( />.  b?)  into  the 
egg;    development  of  the  egg-amphiaster.       C.    Union  of  the  egg-nucleus  (?)  with  the  second 
polar   nucleus    {p.  b?-).     D.   Cleavage-nucleus   and   amphiaster.     /:".    First    .l.Mvage-figure    wiili 
equatorial  plate  containing  168  chromosomes  in  two  groups  of  84  each. 

lengthwise,  and  are  distributed  in  the  usual  manner,  and  reappear 
in  the  same  number  in  later  stages.  In  other  words,  the  second  polar 
body  here  plays  the  part  of  a  sperm-nucleus  precisely  as  maintained 

by  Boveri. 

In  all  individuals  arising  from  eggs  of  the  first  type,  therefore,  the 
somatic  number  of  chromosomes  is  eighty-four;  in  all  those  arising 
from  eggs  of  the  second  type,  it  is  one  hundred  and  sixty-eight.     This 


'284  REDUCTION   OF   THE    CHROMOSOMES 

difference  is  clearly  due  to  the  fact  that  in  the  latter  case  the  chromo- 
somes are  single  or  univalent,  while  in  the  former  they  are  bivalent 
(actually  arising  from  dyads  or  double  chromosomes).  The  remark- 
able feature,  on  which  too  much  emphasis  cannot  be  laid,  is  that  the 
numerical  difference  should  persist  despite  the  fact  that  the  mass,  and, 
as  far  as  we  can  see,  the  quahty,  of  the  chromatin  is  the  same  in  both 
cases.  In  this  fact  we  must  recognize  a  strong  support,  not  only  of 
Hacker's  and  Vom  Rath's  conception  of  bivalent  chromosomes,  but 
also  of  the  more  general  hypothesis  of  the  individuality  of  chromo- 
somes (Chapter  VI.). 

I .    Accessory  Cells  of  the  Testis 

It  is  necessary  to  touch  here  on  the  nature  of  the  so-called  "  Sertoli-cells,"  or  sup- 
porting cells  of  the  testis  in  mammals,  partly  because  of  the  theoretical ^  significance 
attached  to  them  by  Minot.  partly  because  of  their  relations  to  the  question  of  amito- 
sis  in  the  testis.  In  the  seminiferous  tubules  of  the  mammalian  testis,  the  parent- 
cells  of  the  spermatozoa  develop  from  the  periphery  inwards  toward  the  lumen,  where 
the  spermatozoa  are  finally  formed  and  set  free.  At  the  periphery  is  a  layer  of  cells 
next  the  basement-membrane,  having  flat,  oval  nuclei.  Within  this,  the  cells  are 
arranged  in  columns  alternating  more  or  less  regularly  with  long,  clear  cells,  con- 
tainin'g  large  nuclei.  The  latter  are  the  Sertoli-cells,  or  supporting  cells  :^  they  extend 
nearly^throligh  from  the  basement-membrane  to  the  lumen,  and  to  their  inner  ends 
the  young  sp'ermatozoa  are  attached  by  their  heads,  and  there  complete  their  growth. 
The' spermatozoa  are  developed  from  cells  which  lie  in  columns  between  the  Sertoh- 
cells.  and  which  undoubtedly  represent  spermatogonia,  spermatocytes,  and  sperma- 
tids, though  their  precise  relationship  is,  to  some  extent,  in  doubt.  The  innermost 
of  these  cells,  next  the  lumen,  are  spermatids,  which,  after  their  formation,  are  found 
attached  to  the  Sertoli-cells,  and  are  there  converted  into  spermatozoa  without  further 
division.  The  deeper  cells  from  which  they  arise  are  spermatocytes,  and  the  sper- 
matogonia lie  deeper  still,  being  probably  represented  by  the  large,  rounded  cells. 

Two  entirely  diiferent  interpretations  of  the  Sertoli-cells  were  advanced  as  long 
ago  as  1 87 1,  and  both  views  still  have  their  adherents.  Von  Ebner  ("71)  at  first 
regarded  the'Sertoli-cell  as  the  parent-cell  of  the  group  of  spermatozoa  attached  to  it, 
and  the  same  view  was  afterward  especially  advocated  by  Biondi  ("85)  and  by  Minot 
(■92).  the  latter  of  whom  regarded  the  nucleus  of  the  Sertoli-cell  as  the  physiological 
analogue  of  the  polar  bodies,  i.e.  as  containing  the  female  nuclear  substance  (92. 
p.  77).  According  to  the  opposing  view,  first  suggested  by  Merkel  ('70-  the  Sertoli- 
cell  is  not  the  parent-cell,  but  a  nurse-cell,  the  spermatozoa  developing  from  the 
columns  of  rounded  cells,  and  becoming  secondarily  attached  to  the  Sertoli-cell, 
which  serves  merely  as  a  support  and  a  means  of  conveying  nourishment  to  the 
growing  spermatozoa.  This  view  was  advocated  by  Brown  ('85),  and  especially  by 
Benda  ("87).  In  the  following  year  ("88),  von  Ebner  himself  abandoned  his  early 
hypothesis  and  strongly  advocated  Benda  s  views,  adding  the  very  significant  result 
that/^;/r  spermatids  arise  from  each  spermatocyte,  precisely  as  was  afterward  shown 
to  be  the  case  in  Ascaris,  etc.  The  very  careful  and  thorough  work  of  Benda  and 
von  Ebner.  confirmed  by  that  of  Lenhossek  (98.  2).  leaves  no  doubt  that  mamma- 
lian spermatogenesis  conforms,  in  its  main  outlines,  with  that  of  Ascafis.  the  sala- 
mander, and  other  forms,  and  that  Biondi's  account  is  untenable.  Minot's  theoretical 
interpretation  of  the  Sertoli-cell.  as  the  physiological  equivalent  of  the  polar  bodies, 
therefore  collapses. 


SUMMARY  AND    CONCLUSION  285 

2.    Ajuitosis  in  the  Early  Sex-cells 

Whether  the  progenitors  of  the  germ-cells  ever  divide  amitotically  is  a  question  of 
high  theoretical  interest.  Numerous  observers  have  described  amitotic  division  in 
testis-cells.  and  a  few  also  in  those  of  the  ovary.  The  recent  observations  of  Meves 
('91),  Vom  Rath  ('93),  and  others  leave  no  doubt  whatever  that  such  divisions 
occur  in  the  testis  of  many  animals.  Vom  Rath  maintains,  after  an  extended  inves- 
tigation, that  all  cells  so  dividing  do  not  belong  in  the  cycle  of  develo]jmcnt  of  the 
germ-cells  ('93,  p.  164)  :  that  amitosis  occurs  only  in  the  supporting  (;r  nutritive  cells 
(Sertoli-cells,  etc.),  or  in  such  as  are  destined  to  degenerate,  like  the  ••  residual 
bodies''  of  Van  Beneden.  Meves  has,  however,  produced  strong  evidence  ("94)  that 
in  the  salamander  the  spermatogonia  may,  in  the  autumn,  divide  by  amitosi.s,  and  in 
the  ensuing  spring  may  again  resume  the  process  of  mitotic  division,  and  give  rise  to 
functional  spermatozoa.  On  the  strength  of  these  observations  Flemming  (93)  him- 
self now  admits  the  possibility  that  amitosis  may  form  part  of  a  normal  cycle  of  devel- 
opment.^ 

H.     Summary  and  Conclusion 

The  one  fact  of  maturation  that  stands  out  with  perfect  clearness 
and  certainty  amid  all  the  controversies  surrounding  it  is  a  rcdiiction 
of  the  number  of  chromosoiiies  in  tJie  tdtiniate  gerni-cells  to  one-lialf  tJic 
nnniber  cJiaracteristic  of  the  somatic  cells.  It  is  equally  clear  that  this 
reduction  is  a  preparation  of  the  germ-cells  for  their  subsequent  union, 
and  a  means  by  which  the  number  of  chromosomes  is  held  constant 
in  the  species.  With  a  few  exceptions  the  first  indication  of  the 
numerical  reduction  appears  through  the  segmentation  of  the  spireme- 
thread,  or  the  resolution  of  the  nuclear  reticulum,  into  a  number  of 
masses  one-Jialf  that  of  the  somatic  cJiromosomes.  In  nearly  all  higher 
animals  this  process  first  takes  place  two  cell-generations  before  the 
formation  of  the  definitive  germ-cells,  and  the  process  of  reduction  is 
completed  by  two  rapidly  succeeding  "maturation-divisions,"  giving 
rise  to  four  cells,  all  of  which  become  functional  in  the  male,  while  in 
the  female  only  one  becomes  the  ^gg,  while  the  other  three  —  the 
polar  bodies  or  their  analogues  —  are  cast  aside.  During  these  two 
divisions  each  of  the  original  chromatin-masses  gives  rise  to  four 
chromosomes,  of  which  each  of  the  four  daughter-cells  receives  one  ; 
hence,  each  of  the  latter  receives  one-half  the  somatic  number  of 
chromosomes.  In  the  higher  plants,  however,  the  two  maturation- 
divisions  are  followed  by  a  number  of  others,  in  which  the  reduced 
number  of  chromosomes  persists,  a  process  most  strikingly  shown  in 
the  pteridophytes,  where  a  separate  sexual  generation  (prothallium) 
thus  arises,  all  the  cells  of  which  show  the  reduced  number. 

Two  general  types  of  maturation  may  be  distinguished  according 
to  the  manner  in  which  the  primary  chromatin-masses 'divide.     In  one, 

1  For  more  recent  literature  on  this  subject  see  Meves,  Zelltheilung,  in  Mcrkel  and  Bon- 
net's Ergebnisse,  VIIL,  1 898. 


2S6  REDUCTION  OF   THE    CHROMOSOMES 

typically  represented  by  Ascaris  and  the  arthropods,  each  of  these 
masses  divides  into  four  to  form  a  tetrad,  thus  preparing  at  once  for 
two  rapidly  succeeding  divisions,  which  are  not  separated  by  a  recon- 
struction of  the  daughter-nuclei  during  an  intervening  resting  period. 
In  the  other,  examples  of  which  are  given  by  the  flowering  plants  and 
the  spermatogenesis  of  the  Amphibia,  no  true  tetrads  are  formed,  the 
primary  chromatin-masses  dividing  separately  for  each  of  the  matura- 
tion-divisions, which  are  separated  by  a  period  in  which  the  nuclei 
regress  toward  the  resting  state,  though  often  not  completely  return- 
ing to  the  reticular  condition.  These  two  types  differ,  however,  only 
in  degree,  and  with  few  exceptions  they  agree  in  the  fact  that  during 
the  prophases  of  the  first  division  the  chromatin-bodies  assume  the 
form  of  rings,  the  mitosis  thus  being  of  the  heterotypical  form,  and 
each  ring  having  the  prospective  value  of  four  chromosomes. 

Thus  far  the  phenomena  present  no  difficulty,  and  they  give  us  a 
clear  view  of  the  process  by  which  the  numerical  reduction  of  the 
chromosomes  is  effected.  The  confusion  of  the  subject  arises,  on  the 
one  hand,  from  its  complication  with  theories  regarding  the  individu- 
ality of  the  chromosomes  and  the  functions  of  chromatin  in  inheri- 
tance, on  the  other  through  conflicting  results  of  observation  on  the 
mode  of  tetrad-formation  and  the  character  of  the  maturation-divisions. 
Regarding  the  latter  question  nearly  all  observers  are  now  agreed  that 
one  of  these  divisions,  usually  the  first,  is  a  longitudinal  or  equation- 
division,  essentially  like  that  occurring  in  ordinary  mitosis.  The  main 
question  turns  upon  the  other  division,  which  has  been  shown  in  some 
cases  to  be  transverse  and  not  longitudinal,  and  thus  separates  what 
were  originally  different  regions  of  the  spireme-thread  or  nuclear 
substance.  The  evidence  in  favour  of  such  a  division  seems  at  present 
well-nigh  demonstrative  in  the  case  of  insects  and  copepods,  and 
hardly  less  convincing  in  the  turbellarians,  annelids,  and  mollusks. 
On  the  other  hand,  both  divisions  are  regarded  as  longitudinal  by  most 
of  those  who  have  investigated  the  phenomena  in  Ascaris  and  in  the 
vertebrates,  and  by  some  of  the  most  competent  investigators  of  the 
flowering  plants. 

The  evidence  as  it  stands  is  so  evenly  balanced  that  the  subject  is 
hardly  yet  ripe  for  discussion.  The  principle  for  which  Weismann 
contended  in  his  theory  of  reducing  div^ision  has  received  strong 
support  in  fact;  yet  should  it  be  finally  estabUshed  that  numerical 
reduction  may  be  effected  either  with  or  without  transverse  division, 
as  now  seems  probable,  not  only  will  that  theory  have  to  be  aban- 
doned or  wholly  remodelled,  but  we  shall  have  to  seek  a  new  basis 
for  the  interpretation  of  mitosis  in  general.  Weismann's  theory  is 
no  doubt  of  a  highly  artificial  character ;  but  this  should  not  close  our 
eyes  to  the  great  interest  of  the  problem  that  it  attempted  to  solve. 


LITERATURE  287 

The  existing  contradiction  of  results  has  led  to  the  opinion,  expressed 
by  a  number  of  recent  writers,  that  the  difference  between  longitudinal 
or  transverse  division  is  of  minor  importance,  and  that  the  entire 
question  of  reduction  is  a  barren  one.  This  opinion  fails  to  reckon 
with  the  facts  on  which  rests  the  hypothesis  of  the  individuality  of 
chromosomes  (Chap.  VI.);  but  these  facts  cannot  be  left  out  of 
account.  We  must  find  a  common  basis  of  interpretation  for  them 
and  for  the  phenomena  of  reduction ;  yet  how  shall  we  reconcile 
them  with  reduction  by  longitudinal  division  only  }  I  cannot,  there- 
fore, share  the  opinion  that  we  are  dealing  with  a  barren  problem. 
The  peculiarities  of  the  maturation-mitoses  are  obviously  correlated 
in  some  way  with  the  numerical  reduction,  and  the  fact  that  they 
differ  in  so  many  ways  from  the  characters  of  ordinary  mitosis  gives 
ground  to  hope  that  their  exhaustive  study  will  throw  further  light 
not  only  on  the  reduction-problem  itself  but  also  on  mitosis  in  general 
and  on  still  wider  problems  relating  to  the  individuality  of  the  chromo- 
somes and  the  morphological  organization  of  the  nucleus.  It  is  indeed 
very  probable  that  Weismann's  theory  is  but  a  rude  attempt  to  attack 
the  problem,  and  one  that  may  prove  to  have  been  futile.  The  prob- 
lem itself  cannot  be  ignored,  nor  can  it  be  dissociated  from  the  series 
of  kindred  problems  of  which  it  forms  a  part. 


LITERATURE.     V 1 

Van  Beneden,  E.  —  Recherches  sur  la  maturation  de  Toeuf,  la  fdcondation  et  la  division 

cellulaire  :  Arch.  Biol..  IV.     1883. 
Boveri.   Th.  —  Zellenstudien,    I.,  III.      Jena,    1887-90.      See    also   "  Befruchtung" 

(List  IV.). 
Brauer,  A.  —  Ziir  Kenntniss  der  Spermatogenese  von  Ascaris  viegaloccphala  :  An/i. 

mik.  Anat.,  XLII.     1893. 
Id.  —  Zur  Kenntniss  der  Reifung  der  parthenogenetlsch  sich  entwickelnden  Lies  von 

Artemui  Salina  :  Arch.  mik.  A?iat.,  XLIII.      1894. 
Guignard,  L.  — Le  developpement   du   pollen  et  la  reduction  chromatique  dans  Ic 

Naias:  Arch.  Anat.  Mic.  II.      1899.      (Full  literature  on  reduction  in  plants.) 
Griffin,  B.  B.  —  See  Literature,  IV. 
Hacker,  v.  — Die  Vorstadien  der  Eireifung  (General  Review):  Arch.  mik.  A/uii., 

XLV.  2.     1895. 
Id.  — Uber  weitere  Ubereinstimmungen  zwischen  den  Fertpflan/.ungsvorgangen  der 

Thiere  und  Pflanzen  :  Bio/.  Centrally..  X\' II.      1897. 
Id. —  Uber    vorbereitende    Theilungsvorgange    bei    Thieren  und  Prianzcn  :     \  erh. 

dentsch.  Zool.  Ges.,  VIII.     1898. 
Id.  —  Die  Reifungserscheinungen  :  Merkel  nnd  Bonnet's  Ergebnisse.  \' 1 1 1 .     i  S9S . 
Hertwig,  0.— Vergleich  der  Ei-  und  Samenbildung  bei  Nematoden.     Eine  Crund- 

lage  fiir  cellulaire  Streitfragen  :  Arch.  mik.  Anat..  XXWI.     1890. 
Mark,  E.  L.  — (See  List  IV.) 
Peter,  K.  — Die  Bedeutung  der  Nahrzcllen  im  Hoden  :  Arch.  mik.  A>i<7f..  LI  11.     ibgb 

1  See  also  Literature,  IV.,  p.  231. 


288  REDUCTION  OF  THE    CHROMOSOMES 

Plainer,  G.— Uberdie  Bedeutung  der  Richtungskorperchen :  Biol.  Centralb.,\\\\. 

Vom  Rath,  0.  —  Zur  Kenntniss  der  Spermatogenese  von  Gryllotaipa  vulgaris :  Arch. 

inik.  Anat..  XL.      1892.  ,,..-,      c^  a  ^-     -f 

Id.  — Neue  Beitrage  zur  Frage  der  Chromatinreduktion  in  der  Samen-  und  Eireife  : 

Arch.  mik.  Anat.,  XLVI.     1895.  ,  ,  .      -r^        •  1  , 

Riickert,  J.— Die  Chromatinreduktion  der  Chromosomenzahl  nn  Entwicklungsgang 

der  Oro-anismen:  Ergebn.  d.  Anat.  u.  Entwick..  III.     1893  (1894). 
Strasburgert  E.  —  Uber  periodische  Reduktion  der  Chromosomenzalil  im  Entwick- 

luno-'igang  der  Organismen :  Biol.  Centralb.,  XIV.     1894. 
Id.  — Redtktionstheilung.  Spindelbildung,  etc. :  Jena,  Fischer,  1900. 


CHAPTER   VI 

SOME   PROBLEMS   OF  CELL-ORGANIZATION 

"  Wir  miissen  deshalb  den  lebenden  Zellen,  abgesehen  von  der  Molecularstructur  der 
organischen  Verbindungen,  welche  sie  enthalt,  noch  eine  andere  und  in  anderer  Wcise  coni- 
plicirte  Structur  zuschieiben,  und  diese  es  ist,  welche  wir  mit  dem  Namen  Organization 
bezeichnen."  Brl'cke.^ 

"  Was  diese  Zelle  eigentlich  ist,  dariiber  existieren  sehr  verschiedene  Ansichten." 

IIackkl.^ 

The  remarkable  history  of  the  chromatic  substance  in  the  matura- 
tion of  the  germ-cells  forces  upon  our  attention  the  problem  of  the 
ultimate  morphological  organization  of  the  nucleus,  and  this  in  its 
turn  involves  our  whole  conception  of  protoplasm  and  the  cell.  The 
grosser  and  more  obvious  organization  is  revealed  to  us  by  the  micro- 
scope as  a  differentiation  of  its  substance  into  nucleus,  cytoplasm, 
and  the  like.  But,  as  Strasburger  has  well  said,  it  would  indeed  be  a 
strange  accident  if  the  highest  powers  of  our  present  microscopes  had 
laid  bare  the  ultimate  organization  of  the  cell.  Briicke  insisted  more 
than  thirty  years  ago  that  protoplasm  must  possess  a  far  more  com- 
plicated morphological  organization  than  is  revealed  to  us  in  the 
visible  structure  of  the  cell,  repeating,  though  without  accepting,  an 
earlier  suggestion  of  Henle's('4i)  that  the  cell  might  be  composed  of 
more  elementary  vital  units  ranking  between  the  molecule  and  the 
cell.  Many  biological  thinkers  since  Briicke's  time  have  in  one  form 
or  other  accepted  this  conception,  which  indeed  lies  at  the  root  of 
nearly  all  recent  attempts  to  analyze  exhaustively  the  phenomena  of 
cell-life.  Without  attempting  to  follow  out  the  history  of  opinion  in 
detail  or  to  give  any  extended  review  of  the  various  theories,^  it  may 
be  pointed  out  that  this  conception  was  based  both  on  theoretical 
a  priori  grounds  and  on  the  observed  facts  of  cell-structure.  On  the 
former  basis  it  was  developed  by  Herbert  Spencer''  in  his  theory  of 
''  physiological  units  "  by  which  he  endeavoured  to  explain  the  phe- 
nomena of  regeneration,  development,  and  heredity  ;  while  Xageli 
('84)  developed  on  the  same  general  lines  his  theory  of  micclUc  which 

1  Elementarorganisjnen,  1861,  p.  386. 

2  Anthropogenie,  189 1,  p.  104. 

3  For  an  exhaustive  review  see  Yves  Delage,  La  structure  du  protoplasma  et  Us  theories  sui 
Vheredite.     Paris,  1895.  *  Principles  of  Biology,  1864. 

U  2S9 


290  SOME  PROBLEMS   OF  CELL-ORGANIZATION 

has  been  so  widely  accepted  by  botanists.  In  the  meantime  Darwin  ^ 
introduced  a  new  element  into  the  speculative  edifice  in  his  celebrated 
hypothesis  of  pangenesis,  where  for  the  first  time  appear  the  two 
assumptions  of  specific  differences  in  the  ultra-microscopic  corpuscles 
C'gemmules  ")  and  the  power  of  self-propagation  by  division.  Dar- 
win did  not,  however,  definitely  maintain  that  protoplasm  was  actually 
built  of  such  bodies.  The  latter  hypothesis  was  added  by  De  Vries 
('89),  who  remodelled  the  theory  of  pangenesis  on  this  assumption, 
thus  laying  the  basis  for  the  theories  of  development  which  reached 
their  climax  in  the  writings  of  Hertwig  and  Weismann. 

The  views  of  Spencer  and  Darwin  were  based  on  purely  theoretical 
grounds  derived  from  the  general  phenomena  of  growth  and  inheri- 
tance.^  Those  of  Nageh,  De  Vries,  Wiesner,  Altmann,  and  others 
were  more  directly  based  on  the  results  of  microscopical  investigation. 
The  view  was  first  suggested  by  Henle  ('41),  and  at  a  later  period 
developed  by  Bechamp  and  Estor,  by  Maggi  and  especially  by  Alt- 
mann, that  the  protoplasmic  granules  might  be  actually  organic  units 
or  bioblasts,  capable  of  assimilation,  growth,  and  division,  and  hence 
to  be  regarded  as  elementary  units  of  structure  standing  between  the 
cell  and  the  ultimate  molecules  of  living  matter.  By  Altmann,  espe- 
cially, this  view  was  pushed  to  an  extreme  limit,  which  lay  far  beyond 
anything  justified  by  the  known  facts;  and  the  theory  of  genetic  con- 
tinuity expressed  by  Redi  in  the  aphorism  ^^  oiniie  viviini  ex  vivo,'' 
reduced  by  Virchow  to  '^  omnis  cellula  e  celhda,'"  finally  appears  in 
the  writings  of  Altmann  as  ^^  omne  gramdiini  e  graiuilo''  /^ 

Altmann's  premature  generalization  rested  upon  a  very  insecure 
foundation  and  was  received  with  just  scepticism.  Except  in  the  case 
of  plastids,  the  division  of  the  cytoplasmic  granules  was  and  still 
remains  a  pure  assumption,  and  furthermore  many  of  Altmann's 
**  granules"  (zymogen-granules  of  gland-cells,  etc.)  are  undoubtedly 
metaplasmic  bodies.'^  Yet  the  beautiful  discoveries  of  Schimper  ('85) 
and  others  on  the  origin  of  plastids  in  plant-cells  give  evidence  that 
these  cells  do  in  fact  contain  large  numbers  of  bodies,  other  than  the 
nuclei,  that  possess  the  power  of  growth  and  division.  The  division 
of  the  chlorophyll-bodies,  observed  long  ago  by  Mohl,  was  shown  by 
Schmitz  and  Schimper  to  be  their  usual  if  not  their  only  mode  of  ori- 
gin ;  and  Schimper  was  able  to  trace  them  back  to  minute  colourless 
plastids,  scarcely  larger  than  '*  microsomes,"  that  are  present  in  large 
numbers  in  the  protoplasm  of  the  embryonic  cells  and  of  the  Qgg,  and 
give  rise  not  only  to  chlorophyll-bodies  but  also  to  the  amyloplasts  or 
starch-formers  and  the  chromoplasts  or  pigment-bodies.  While  it  still 
remains  doubtful  whether  the  plastids  arise  solely  by  division  or  also 

1  Variation  of  Atiimals  and  Plants,  1868.  *  Cf.  Introduction,  p.  12. 

^  Die  Elemeiitarorganismen,  Leipsic,  1894,  p.  155.  ^  Cf.  Lazarus,  '98. 


THE  NATURE    OE   CELL-ORGANS  201 

by  new  formation  (as  now  seems  to  be  the  case  with  the  centrosome), 
the  foregoing  observations  on  the  plastids  give  a  substantial  basis  for 
the  hypothesis  that  protoplasm  may  be  built  of  minute  dividing  bodies 
which  form  its  ultimate  structural  basis.  It  was  these  facts,  taken  in 
connection  with  the  phenomena  of  particulate  inheritance  and  varia- 
tion (Galton),  that  led  De  Vries  and  his  followers  to  the  fundamental 
assumption  of  '' pangens,"  ''plasomes,"  "  biophores,"  and  the  like  as 
final  protoplasmic  units ;  ^  but  these  were  conceived  not  as  the  visible 
granules,  plastids,  etc.,  but  as  much  smaller  bodies,  lying  far  bevond 
the  limits  of  present  microscopical  vision,  through  the  growth  or 
aggregation  of  which  the  visible  structures  arise.  This  assumption 
has  been  harshly  criticised;  yet  when  we  recall  that  in  one  form  or 
another  it  has  been  accepted  by  such  men  as  Spencer,  Darwin,  Beale, 
Hackel,  Michael  Foster,  Nageli,  De  Vries,  Wiesner,  Roux,  Weis- 
mann,  Oscar  Hertwig,  Verworn,  and  Whitman,  and  on  evidence  drawn 
from  sources  so  diverse,  we  must  admit  that  despite  its  highly  specula- 
tive character  it  is  not  to  be  lightly  rejected.  In  the  present  chapter 
we  may  inquire  how  far  the  known  facts  of  cell-structure  speak  for  or 
against  this  hypothesis,  incidentally  considering  a  number  of  detailed 
questions  of  cell-morphology  which  have  not  hitherto  found  a  place. 

A.     The  Nature  of  Cell-organs 

The  cell  is,  in  Briicke's  words,  an  elementary  organism,  which  may 
by  itself  perform  all  the  characteristic  operations  of  Hfe,  as  is  the  case 
with  the  unicellular  organisms,  and  in  a  sense  also  with  the  germ-cells. 
Even  when  the  cell  is  but  a  constituent  unit  of  a  hifrher  sfrade  of 
organization,  as  in  multicellular  forms,  it  is  no  less  truly  an  organism, 
and  in  a  measure  leads  an  independent  life,  even  though  its  functions 
be  restricted  and  subordinated  to  the  common  life.  It  is  true  that  the 
earlier  conception  of  the  multicellular  body  as  a  colony  of  one-celled 
forms  cannot  be  accepted  without  certain  reservations.-  Neverthe- 
less, all  the  facts  at  our  command  indicate  that  the  tissue-cell  possesses 
the  same  morphological  organization  as  the  egg-cell,  or  the  protozoan, 
and  the  same  fundamental  physiological  properties  as  well.  Like 
these  the  tissue-cell  has  its  differentiated  structural  parts  or  organs, 
and  we  have  now  to  inquire  how  these  cell-organs  are  to  be  conceived. 

1  The  following  list  includes  only  some  of  the  various  names  that  have  been  «;ivcn  to 
these  hypothetical  units  by  modern  writers :  Physiological  iDiiis  (Spencer) ;  geinmuU's 
(Darwin);  pangens  (De  Vries);  plasomes  (Wiesner);  miccllie  (Nageli);  ,plastiiiules 
(Hackel  and  Elssberg) ;  inotagmata  (Engelmann);  hiophores  (Weismann);  bioblasfs 
(Beale);  so7nacitles  {Yo?X.(tx)\  idioblasts  (Hertwig);  icUosomes  (Whitman);  biogens  (Ver- 
worn); microzymas  (Bechamp  and  Estor) ;  gemnuc  (Haacke).  These  names  are  not 
strictly  synonymous,  nor  do  all  of  the  writers  cited  assume  the  power  of  division  in  the 
units.  -  Cf.  p.  58. 


292  SOME  PROBLEMS   OF  CELL-ORGANIZATION 

The  visible  organs  of  the  cell  fall  under  two  categories,  according  as 
they  are  merely  temporary  structures,  formed  anew  in  each  successive 
cell-generation  out  of  the  common  structural  basis,  or  permanent  struc- 
tures whose  identity  is  never  lost,  since  they  are  directly  handed  on  by 
division  from  cell  to  cell.^  To  the  former  category  belong,  in  general, 
such  structures  as  cilia,  pseudopodia,  and  the  like ;  to  the  latter,  the 
nucleus,  perhaps  also  the  centrosomes,  and  the  plastids  of  plant-cells. 
A  peculiar  interest  attaches  to  the  permanent  cell-organs.  Closely 
interrelated  as  these  organs  are,  they  nevertheless  have  a  remarkable 
degree  of  morphological  independence.  They  assimilate  food,  grow, 
divide,  and  perform  their  own  characteristic  actions  like  coexistent  but 
independent  organisms,  of  a  lower  grade  than  the  cell,  living  together 
in  colonial  or  symbiotic  association.  So  striking  is  this  morphological 
and  physiological  autonomy  in  the  case  of  the  green  plastids  or  chro- 
matophores  that  neither  botanists  nor  zoologists  are  as  yet  able  to  dis- 
tinguish with  absolute  certainty  between  those  that  form  an  integral 
part  of  the  cell,  as  in  the  higher  green  plants,  and  those  that  are 
actually  independent  organisms  living  symbiotically  within  it,  as  is 
probably  the  case  with  the  yellow  cells  of  Radiolaria.  Even  so 
acute  an  investigator  as  Watase  ('93,  i)  has  seriously  propounded  the 
view  that  the  nucleus  itself  —  or  rather  the  chromosome  —  should  be 
regarded  as  a  distinct  organism  living  in  symbiotic  association  with 
the  cytoplasm,  but  having  had,  in  an  historical,  sense,  a  different  origin. 
This  rather  fantastic  view  has  not  found  much  favour,  and  even  were 
it  true  w^ould  teach  us  nothing  of  the  origin  of  the  power  of  division, 
which  must  for  the  present  be  taken  as  an  elementary  process  forming 
one  of  the  primary  data  of  biology.  Yet  we  may  still  inquire  whether 
the  power  of  division  shown  by  such  protoplasmic  masses  as  plastids, 
chromosomes,  centrosomes,  nucleoli,  and  nuclei  may  not  have  its  root 
in  a  like  power  residing  in  ultimate  protoplasmic  units  of  which  they 
are  made  up.  Could  we  accept  such  a  view,  we  might  much  more 
easily  meet  some  puzzling  cytological  difficulties.  For  under  this 
assumption  the  difference  between  transient  and  permanent  cell- 
organs  would  become  only  one  of  degree,  depending  on  the  degree  of 
cohesion  between  their  structural  components  ;  and  we  could  thus  con- 
ceive, for  example,  how  such  a  body  as  a  centrosome  might  form,  per- 
sist by  division  for  a  number  of  generations,  and  finally  disintegrate. 
In  connection  with  this  it  may  be  pointed  out  that  even  such  a  typical 
permanent  organ  as  the  nucleus  does  not  persist  as  siicJi  during  the 
ordinary  form  of  division  ;  for  it  loses  its  boundary  and  many  of  its 
other  structural  characters,  becoming  resolved  into  a  group  of  sepa- 
rate chromosomes.  What  persists  is  here  not  the  structural  unit,  but 
the  characteristic  substance  which  forms  its  essential  constituent,  and 

1  Cf.  footnote,  p.  30. 


STRUCTURAL  BASIS   OF  THE   CELL  293 

a  large  part  even  of  this  substance  may  be  lost  in  the  process.  The 
term  ''persistent  organ"  is  therefore  used  in  rather  a  figurative  sense, 
and  if  too  literally  understood  may  easily  mislead  us. 

With  the  foregoing  considerations  in  mind  let  us  turn  to  the  actual 
structural  relation  of  the  cell-orgfans. 


B.  Structural  Basis  of  the  Cell 

In  Chapter  I.  some  of  the  reasons  have  been  given  for  the  conclu- 
sion that  none  of  the  obvious  structural  features  of  protoplasm  (fibrillce, 
alveoli,  granules,  and  the  like)  can  be  regarded  as  necessary  or  uni- 
versal ;  and  we  may  now  inquire  whether  there  is  any  evidence  that 
such  structures  may  have  such  a  common  structural  basis  as  De  \'ries's 
theory  assumes.  I  shall  here  take  as  a  point  of  departure  my  observa- 
tions on  the  structure  of  protoplasm  in  echinoderm-eggs,' already  briefly 
reviewed  at  page  28.  The  beautiful  alveolar  structure  of  these  eggs  is 
entirely  of  secondary  origin,  and  all  the  visible  structural  elements 
arise  during  the  growth  of  the  eggs  by  the  deposit  and  subsequent 
enlargement  of  minute  spherical  bodies,  all  apparently  liquid  drops, 
in  a  homogeneous  or  finely  granular  basis  which  is  itself  a  liquid. 
Some  of  these  spheres  enlarge  to  form  the  alveolar  spheres,  whiie  the 
homogeneous  basis  or  continuous  substance  remains  as  the  interalve- 
olar  material.  Others  remain  much  smaller  to  constitute  the  "  micro- 
somes "  scattered  through  the  interalveolar  walls  ;  and  these  bodies, 
like  the  alveolar  spheres,  are  perfectly  visible  in  life,  as  well  as  in 
section  ;  they  are  therefore  not  coagulation-products  or  artifacts.  From 
these  three  elements  arise  all  the  other  structures  observed  in  these 
eggs,  deutoplasm-spheres  {Ophiiira)  and  pigment-bodies  {Arbncia) 
being  formed  by  further  enlargement  and  chemical  alteration  of  the 
alveolar  spheres,  while  astral  rays  and  spindle-fibres  are  differentiated 
out  of  the  inter-alveolar  material  and  microsomes.^  These  various 
elements  show  a  continuous  gradation  in  size  from  the  largest  deuto- 
plasm-spheres down  to  the  smallest  visible  granules,  the  latter  being 
the  source  of  all  the  larger  elements,  and  in  their  turn  emerging  into 
view  from  the  "  homogeneous  "  basis.  Clearly,  then,  none  of  these 
bodies  can  be  regarded  as  the  ultimate  structural  units  ;  for  the  latter, 
if  they  exist,  must  lie  in  a  region  at  present  inaccessible  to  the  micro- 
scope. This  fact,  however,  no  more  disproves  their  existence  than  it 
does  that  of  molecules  and  atoms.  It  only  shows  the  futiHty  of  such 
attempts  as  those  of  Altmann  and  his  predecessors  to  identify  "  gran- 
ules "  or  ''microsomes  "  as  final  morphological  units,  and  compels  us  to 
turn  to  indirect  instead  of  direct  evidence.  It  may,  however,  again  be 
pointed  out  that  it  would  be  quite  irrational  to  conclude  that  the  small- 

1  Cf.  Wilson,  '99. 


294  SOME  PROBLEMS   OF  CELL-ORGAXIZATION 

est  visible  granules  first  come  into  existence  when  they  first  come 
within  view  of  the  microscope.  The  **  homogeneous  "  substance  must 
itself  contain  or  consist  of  granules  still  smaller.  The  real  question 
is  not  whether  such  ultra-microscopical  bodies  exist,  but  whether  they 
are  permanent  organize dho^i^?.  possessing  besides  the  power  of  growth 
also  the  power  of  division.  This  question  can  be  only  indirectly  ap- 
proached ;  and  we  shall  find  it  convenient  to  do  so  by  beginning  at 
the  opposite  end  of  the  series,  through  a  reconsideration  of  the 
phenomena  of  nuclear  division. 

C.     Morphological  Composition  of  the  Nucleus 

I.    TJie  Chromatin 

(a)  HypotJiesis  of  the  Individuality  of  the  CJiromosomes.  —  It  may 
now  be  taken  as  a  well-estabhshed  fact  that  the  nucleus  is  never 
formed  de  novo,  but  always  arises  by  the  division  of  a  preexisting 
nucleus.  In  the  typical  mode  of  division  by  mitosis  the  chromatic 
substance  is  resolved  into  a  group  of  chromosomes,  always  the  same 
in  form  and  number  in  a  given  species  of  cell,  and  having  the  power 
of  assimilation,  growth,  and  division,  as  if  they  were  morphological 
individuals  of  a  lower  order  than  the  nucleus.  That  they  are  such 
individuals  or  units  has  been  maintained  as  a  definite  hypothesis,  es- 
pecially by  Rabl  and  Boveri.  As  a  result  of  careful  study  of  mitosis 
in  epithelial  cells  of  the  salamander,  Rabl  ('85)  concluded  that  the 
cJironiosomes  do  not  lose  their  individuality  at  the  close  of  division,  but 
persist  in  the  chromatic  reticnhim  of  the  resting  nnclens.  The  reticu- 
lum arises  through  a  transformation  of  the  chromosomes,  which  give 
off  anastomosing  branches,  and  thus  give  rise  to  the  appearance  of  a 
network.  Their  loss  of  identity  is,  however,  only  apparent.  They 
come  into  view  again  at  the  ensuing  division,  at  the  beginning  of 
which  "the  chromatic  substance  flows  back,  through  predetermined 
paths,  into  the  primary  chromosome-bodies  "  (Kernfaden),  which  re- 
appear in  the  ensuing  spireme-stage  in  nearly  or  quite  the  same  posi- 
tion they  occupied  before.  Even  in  the  resting  nucleus,  Rabl  believed 
that  he  could  discover  traces  of  the  chromosomes  in  the  configuration 
of  the  network,  and  he  described  the  nucleus  as  showing  a  distinct 
polarity  having  a  ''pole"  corresponding  with  the  point  toward  which 
the  apices  of  the  chromosomes  converge  {i.e.  toward  the  centrosome), 
and  an  ''  antipole"  (Gegenpol)  at  the  opposite  point  {i.e.  toward  the 
equator  of  the  spindle)  (Fig.  22).  Rabl's  hypothesis  was  precisely 
formulated  and  ardently  advocated  by  Boveri  in  1887  and  1888,  and 
again  in  1891,  on  the  ground  of  his  own  studies  and  those  of  Van 
Beneden  on  the  early  stages  of  Ascaris.    The  hypothesis  was  supported 


MORPHOLOGICAL    COMPOSITION  OF  THE  NUCLEUS 


295 


by  extremely  strong  evidence,  derived  especially  from  a  study  of  ab- 
normal variations  in  the  early  development  of  Ascaris,  the  force  of 
which  has,  I  think,  been  underestimated  by  the  critics  of  the  hypothesis. 
Some  of  this  evidence  may  here  be  briefly  reviewed.  In  some  cases, 
through  a  miscarriage  of  the  mitotic  mechanism,  one  or  both  of  the 
chromosomes  destined  for  the  second  polar  body  are  accidentally  left 


Fig.  143.  —  Evidence  of  the  individuality  of  the  chromosomes.  Abnormalities  in  the  fertiliza- 
tion of  Ascaris.     [BOVERI.J 

A.  The  two  chromosomes  of  the  egg-nucleus,  accidentally  separated,  have  given  rise  each  to  a 
reticular  nucleus  (?,  ?)  ;  the  sperm-nucleus  below  (-T).  B.  Later  stage  of  the  same,  a  single 
chromosome  in  each  egg-nucleus,  two  in  the  sperm-nucleus.  C.  An  egg  in  which  the  second 
polar  body  has  been  retained;  p.b'^  the  two  chromosomes  arising  from  it;  9  the  egg-chromo- 
somes;  d"  the  sperm-chromosomes.     D.   Resulting  equatorial  plate  with  six  chromosomes. 


in  the  ^gg.  These  chromosomes  give  rise  in  the  Q.gz  to  a  reticular  nu- 
cleus, indistinguishable  from  the  egg-nucleus.  At  a  later  period  this 
nucleus  gives  rise  to  the  same  number  of  chromosomes  as  those  that 
entered  into  its  formation,  i.e.  either  one  or  two.  These  are  drawn 
into  the  equatorial  plate  along  with  those  derived  from  the  germ- 
nuclei,  and  mitosis  proceeds  as  usual,  the  number  of  chromosomes 
being,  however,  abnormally  increased  from  four  to  five  or  six  (Fig.  143, 


296 


SOME  PROBLEMS   OF  CELL-ORGANIZATION 


C,  D).  Again,  the  two  chromosomes  left  in  the  ^gg  after  removal  of 
the  second  polar  body  may  accidentally  become  separated.  In  this 
case  each  chromosome  gives  rise  to  a  reticular  nucleus  of  half  the 
usual  size,  and  from  each  of  these  a  single  chromosome  is  afterward 
formed  (Fig.  143,  A,  B).  Finally,  it  sometimes  happens  that  the  two 
germ-nuclei  completely  fuse,  while  in  the  reticular  state,  as  is  normally 
the  case  in  sea-urchins  and  some  other  animals  (p.  188).  From  the 
cleavage-nucleus  thus  formed  arise  four  chromosomes. 

The  same  general  result  is  given  by  the  observations  of  Zur  Strassen 
('98)  on  the  history  of  giant  embryos  in  Ascaris.  These  embryos 
arise  by  the  fusion,  either  before  or  after  the  fertilization,  of  previ- 
ously separate  eggs,  and  have  been  shown  to  be  capable  of  develop- 
ment up  to  a  late  stage.  Not  only  in  the  first  but  also  in  some,  at 
least,  of  the  later  mitoses,  such  eggs  show  an  increased  number  of 

chromosomes  proportional  to  the  number 
of  nuclei  that  have  united.  Thus  in 
monospermic  double  eggs  (variety  bi- 
valeiis)  the  number  is  six  instead  of  four ; 
in  dispermic  double  eggs  the  number  is 
increased  to  eight  (Fig.  144). 

These  remarkable  observations  show 
that  whatever  be  the  imniber  of  eJironio- 
sonies  entering  into  the  forniation  of  a 
reticular  nnclens,  the  same  nnniber  after- 
ward issnes  from  it  —  a  result  which  de- 
monstrates that  the  number  of  chromo- 
somes is  not  due  merely  to  the  chemical 
composition  of  the  chromatin-substance, 
but  to  a  morphological  organization  of 
Fig.  144.  — Giant-embryo  of  y4j^ar/>,   the  nuclcus.     A  bcautiful   Confirmation 

of  this  conclusion  was  afterward  made 
by  Boveri  ('93,  '95,  i)  and  IMorgan  (95, 
4),  in  the  case  of  echinoderms,  by  rear- 
ing larvae  from  enucleated  egg-fragments,  fertilized  by  a  single  sper- 
matozoon (p.  194).  All  the  nuclei  of  such  larvae  contain  but  half  the 
typical  number  of  chromosomes,  —  i.e.  in  Echinns  nine  instead  of 
eighteen,  —  since  all  are  descended  from  one  germ-nucleus  instead 
of  two ! 

Equally  striking  is  the  remarkable  fact,  described  at  page  275,  that 
all  of  the  cells  in  the  sexual  generation  (oophore)  of  the  higher 
cryptogams  show  half  the  number  of  chromosomes  characteristic  of 
the  sporophyte,  the  explanation  being  that  while  reduction  occurs 
at  the  time  of  spore-formation,  the  spores  develop  without  fertilization, 
the  reduced  chromosome-number  persisting  until  fertilization  occurs 


var.  bivaleiis,  arising  from  a  double- 
fertilized  double  egg,  showing  eight 
chromosomes  {7.ur  Strasseti). 


MORPHOLOGICAL    COMPOSITION  OF  THE  NUCLEUS 


297 


long  afterward.  Attention  may  be  again  called  to  the  surprising  case 
of  Arteniia,  described  at  page  281,  which  gives  a  strong  argument  in 
favour  of  the  hypothesis. 

In  addition  to  the  foregoing  evidence,  Van  Beneden  and  Boveri 
were  able  to  demonstrate  in  Ascaris  that  in  the  formation  of  the 
spireme  the  chromosomes  reappear  in  the  same  position  as  those 
which  entered  into  the  formation  of  the  reticulum,  precisely  as  Rabl 


Fig.  145.  —  Evidence  of  the  individuality  of  the  chromosomes  in  the  egg  of  Ascaris.     [Boveri.] 

E.  Anaphase  of  the  first  cleavage.  F.  Two-cell  stage  with  lobed  nuclei,  the  lobes  formed  by 
the  ends  of  the  chromosomes.  G.  Early  prophase  of  the  ensuing  division  ;  chromosomes  re-form- 
ing, centrosomes  dividing.  H.  Later  prophase,  the  chromosomes  lying  with  their  ends  in  the 
same  position  as  before ;  centrosomes  divided. 

maintained.  As  the  long  chromosomes  diverge,  their  free  ends  arc 
always  turned  toward  the  middle  plane  (Fig.  31),  and  upon  the  re- 
construction of  the  daughter-nuclei  these  ends  give  rise  to  corresjiond- 
ing  lobes  of  the  nucleus,  as  in  Fig.  145,  which  persist  throughout  the 
resting  state.  At  the  succeeding  division  the  chromosomes  reappear 
exactly  in  the  same  position,  their  ends  lyiuj^  in  the  nuclear  lobes  as 
before  {¥ig.  145,  G,  H).  On  the  strength  of  these  facts  Boveri  con- 
cluded that  the  chromosomes  must  be  regarded  as  "  individuals  "  or 
''  elementary  organisms,"  that  have  an  independent  existence  in  the 


298 


SOME  PROBLEMS   OF  CELL-ORGANIZATION- 


cell.  During  the  reconstruction  of  the  nucleus  they  send  forth  pseu- 
dopodia  which  anastomose  to  form  a  network  in  which  their  identity 
is  lost  to  view.  As  the  cell  prepares  for  division,  however,  the  chro- 
mosomes contract,  withdraw  their  processes,  and  return  to  their 
"resting  state,"  in  which  fission  takes  place.  Applying  this  con- 
clusion to  the  fertilization  of  the  Qgg,  Boveri  expressed  his  belief  that 


Fig.  146.— Independence  of  paternal  and  maternal  chromatin  in  the  segmenting  eggs  of 
Cyclops.     [A-C,  from  RucKERT;  D,  from  Hacker.] 

A.  First  cleavage-figure  in  C.  strenuus ;  complete  independence  of  paternal  and  maternal 
chromosomes.  B.  Resulting  two-cell  stage  with  double  nuclei.  C.  Second  cleavage;  chromosomes 
still  in  double  groups.   D.  Blastomeres  with  double  nuclei  from  the  eight-cell  stage  of  C  brevicornis. 

"  we  may  identify  every  chromatic  element  arising  from  a  resting 
nucleus  with  a  definite  element  that  entered  into  the  formation  of 
that  nucleus,  from  which  the  remarkable  conclusion  follows  that  in 
all  cells  derived  in  the  regular  course  of  division  from  the  fertilized 
egg,  one-half  of  the  chromosojnes  are  of  strictly  pater^ial  origin,  the 

other  half  of  maternal T  ^ 

i'9i,  p.  410. 


MORPHOLOGICAL    COMPOSITION  OF   THE  NUCLEUS  2Qg 

Boveri's  hypothesis  has  been  criticised  by  many  writers,  especially 
by  Hertwig,  Guignard,  and  Brauer,  and  I  myself  have  urged  some 
objections  to  it.  Recently,  however,  it  has  received  a  support  so 
strong  as  to  amount  almost  to  a  demonstration,  through  the  remark- 
able observations  of  Ruckert,  Hacker,  Herla,  and  Zoja  on  the 
independence  of  the  paternal  and  maternal  chromosomes.  These 
observations,  already  referred  to  at  page  208,  may  be  more  fully  re- 
view^ed  at  this  point.  Hacker  ('92,  2)  first  showed  that  in  Cyclops 
stre7iiiiis,  as  in  Ascaris  and  other  forms,  the  germ-nuclei  do  not  fuse 
but  give  rise  to  two  separate  groups  of  chromosomes  that  lie  side  by 
side  near  the  equator  of  the  cleavage-spindle.  In  the  two-cell  stage 
(of  Cyclops  tciiiiicornis)  each  nucleus  consists  of  two  distinct  thou^'-h 
closely  united  halves,  which  Hacker  believed  to  be  the  derivatives  of 
the  two  respective  germ-nuclei.  The  truth  of  this  surmise  was  demon- 
strated three  years  later  by  Ruckert  ('95,  3)  in  a  species  of  Cyclops, 
likewise  identified  as  C.  strcimiis  (Fig.  146).  The  number  of  chromo- 
somes in  each  germ-nucleus  is  here  twelve.  Ruckert  was  able  to 
trace  the  paternal  and  maternal  groups  of  daughter-chromosomes  not 
only  into  the  respective  halves  of  the  daughter-nuclei  of  the  two-cell 
stage,  but  into  later  cleavage -stages.  From  the  bilobed  nuclei  of  the 
two-cell  stage  arise,  in  each  cell,  a  double  spireme  and  a  double 
group  of  chromosomes,  from  which  are  formed  bilobed  or  double 
nuclei  in  the  four-cell  stage.  This  process  is  repeated  at  the  ne.xt 
cleavage,  and  the  double  character  of  the  nuclei  was  in  nian\-  cases 
distinctly  recognizable  at  a  late  stage  when  the  germ-layers  were 
being  formed. 

Finally  Victor  Herla's  ('93)  and  Zoja's  ('95,  2)  remarkable  obser- 
vations on  Ascaris  showed  that  in  Ascaris  not  only  the  chromatin  of 
the  germ-nuclei,  but  also  the  paternal  and  maternal  cliroviosonics, 
remain  perfectly  distinct  as  far  as  the  twelve-cell  stage  —  certainly  a 
brilliant  confirmation  of  Boveri's  conclusion.  Just  how  far  the  dis- 
tinction is  maintained  is  still  uncertain,  but  Hacker's  and  Riickert's 
observations  give  some  ground  to  believe  that  it  may  ])ersist  through- 
out the  entire  life  of  the  embryo.  Both  these  observers  have  shown 
that  the  chromosomes  of  the  germinal  vesicle  appear  in  ti^'o  (lisfinct 
groups,  and  Ruckert  suggests  that  these  may  represent  the  paternal 
and  maternal  elements  that  have  remained  distinct  throughout  the 
entire  cycle  of  development,  even  down  to  the  formation  of  the  (t^^'g ! 

Leaving  aside  all  doubtful  cases  (such  as  the  above  suggestion  of 
Riickert's),  the  well-determined  facts  form  an  irresistible  proof  of  the 
general  hypothesis  ;  and  it  is  one  with  which  every  general  analysis 
of  the  cell  has  to  reckon.  I  beheve,  however,  that  the  hypothesis  has 
received  an  unfortunate  name ;  for,  except  in  a  few  special  cases,^ 


(f/p. 


■16- 


30O 


SOME  PROBLEMS   OF  CELL-ORGANIZATION 


almost  no  direct  evidence  exists  to  show  that  the  chromosomes  persist 
as  *'  individuals  "  in  the  chromatin-reticulum  of  the  resting  cell.  The 
facts  indicate,  on  the  contrary,  that  in  the  vast  majority  of  cases  the 
identity  of  the  chromosomes  is  wholly  lost  in  the  resting  nucleus,  and 
the  attempts  to  identify  them  through  the  polarity  or  other  morpho- 
logical features  of  the  nuclear  network  have  on  the  whole  been  futile. 
It  is  therefore  an  abuse  of  language  to  speak  of  a  persistent  ''individ- 


B 


C 

Fig.  147.  —  Hybrid  fertilization  of  the  egg  oi  As  car  is  megalocephala,  var.  bivalens,  by  the  sper- 
matozoon of  van  z^«/^'a/^«i'.     [Herla.] 

A.  The  germ-nuclei  shortly  before  union.  B.  The  cleavage-figure  forming;  the  sperm-nucleus 
has  given  rise  to  one  chromosome  (J'),  the  egg-nucleus  to  two  (9).  C.  Two-cell  stage  dividing, 
showing  the  three  chromosomes  in  each  cell.  D.  Twelve-cell  stage,  with  the  three  distinct  chro- 
mosomes still  shown  in  the  primordial  germ-cell  or  stem-cell. 

uality  "  of  chromosomes.  But  this  verbal  difficulty  should  not  blind 
us  to  the  extraordinary  interest  and  significance  of  the  facts.  It  is 
difficult  to  suppose  that  the  tendency  of  the  chromatin  to  resolve 
itself  into  a  particular  number  of  chromosomes  is  directly  due  to  its 
chemical  or  molecular  structure,  or  is  analogous  to  crystallization  ;  for 
in  the  chromatin  of  the  same  species,  or  even  in  that  of  the  same  ^^%^ 
this  tendency  varies,  not  with  chemical,  but  with  purely  morphological 


MORPHOLOGICAL    COMPOSFnON   OF   TLIE  NUCLEUS  ^o\ 

conditions,  i.e.  with  the  number  of  chromosomes  that  enter  the 
nucleus.  Neither  can  we  assume  that  it  is  due  merely  to  the  total 
mass  of  the  chromatin  in  each  case  ;  for  this  varies  in  different  nuclei 
of  the  same  species,  or  even  in  the  nucleus  of  the  same  cell  at  dif- 
ferent periods  (as  in  the  egg-cell),  yet  the  same  number  of  chromo- 
somes is  characteristic  of  all.  Indeed,  we  seek  in  vain  for  an  analogy 
to  these  phenomena  and  can  only  admit  our  entire  inability  to  explain 
them.  No  phenomena  in  the  history  of  the  cell  more  clearly  indicate 
the  existence  of  a  morphological  organization  which,  though  resting 
upon,  is  not  to  be  confounded  with,  the  chemical  and  molecular 
structure  that  underlies  it  ;  and  this  remains  true  even  though  we  are 
wholly  ignorant  what  that  organization  is. 

{b^  Coviposition  of  the  CJiromosonies.  —  We  owe  to  Roux  ^  the  first 
clear  formulation  of  the  view  that  the  chromosomes,  or  the  chr(jmatin- 
thread,  consist  of  successive  regions  or  elements  that  are  qualitatively 
different  (p.  244).  This  hypothesis,  which  has  been  accepted  by 
Weismann,  Strasburger,  and  a  number  of  others,  lends  a  peculiar 
interest  to  the  morphological  composition  of  the  chromatic  substance. 
The  facts  are  now  well  estabhshed  (i )  that  in  a  large  number  of  cases 
the  chromatin-thread  consists  of  a  series  of  granules  (chromomeres) 
embedded  in  and  held  together  by  the  linin-substance,  (2)  that  the 
splitting  of  the  chromosomes  is  caused  by  the  division  of  these  more 
elementary  bodies,  (3)  that  the  chromatin-grains  may  divide  at  a  time 
when  the  spireme  is  only  just  beginning  to  emerge  from  the  reticulum 
of  the  resting  nucleus.  These  facts  point  unmistakably  to  the  conclu- 
sion that  these  granules  are  perhaps^ to  be  regarded  as  independent 
morphological  elements  of  a  lower  grade  than  the  chromosomes. 
That  they  are  not  artifacts  or  coagulation-products  is  proved  by  their 
uniform  size  and  regular  arrangement  in  the  thread,  especially  when 
the  thread  is  split.  A  decisive  test  of  their  morphological  nature  is, 
however,  even  more  difficult  than  in  the  case  of  the  chromosomes ; 
for  the  chromatin-grains  often  become  apparently  fused  together  so 
that  the  chromatin-thread  appears  perfectly  homogeneous,  and  whether 
they  lose  their  individuality  in  this  close  union  is  undetermined. 
Observations  on  their  number  are  still  very  scanty,  but  they  point  to 
some  very  interesting  conclusions.  In  Boveri's  figures  of  the  egg- 
maturation  of  Ascaris  each  element  of  the  tetrad  consists  of  six  chro- 
matin-discs  arranged  in  a  linear  series  (Van  Beneden's  figures  of  the 
same  object  show  at  most  five)  which  finally  fuse  to  form  an  appar- 
ently homogeneous  body.  In  the  chromosomes  of  the  germ-nuclei 
the  number  is  at  least  double  this  (Van  Beneden).  Their  number  has 
been  more  carefully  followed  out  in  the  spermatogenesis  of  the  same 
animal  (variety  bivalens)  by  Brauer.     At  the  time  the  chromatin-grains 

1  Bedeututtg  der  Kerntheilungsfigtiren,  1SS3,  p.  15. 


302  SOME  PROBLEMS    OF  CELL-ORGAXIZATION- 

divide,  in  the  reticulum  of  the  spermatocyte-nucleus,  they  are  very 
numerous.  His  figures  of  the  spireme-thread  show  at  first  nearly 
forty  granules  in  linear  series  (Fig.  120,  />).  Just  before  the  breaking 
of  the  thread  into  two  the  number  is  reduced  to  ten  or  twelve  (Fig. 
120,  C).  Just  after  the  division  to  form  the  two  tetrads  the  number 
is  four  or  five  (Fig.  120,  Z>),  which  finally  fuse  into  a  homogeneous 
body.^ 

It  is  certain,  therefore,  that  the  number  of  chromomeres  is  not  con- 
stant in  a  given  species,  but  it  is  a  significant  fact  that  in  Ascaris  the 
final  number,  before  fusion,  appears  to  be  nearly  the  same  (four  to 
six)  both  in  the  oogenesis  and  the  spermatogenesis.  The  facts  re- 
garding bivalent  and  plurivalent  chromosomes  (p.  '^j)  at  once  sug- 
gest themselves,  and  one  cannot  avoid  the  thought  that  the  smallest 
chromatin-grains  may  successively  group  themselves  in  larger  and 
larsfer  combinations  of  which  the  final  term  is  the  chromosome. 
Whether  these  combinations  are  to  be  regarded  as  **  individuals  "  is  a 
question  which  can  only  lead  to  a  barren  play  of  words.  The  fact 
that  cannot  be  escaped  is  that  the  history  of  the  chromatin-substance 
reveals  to  us,  not  a  homogeneous  substance,  but  a  definite  morpho- 
logical organization  in  which,  as  through  an  inverted  telescope,  we 
behold  a  series  of  more  and  more  elementary  groups,  the  last  visible 
term  of  which  is  the  smallest  chromatin-granule,  or  nuclear  microsome, 
beyond  which  our  present  optical  appliances  do  not  allow  us  to  see. 
Are  these  the  ultimate  dividing  units,  as  Brauer  suggests  (p.  113)? 
Here  again  we  may  well  recall  Strasburger's  warning,  and  hesitate  to 
identify  the  end  of  the  series  with  the  Hmits  reached  by  our  best 
lenses.  Somewhere,  however,  the  series  must  end  in  final  chromatic 
units  which  cannot  be  further  subdivided  without  the  decomposition 
of  chromatin  into  simpler  chemical  substances  ;  and  these  units  must 
be  capable  of  assimilation,  growth,  and  division  without  loss  of  their 
specific  character.  It  is  in  these  ultimate  units  that  we  must  seek  the 
**  qualities,"  if  they  exist,  postulated  in  Roux's  hypothesis;  but  the 
existence  of  such  qualitative  differences  is  a  physiological  assumption 
that  in  no  manner  prejudices  our  conclusion  regarding  the  ultimate 
vioi'pJiological  composition  of   the  chromatin. 

D.     Chromatin,  Lixin,  and  Cytoplasm 

What,  now,  is  the  relation  of  the  chromatin-grains  to  the  linin-net- 
work  and  the  cytoplasm }     Van  Beneden  long  ago  maintained  ^  that 

1  Eisen  ('99)  finds  that  the  chromosomes  of  the  spermatogonia  of  Batrachoseps  always 
consist  of  six  "chromomeres,"  each  of  which  consists  of  three  smaller  granules  or  "  chromi- 
oles."  The  latter  persist  as  the  chromatin-granules  of  the  resting  nucleus;  and  it  is  through 
their  successive  aggregation  that  the  chromumeres  and  chromosomes  are  formed. 

2  '83,  pp.  580,  583. 


CHROMATIN,  LININ,   AND   CYTOPLASM  303 

the  achromatic  network,  the  nuclear  membrane,  and  the  cell-mesh- 
work  have  essentially  the  same  structure,  all  consisting  of  microsomes 
united  by  connective  substance,  and  being  only  "  parts  of  one  and  the 
same  structure."  But,  more  than  this,  he  asserted  that  the  chromatic 
and  acJiroinatic  7nicrosomcs  might  be  transformed  into  07ie  another,  and 
were  therefore  of  essentially  the  same  morphological  nature.  "  They 
pass  successively,  in  the  course  of  the  nuclear  evolution,  through  a 
chromatic  or  an  achromatic  stage,  according  as  they  imbibe  or  *^nve 
off  the  chromophilous  substance."  ^  Both  these  conclusions  are  borne 
out  by  recent  researches.  Heidenhain  ('93,  '94),  confirmed  by  Reinke 
and  Schloter,  finds  that  the  nuclear  network  contains  granules  of  two 
kinds  differing  in  their  staining-capacity.  The  first  are  the  basi-chro- 
matin  granules,  which  stain  with  the  true  nuclear  dyes  (basic  tar-col- 
ours, etc.),  and  are  identical  with  the  '' chromatin-granules  "  of  other 
authors.  The  second  are  the  oxychromatin-granules  of  the  linin-net- 
work,  which  stain  with  the  plasma-stains  (acid  colours,  etc.),  and  are 
closely  similar  to  those  of  the  cytoreticulum.  These  two  forms  gradu- 
ate into  one  ajiother,  and  are  conjectured  to  be  diffeirnt  phases  of  the 
same  elements.  This  conception  is  furthermore  supported  by  many 
observations  on  the  behaviour  of  the  nuclear  network  as  a  whole. 
The  chromatic  substance  is  known  to  undergo  very  great  changes  in 
staining-capacity  at  different  periods  in  the  life  of  the  nucleus  (p.  ll^\ 
and  is  known  to  vary  greatly  in  bulk.  In  certain  cases  a  very  large 
amount  of  the  original  chromatic  network  is  cast  out  of  the  nucleus 
at  the  time  of  the  division,  and  is  converted  into  cytoplasm.  And, 
finally,  in  studying  mitosis  in  sea-urchin  eggs  I  found  reason  to  con- 
clude ('95,  2)  that  a  considerable  part  of  the  linin-network,  from  which 
the  spindle-fibres  are  formed,  is  actually  derived  from  the  chromatin. 

From  the  time  of  the  earlier  writings  of  Frommann  ('65,  '67), 
Arnold  {^^j\  Heitzmann  ('73),  and  Klein  {^J^\  down  to  the  present, 
an  increasing  number  of  observers  have  held  that  the  nuclear  reticu- 
lum is  to  be  conceived  as  a  modification  of  the  same  structural  basis 
as  that  which  forms  the  cytoplasm.  The  latest  researches  indicate, 
indeed,  that  true  chromatin  (nuclein)  is  confined  to  the  nucleus.-  I^ut 
the  whole  weight  of  the  evidence  now  goes  to  show  that  the  linin- 
network  is  of  the  same  nature  as  the  cell-mesh  work,  and  that  the 
achromatic  nuclear  membrane  is  formed  as  a  condensation  of  the  same 
substance.  Many  investigators,  among  whom  may  be  named  From- 
mann, Leydig,  Klein,  Van  Beneden,  Carnoy,  and  Reinke,  have  de- 
scribed the  fibres  of  both  the  intra-  and  extra-nuclear  network  as 
terminating  in  the  nuclear  membrane ;  and  the  membrane  itself  is 
described  by  these  and  other  observers  as  being  itself  reticular  in 
structure,  and  by  some  (Van  Beneden)  as  consisting  of  closely  crowded 

1  I.e.  p.  583.  2  Cf.  Hammarsten  ('95). 


304  SOME  PROBLEMS   OF  CELL-ORGAXIZATION 

microsomes  arranged  in  a  network.  The  clearest  evidence  is,  however, 
afforded  by  the  origin  of  the  spindle-fibres  in  mitotic  division  ;  for  it 
is  now  well  established  that  these  may  be  formed  either  inside  or  out- 
side the  nucleus,  and  at  the  close  of  mitosis  the  central  portion  of  the 
spindle  appears  always  to  give  rise  to  a  portion  of  the  cytoplasm 
lying  between  the  daughter-nuclei.  In  such  a  case  as  that  of  the 
sea-urchin  (see  above)  we  have,  therefore,  evidence  of  a  direct  trans- 
formation of  chromatin  into  linin-substance,  of  the  latter  into  spindle- 
fibres,  and,  finally,  of  these  into  cytoplasm. 

When  all  these  facts  are  placed  in  connection,  we  find  it  difficult  to 
escape  the  conclusion  that  no  definite  line  can  be  drawn  between  the 
cytoplasmic  granules  at  one  extreme  and  the  chromatin-granules  at 
the  other.  And  inasmuch  as  the  latter  are  certainly  capable  of 
growth  and  division,  we  cannot  deny  the  possibility  that  the  former 
may  themselves  have,  or  arise  from  elements  having  like  powers. 
But  while  we  may  take  this  as  a  fair  working  hypothesis,  we  should 
clearly  recognize  that  the  base  of  well-determined  fact  on  which  it 
rests  is  approached  by  a  circuitous  route ;  that  in  case  of  most  of  the 
cytoplasmic  granules  there  is  not  the  slightest  evidence  that  they 
multiply  by  division ;  and  that  even  though  some  of  them  may  have 
such  powers,  we  cannot  regard  them  as  the  ultimate  structural  units, 
for  the  latter  must  be  bodies  far  more  minute. 

E.     The    Centrosome 

From  our  present  point  of  view  the  centrosome  possesses  a  peculiar 
interest  as  a  cell-organ  which  may  be  scarcely  larger  than  a  cytomi- 
crosome,  yet  possesses  specific  physiological  properties,  assimilates, 
grows,  divides,  and  may  persist  from  cell  to  cell  without  loss  of  identity. 
Nearly  all  observers  of  the  centrosome  have  found  it  lying  in  the 
cytoplasm,  outside  the  nucleus;  but  apart  from  the  Protozoa  (p.  94) 
there  is  at  least  one  well-established  case  in  which  it  lies  within  the 
nucleus,  namely,  that  of  Ascaris,  where  Brauer  made  the  interesting 
discovery  that  in  one  variety  {iinivalens)  the  centrosome  lies  inside  the 
nnclens^  in  the  other  variety  {bivalens)  outside  —  a  fact  which  proves 
that  its  position  is  non-essential  ((/.  Figs.  120  and  148). 

An  intra-nuclear  origin  of  the  centrosome  has  also  been  asserted  by 
Julin  ('93)  in  the  primary  spermatocytes  of  Styleopsis,  by  Riickert 
('94)  in  the  eggs  of  Cyclops,  Mathews  ('95)  in  those  of  Asterias,  Car- 
noy  and  Le  Brun  ('97,  2)  in  Ascaris,  Van  der  Stricht  ('98)  in  the  eggs 
of  Thysanozodn,  by  R.  Hertwig  ('98)  in  ActinospJicerinm,  Calkins 
('98,  I )  in  Noctiluca,  and  Schaudinn  ('96,  3)  in  spore-producing  buds  of 
Acanthocystis,  though  in  the  last-named  form  the  centrosome  of  the 
vegetative  forms  is  extra-nuclear  (p.  92). 


1 


THE    CEXTROSOME  305 


As  already  stated, ^  it  is  still  undetermined  whether  a  true  centro- 
some  may  ever  arise  de  novo,  but  the  evidence  in  favour  of  such  a 
possibility  has  of  late  rapidly  increased.  Carnoy  ('86)  long  since 
showed  that  the  ^g^  of  Ascaris,  during  the  formation  of  the  polar 
bodies,  sometimes  showed  numerous  accessory  asters  scattered 
through  the  cytoplasm.  Reinke  ('94)  described  somewhat  similar 
asters  in  peritoneal  cells  of  the  salamander,  distinguishing  among 
them  three  orders  of  magnitude,  the  largest  containing  distinct 
centrosomes  or  "primary  centres,"  while  the  smaller  contained 
"secondary"  and  "tertiary"  centres,  the   last   named   being   single 


H 


D 


E 


a: 


F  ^ 

Fig.  148.  —  Mitosis  witli  intra-nuclear  centrosome,  in  the  spermatocytes  of  Ast\iris  tttfx<^h- 
cephala,  var.  univalens.      [Brauer.] 

A.  Nucleus  containing  a  quadruple  group  or  tetrad  of  chromosomes  (/).  nucleolus  («).  and 
centrosome  {c).  B.  C.  Division  of  the  centrosome,  D.  E.F.  G.  Formation  of  the  mitotic  tiijure. 
centrosomes  escaping  from  the  nucleus  in  G. 

microsomes  at  the  nodes  of  the  cytoreticulum.  By  successive  aggre- 
gations of  the  tertiary  and  secondary  centres  arise  true  centrosomes 
as  new  formations.  Watase  ('94-95)  ^^^o  finds  in  the  q^^  of  Mturo- 
bdella,  besides  the  normal  aster  containing  an  undoubted  centrosome. 
numerous  smaller  asters  graduating  downwards  to  such  "tertiary 
asters"  as  Reinke  describes  with  a  microsome  at  the  centre  of  each, 
and  on  this  basis  concludes  that  the  true  centrosome  differs  from  a 
microsome  only  in  degree  and  may  arise  dc  novo.  iMottier  ('97.  2) 
finds  in  pollen-mother-cells  numerous  minute  "  cyto-asters "  having 
no  direct  relation  to  the  spindle-formation  (Fig.    133).     Again   Juel 

1  r/pp.  52.  214. 


3o6 


SOME  PROBLEMS   OF  CELL-ORGANIZATION 


('97)  finds  that  an  isolated  chromosome,  accidentally  separated  from 
the  equatorial  plate  (pollen-mother-cells  of  Hcmei'ocallis),  may  give 
rise  to  a  small  vesicular  nucleus  which  may  subsequently  divide  by 
mitosis,  though  it  is  quite  out  of  relation  to  the  spindle-poles  of  the 
preceding  mitosis  (Fig.  149).  Strong  evidence  of  the  same  character 
as  the  last  is  given  by  the  facts  in  the  heliozoon  AauitJiocystis,  as 
shown  by  Schaudinn  ('96,  3),  the  ordinary  vegetative  cells  containing 
a  persistent  extra-nuclear  centrosome,  while  in  the  bud-formation  of 
the  swarm-spores  a  centrosome  is  formed  dc  novo,  ivitJiojit  relation  to 
that  of  the  mother-cell,  inside  the  nucleus  of  the  bud  (Fig.  41). 

The  strongest  case  in  favour  of  the  independent  origin  of  centro- 
somes  is,  however,  given  by  the  observations  of  Mead  on  CJiatopteriis 
('98)  and  the  remarkable  experiments  of  R.  Hertwig  ('95,  '96)  and 


A 


C 


Fig.  149.  —  Abnormal  mitosis  in  pollen-mother-cells  of  Hemerocallis,  showing   formation  of 
small  nucleus  from  one  or  two  stray  chromosomes  and  its  subsequent  division.     [JUEL.] 


Morgan  ('96,  i  ;  '99,  i)on  the  eggs  of  echinoderms  and  other  animals. 
When  eggs  of  CJicEtopterns  are  taken  from  the  body-cavity  and  placed 
in  sea-water,  a  multitude  of  small  asters  appear  in  the  cytoplasm,  two 
of  which  are  believed  to  persist  as  those  of  the  polar  spindle,  while 
the  others  degenerate  (Fig.  150).  Mead  is  therefore  convinced 
that  the  polar  centrosomes  arise  in  this  case  separately  and  de  novo} 
R.  Hertwig  showed  that  when  unfertilized  eggs  of  sea-urchins 
{ Strongylocentrotus ,  EcJiiniis)  are  kept  for  some  time  in  sea-water  or 
treated  with  dilute  solutions  of  strychnine  the  nuclei  undergo  some  of 


1  A  number  of  other  authors  {^e.g.  Griffin,  Thalassema,  Coe,  Cerebratuhis)  have  likewise 
found  the  first  polar  asters  widely  separated  at  their  first  appearance.  On  the  other  hand, 
Mathews  ('95),  whose  preparations  I  have  seen,  finds  the  polar  centrosomes  in  Asto-ias 
close  together,  and  Francotte  ('97,  '98)  has  demonstrated  that  in  Cydopoi-iis  and  Prosthece- 
ra:iis\\v^v  arise  by  the  division  of  a  single  primary  centrosome.  The  same  is  stated  by  Gar- 
diner ('98)  to  be  the  case  in  Polychccrus.  It  should  be  noted,  further,  that  Mead  could  find 
no  undoubted  centrosomes  save  in  the  "  primary  "  or  definitive  polar  asters. 


THE    CENTROSOME 


30; 


the  changes  of  mitosis,  the  chromatin-nctwork  giving  rise  to  a  group 
of  chromosomes  and  a  spindle,  or  more  frequently  a  fan-shaped 
half-spindle,  arising  from  the  achromatic  substance.  In  some  cases 
not  only  a  complete  spindle  appeared  but  also  asters  at  the  poles, 
though  no  centrosomes  were  observed  (Fig.  151).  Morgan's  experi- 
ments along  the  same  lines  were  mainly  performed  upon  the  sea- 
urchin  Arbacia,  but  included  also  the  eggs  of  Astcrias,  Sipunculus, 
and  Cerebratnlus  (Figs.  150,  151).  In  these  eggs  numerous  asters 
may  arise  in  the  cytoplasm,  if  they  are  allowed  to  lie  some  time  in  sea- 


v=i-il!.:: 


/::••.. 


9!f^yU -MSB  ■:;::. 


^ 


:'i^M--' 


W-  '"■■■ 


,j->;_^^-  -.^  .— 


•  -:  ^-  •.  '  -.r  V-";  *■;.    \  ■«*  •      ;  ■  ■   ^^^  - 


mimm   c 


■:^^. 


'■:-->J: 


':.-:-:'vk'^^ 


c#i^ 


^ 


B 


••.::-^:: 


"^^kWM-o 


c 


Fig.  150.  — Formation  de  novo  (?)  of  centrosomes.     [A,  B,  MEAD;  C,  MORGAN.] 

A.   Unfertilized  egg  of  ChcBtopterus  with  "  secondary  asters  "  developed  a  few  minutes  after  the 

egg  is  placed  in  sea-water.    D.  Slightly  later  stage  with  two  definitive  polar  asters  and  centrosomes. 

C.    Large  "sun"  (transformed  polar  aster)   containing  numerous  small  "secondary  asters  "  .in  <1 

centrosomes,   from    unfertilized   egg  of   Cerebratnlus  after  22  hours   in  1.5  '•;    sodium  chlorult- 

solution. 

water  or  treated  by  weak  solutions  of  sodium  or  magnesium  chloride. 
These  asters  often  contain  deeply  staining,  central  granules  indistin- 
guishable from  the  centrosomes  of  the  normal  asters  ;  and,  what  is  ot 
high  interest,  such  of  them  as  lie  near  the  nucleus  take  part  in  the 
irregular  nuclear  division  that  ensues,  forming  centres  toward  which 
the  chromosomes  pass.  These  divisions  continue  for  some  time,  the 
chromosomes  being  irregularly  distributed  through  the  it^^^,  and  giving 
rise  to  nuclei  of  various  sizes  apparently  dependent  upon  the  number 
of  chromosomes  each  receives.     After  a  variable   number  of   such 


3o8 


SOME   PROBLEMS   OF  CELL-ORGANIZATION 


divisions  the  asters  disappear,  yet  the  irregular  nuclear  divisions  con- 
tinue, nuclear  spindles  with  distinct  centrosomes  being  formed  at  each 
division,  but  apparently  without  relation  to  the  older  asters,  and  they 


wmi:^^ 


Fig.  151.  —  Formation  of  centrosomes  and  asters  in  unfertilized  echinoderm-eggs.  [A,  B, 
Morgan  ;   C-E,  R.  Hertwig.] 

A.  Arbacia,  after  4V2  hours  in  1.5  %  solution  of  sodium  chloride,  then  5  hours  in  sea-water; 
scattered  chromosomes  and  asters.  B.  Asters  formed  after  6^2  hours  in  XaCl.  C-E.  Echinus 
after  treatment  with  0.5  %  strychnine-solution,  showing  various  forms  of  astral  formations  (fan- 
shaped  aster,  half  spindle,  and  complete  mitotic  figure). 

are  believed  by  Morgan  to  arise  de  novo  from  the  Qgg  substance.^ 
In  the  meantime  irregular  cleavage  of  the  Qgg  occurs,  though  no 
embryo  is  produced.'^    Loeb,  however,  in  the  remarkable  experiments 

^  '99,  p-  479. 

-  Morgan  makes  the  important  observation,  which  harmonizes  with  that  of  Boveri, 
reported  at  page  108,  that  the  divisions  occur  with  respect  to  the  nujuber  and  position  of  the 
nuclei,  not  of  the  asters,  concluding  that  the  former  must  therefore  play  an  essential  rdle  as 
centres  of  division,  and  that  the  activity  of  the  asters  is  in  itself  not  sufficient  to  account 
for  division  of  the  cytoplasm. 


I 


THE    CENTROSOME  309 

referred  to  at  page  215,  finds  that  after  treatment  with  magnesium 
chloride  unfertilized  sea-urchin  eggs  {Arbacia)  may  give  rise  to  perfect 
Phiteus  larvae  —  a  result  which  if  well  founded  seems  to  place  the 
new  formation  of  true  centrosomes  beyond  question. 

Taken  together,  these  researches  give  strong  ground  for  the  con- 
clusion that  true  {i.e.  physiological)  centrosomes  may  arise  de  novo 
from  either  the  cytoplasmic  or  the  nuclear  substance  and  may  play 
the  usual  role  (whatever  that  may  be)  in  mitosis.  If  this  conclusion 
be  sustained  by  future  research,  we  shall  no  longer  be  able  to  accept 
Van  Beneden's  and  Boveri's  conception  of  the  centrosome  as  a  per- 
sistent organ  in  the  same  sense  as  the  nucleus  ;  but  on  the  other  hand 
we  shall  have  gained  important  ground  for  further  inquiry  into  the 
nature  and  source  of  that  power  of  division  which  is  so  characteristic 
of  living  things  and  upon  which  the  law  of  genetic  continuity  rests. 

Morphology  of  the  Centrosome.  —  In  its  simplest  form  (Fig.  152,  A) 
the  centrosome  appears  under  the  highest  powers  as  nothing  more  than 
a  single  granule  of  extraordinary  minuteness  which  stains  intensely 
with  iron-haematoxylin,  and  can  scarcely  be  distinguished  from  the 
cyto-microsomes  except  for  the  fact  that  it  lies  at  the  focus  of  the 
astral  rays.  In  this  form  it  always  appears  at  the  centre  of  the  very 
young  sperm-asters  during  fertilization  (Figs.  97,  99),  in  the  earlv 
phases  of  ordinary  mitosis  (Figs.  27,  32),  and  in  some  cases  also  in  the 
resting  cell,  for  example,  in  leucocytes  and  connective  tissue  corpuscles 
(Figs.  8,  49),  where,  however,  it  is  often  triple  or  quadruple.  In  the 
course  of  division  the  centrosome  often  increases  in  size  and  assumes 
a  more  complex  form,  becoming  also  surrounded  by  various  structures 
involved  in  the  aster-formation.  The  relation  of  these  structures  to 
the  centrosome  itself  has  not  yet  been  fully  cleared  up  and  there 
is  still  much  divergence  of  opinion  regarding  the  cycle  of  changes 
through  which  the  centrosome  passes.  It  is,  therefore,  not  yet  possi- 
ble to  give  a  very  consistent  account  of  the  centrosome,  still  less  to 
frame  a  satisfactory  morphological  definition  of  it. 

It  is  convenient  to  take  up  as  a  starting-point  Boveri's  ('88)  account 
of  the  centrosomes  in  the  Qgg  of  Asearis,  supplemented  by  Brauer's 
('93)  description  of  those  in  the  spermatocytes  of  the  same  animal. 
During  the  early  prophases  of  the  first  cleavage  Boveri  found  the 
centrosome  as  a  minute  granule  which  steadily  enlarges  as  the  spin- 
dle forms,  until  shortly  before  the  metaphase  it  becomes  a  rather  large, 
well-defined  sphere  in  the  centre  of  which  a  minute  central gratni I c  or 
ceiitriole  appears  (Fig.  152,  B,  C).  From  this  time  onward  the  cen- 
trosome decreases  in  size  until  in  the  daughter-cells  it  is  again  reduced 
to  a  small  granule  which  divides  into  two  and  goes  through  a  similar 
cycle  during  the  second  cleavage  and  so  on.  The  centrosome  is 
at  all  stages  surrounded  by  a  clear  zone  ("Heller  Hot")  in  which 


310 


SOME  PROBLEMS   OF  CELL-ORGANIZATION 


the  astral  rays  are  thinner  and  stain  less  deeply  than  farther 
out.  Brauer's  account  is  substantially  the  same,  though  no  definite 
"Heller  Hof  "  was  found,  and  the  astral  rays  were  traced  directly  in 
to  the  boundary  of  the  centrosome.  He  added,  however,  two  impor- 
tant observations,  viz.  (i)  that  the  central  granule  is  visible  at  every 
period  ;  and  (2)  division  of  tJie  centrosome  is  preceded  by  division  of 
tJie  centi'al granule  {Y\g.  148)  —  an  observation  recently  extended  by 
Boveri  to  the  division  of  the  egg-centrosome.^  Van  Beneden  and 
Neyt  {^Zj\  on  the  other  hand,  gave  a  quite  different  account  of  the 


t  ig.  152.  —  Diagrams  illustrating  various  accounts  of  centrosome  and  aster, 

A.  Centrosome,  a  simple  granule  at  the  centre  of  the  aster;  ex.  sperm-aster  in  various  animals. 
B.  "Centrosome,"  a  sphere  enclosing  a  central  granule  or  centriole;  ex.  Brauer's  account  of 
spermatocytes  oi  Ascaris.  C  Like  the  last,  but  "  centrosome  "  surrounded  by  a  "Heller  Hof"; 
ex.  Boveri's  account  of  the  centrosome  of  the  Ascaris  egg.  D.  Central  granule  surrounded  by  a 
radial  sphere  ("centrosome")  bounded  by  a  microsome-circle,  and  lying  in  a  "Heller  Hof"; 
ex.  polar  spindles  of  Thysatiozodn,  Van  der  Stricht.  E.  Central  granule  ("  centrosome  ")  sur- 
rounded by  medullary  and  cortical  radial  zones,  each  bounded  by  a  microsome-circle ;  ex.  polar 
spindle  of  Unio,  Lillie,  F.  Van  Beneden's  representation  of  aster  of  the  Ascaris  egg;  like  the  last, 
but  the  "  corpuscule  central "  consisting  of  a  group  of  granules.  G.  "  Centrosome,"  a  group  of 
granules  surrounded  by  a  "Heller  Hof";  ex.  the  echinoderm-egg.  //.  "Centrosome"  (central 
granule)  surrounded  by  a  vague  larger  body  lying  in  a  reticulated  centrosphere ;  ex.  Thalasseyna. 
[Griffin.] 

Structures  at  the  centre  of  the  aster.  The  "  corpuscule  central " 
(usually  assumed  by  later  writers  to  be  the  centrosome),  described  as 
a  "mass  of  granules,"  is  surrounded  by  two  well-defined  astral  zones, 
formed  as  modifications  of  the  inner  part  of  the  aster,  and  constitut- 
ing the  "attraction-sphere."  These  are  an  inner  "medullary  zone," 
and  an  outer  "  cortical  zone,"  each  bounded  by  a  very  distinct  layer 
of  microsomes  (Fig.  152,  F). 

1  Reported  by  Fiirst,  '98,  p.  in. 


THE    CENTROSOME  311 

The  discrepancy  between  these  results  on  the  part  of  the  two 
pioneer  investigators  of  the  centrosome  has  led  to  great  confusion 
in  the  terminology  of  the  subject,  which  has  not  yet  been  fully 
cleared  away.  Many  of  the  observers  who  followed  Boveri  (Flem- 
ming,  Hermann,  Van  der  Stricht,  Heidenhain,  etc.)  found  the  centro- 
some, in  various  cells,  as  a  much  smaller  body  than  he  had  described, 
often  as  a  single  or  double  minute  granule,  staining  intensely  with 
iron-haematoxylin.  Heidenhain  ('93,  '94)  and  Druner  ('94,  '95)  found 
further  that  the  asters  in  leucocytes  and  other  forms  often  show 
several  concentric  circles  of  microsomes,  and  that  the  sphere  bounded 
by  the  innermost  circle  often  stains  more  deeply  than  the  outer  por- 
tions and  may  appear  nearly  or  quite  homogeneous  (Fig.  156).  To 
this  sphere,  with  its  contained  central  granule  or  granules  Heidenhain 
applies  the  term  microcentrum  ('94,  p.  463),  while  Kostanecki  and 
Siedlecki  suggest  the  term  microsphere  ('96,  p.  217).  Still  later 
Kostanecki  and  Siedlecki  ('97)  found  that  even  in  Ascaris,  as  in  other 
forms,  sufficient  extraction  of  the  colour  (iron-haematoxylin)  reduces  the 
centrosome  to  a  minute  granule  to  which  the  astral  rays  converge, 
and  which  is  presumably  identical  with  Boveri's  ''central  granule." 
Heidenhain  ('93,  '94)  found  that  in  leucocytes  the  central  granule  is 
often  double,  triple,  or  even  quadruple,  while  in  giant-cells  of  certain 
kinds  there  are  numerous  deeply  staining  granules  (Fig.  14).  He 
therefore  proposed  to  restrict  the  term  centrosome  to  the  individ- 
ual granules,  whatever  be  their  number,  applying  the  term  microccn- 
triim  to  the  entire  group  ('94,  p.  463). 

With  these  facts  in  mind  we  can  gain  a  clear  view  of  the  manner 
in  which  both  the  confusion  of  terminology  and  the  contradiction  of 
results  has  arisen.  Brauer  ('93)  found  in  Ascaris  (see  above)  that 
division  of  the  central  grmuile  precedes  division  of  the  ^^  centrosome ,'' 
and  therefore  suggested  that  only  the  former  is  equivalent  to  Wan 
Beneden's  '' corpuscule  central,"  while  the  body  called  "centrosome" 
by  Boveri  is  really  the  medullary  astral  zone,  the  "  Heller  Hof  "  being 
the  cortical  zone.  This  is  substantially  the  same  conclusion  reached 
by  Heidenhain,  Rawitz,  Lenhossek,  Kostanecki  and  Siedlecki,  Krlan- 
ger,  Van  der  Stricht,  Lillie,  and  several  others.  The  confusion  of 
the  subject  is  owing,  on  the  one  hand,  to  the  fact  that  those  who 
have  accepted  this  conclusion  continue  to  use  the  word  centrosome 
in  two  quite  different  senses,  on  the  other  hand  to  the  fact  that  the 
conclusion  is  itself  repudiated  by  Boveri  ('95),  MacFarland  (97),  and 
Fiirst  ('98). 

As  regards  the  terminology  we  find  that  most  recent  writers  agree 
with  Heidenhain,  Kostanecki  and  Siedlecki,  in  restricting  the  word 
centrosome  to  the  minute,  deeply  staining  granules,  whether  one  or 
more,  at  the  centre  of  the  aster.     On  the  other  hand,  Brauer,  Fran- 


312 


SOME  PROBLEMS   OF  CELL-ORGANIZATION 


cotte,  Van  der  Stricht,  Meves,  and  others  apply  the  term  to  the  central 
granule  or  granules  plus  the  surrounding  sphere  ("  centrosome "  of 
Boveri),  which  they  regard  as  equivalent  to  the  medullary  zone  of 
Van  Beneden,  the  "  corpuscule  central"  of  the  last-named  author 
being  identified  with  the  central  granule  or  "centriole"  of  Boveri, 
though  the  latter  structure  is  considerably  smaller  than  the  former 
as  described  by  Van  Beneden. 

The  matter  of  fact  turns  largely  on  the  question  whether  the  astral 
rays  traverse  the  larger  sphere  to  the  central  granule.  That  such  is 
the  case  in  Ascaris  is  positively  asserted  by  Kostanecki  and  Siedlecki, 
('97)  and  as  positively  denied  by  Fiirst  ('98)  with  whose  observations 


Fig.  153.  —  Structure  of  the  centiosome  in  the  polar  asters  of  a  gasteropod,  Dinulula.  [MaC- 
Farland.] 

A.  Mitotic  figure,  formation  of  first  polar  body.  B.  Inner  aster  at  final  anaphase;  central 
granule  double  within  the  "  centrosome."  C.  Elongation  of  old  "  centrosome  "  to  form  second 
polar  spindle. 


those  of  MacFarland  ('97)  on  gasteropod-eggs  agree.  On  the  other 
hand,  in  the  turbellarians  the  observations  of  Francotte  ('97,  '98)  and 
Van  der  Stricht  ('98,  i)  seem  to  leave  no  doubt  that  the  larger  sphere 
("centrosome"),  here  very  sharply  defined  and  staining  deeply  in 
iron-haematoxylin,  is  traversed  by  well-defined  astral  rays  converging 
to  the  central  corpuscle,  and  both  these  observers  agree  further  that 
hotJi  the  corpuscle  and  tJie  sphere  divide  to  persist  as  the  ^^ eentrosonies  " 
of  tJie  daughter-cells  —  a  result  in  conformity  with  Van  Beneden's  con- 
clusion in  the  case  of  Ascaris. 

Lillie's  valuable  observations  on  the  polar  asters  of  Unio  ('98)  afford, 
I  believe,  conclusive  evidence  as  to  the  nature  of  the  sphere.     In  the 


THE    CENTROSOME 


earlier  stages  the  aster  has  cxarM,    .u  ^'^ 


w 


^ 


^ 


6^ 

Fig-  154-  -  Centrosome  and  aster  in 
entosphere  iound^TK'"'  ^^^'^^Ph^-'eJ  ^ones. 


the  polar  mitoses  of  C/n/o.     [Lillie.] 
granule   (centrosome)   surrounded  bv  medullarv 
^.  Late  anaphase  of  second  polar  mitosis    raS 
oftt'oid'rS:,::--"^  ---.•  ^ormatt' 


rays  pass  (Fig-    ic-?    /7    p- 

a  dense  and  deep?y"staininf  s!,!!!'  '^^l  '"""'  '^^"''''  '^°"^'^ting  of 
structure  and  is  b^LderbTa  i's"'  '  f  ''■^'  '"^  ^>-'^''"'  '''-"'^•^^^ 
anaphase)  the  central  grat,fe  di  "deTT^to  f'  '"  ^^^  ^''"'^'"-■^  "^^^^ 
^o-r  or  .ore  granuies,^of  whicJl^^  Co^^  or;rl;^r:cti"l; 


314 


SOME  PROBLEMS    OF  CELL-ORGANIZATION 


persist.  The  inner  sphere  is  now  bounded  by  a  definite  membrane, 
and  its  radiate  structure  becomes  obscure,  the  astral  rays  extending 
only  to  the  boundary  of  the  sphere,  though  a  few  rays  persist  within 
it  (Fig.  154,  B).  It  is  clear  from  this  that  the  inner  sphere  and 
central  granule  pass  through  phases  that  bridge  the  gap  between 
Van  Beneden's  and  Boveri's  descriptions.  LilHe's  observations  fully 
sustain  the  conclusion  that  the  cmtral gmniUe  {^' ceiitriole''  of  Boveri) 
corresponds  to  the  ''  corpiiscide  cejitraV'  of  Van  Beneden,  and  the  inner 
spJiere  {medullary  zone)  to  BoverVs  ''  cejitrosomey  A  comparison  of 
the  polar  aster  of  rm'o  with  that  of  TJiysanozoon,  as  described  by 
Van  der  Stricht  ('98),  leaves  hardly  room  for  doubt  that  the  cortical 
zone  represents  Boveri's  "  Heller  Hof  "  ;  for  in  both  forms  the  rays 
of  the  cortical  zone  are  much  thinner  and  Hghter  than  the  more 
peripheral  portions,  thus  giving  a  clear  zone,  which  in  Uiiio  is  bounded 
by  only  a  fairly  definite  microsome-circle  and  in  TJiysanozoon  by  none. 
Lastly,  we  must  recognize  the  justice  of  the  view  urged  by  Kos- 
tanecki.  Griffin,  Mead,  Lillie,  Coe,  and  others,  that  the  term  centro- 
some  should  be  applied  to  the  central  granule  and  not  to  the  sphere 
surrounding  it  (medullary  zone),  despite  the  fact  that  historically  the 
word  was  first  applied  by  Boveri  to  the  latter  structure.  For  in  both 
Diauhila  (MacFarland)  and  Unio  (Lillie)  the  second  polar  spindle 
arises  from  the  substance  of  the  inner  sphere,  while  the  central 
granule,  becoming  double,  gives  rise  to  the  centrosomes  at  its  poles. 
By  following  Boveri's  terminology,  therefore,  MacFarland  is  driven  to 
the  strange  conclusion  that  the  second  polar  spindle  is  nothing  other 
than  an  enormously  enlarged  '*  centrosome  "  —  a  result  little  short  of 
a  reductio  ad  absnrdnm  when  we  consider  that  in  Ascaris  the  polar 
spindle  arises  by  a  direct  transformation  of  the  germinal  vesicle 
(p.  277).  The  obvious  interpretation  is  that  the  central  granule  is 
the  only  structure  that  should  be  called  a  centrosome,  the  surround- 
ing sphere  being  a  part  of  the  aster,  or  rather  of  the  attraction-sphere. 
Thus  regarded,  the  origin  of  the  spindle  in  Dianlula  presents  nothing 
anomalous  and  a  similar  interpretation  may  be  placed  on  the  polar 
spindles  of  Ascaris  as  described  by  Fiirst  ('98).^ 


1  In  echinoderms  the  concurrent' results  of  Reinke  ('95),  Boveri  ('95),  myself  ('96-'97), 
show  that  the  "  centrosome  "  is  a  well-defined  sphere  containing  a  large  group  (ten  to  twenty) 
of  irregularly  scattered,  deeply  staining  granules.  I  have  shown  in  this  case  that  in  the  early 
prophases  there  is  but  one  such  granule,  which  then  becomes  double  and  finally  multiple, 
forming  a  pluricorpuscular  centrum  (Fig.  52)  not  unlike  that  described  by  Heidenhain 
in  giant-cells.  Kostanecki,  who  asserts  that  the  centrosome  of  echinoderms  is  a  single 
granule  ('96,  i,  '96,  2,  p.  248),  has  not  sufficiently  studied  the  later  phases  of  mitosis. 
Cf.  also  Erlanger  ('98).  The  centrosomes  described  in  nerve  cells  by  Lenhossek  ('95)  are 
apparently  of  somewhat  similar  type.  Until  the  facts  are  more  fully  known  the  exact  nature 
of  these  "centrosomes"  remains  an  open  question.  Lillie's  observations  on  Unio  show 
that  here, too  (first polar  spindle),  the  centrosome  divides  to  form  a  considerable  number  of 


THE    CEiVTROSOME  ^ic 

The  genesis  of  the  concentric  spheres  surrounding  the  centrosome 
will  be  considered  in  the  following  section.  We  may  here  only 
emphasize  the  remarkable  fact  that  the  centres  of  the  dividino- 
system  are  bodies  which  are  in  many  cases  so  small  as  to  lie  almost 
at  the  limits  of  microscopical  vision,  and  which  in  the  absence  of  the 
surrounding  structures  could  not  be  distinguished  from  other  proto- 
plasmic granules.  Full  weight  should  be  given  to  this  fact  in  every 
estimate  of  the  centrosome  theory,  and  it  is  no  less  interestin"-  in  its 
bearing  upon  the  corpuscular  theory  of  protoplasm. 

Watase  ('93,  '94)  made  the  very  interesting  suggestion  that  t/ie  cen- 
trosome is  itself  notJiing  other  tJian  a  microsome  of  the  same  morj^ho- 
logical  nature  as  those  of  the  astral  rays  and  the  general  meshwork, 
differing  from  them  only  in  size  and  in  its  peculiar  powers.^  Despite 
the  vagueness  of  the  word  "microsome,"  which  has  no  well-defined 
meaning,  Watase's  suggestion  is  full  of  interest,  indicating  as  it  does 
that  the  centrosome  is  morphologically  comparable  to  other  elemen- 
tary bodies  existing  in  the  cytoplasmic  sti-ucture,  and  which,  minute 
though  they  are,  may  have  specific  chemical  and  physiological  prop- 
erties. 

An  interesting  hypothesis  regarding  the  historical  origin  of  centrosome  is  that  of 
Rutschli  ("91)  and  R.  Hertwig  ("92),  who  suggest  that  it  may  be  a  derivative  of  a 
body  comparable  with  the  micro-nucleus  of  Infusoria,  which  has  lost  its  chromatin 
but  retained  the  power  of  division  ;  and  the  last-named  author  has  suggested  further 
that  the  so-called  " archoplasmic  loops''  discovered  by  Platner  in  puhnonates  mav 
be  remnants  of  the  chromatic  elements.  A  similar  view  has  been  advocated  bv 
Heidenhain  (''93,  '94)  and  Lauterborn  ('96).  Heidenhain  regards  central  spindle 
and  centrosomes  as  forming  essentially  a  unit  ("-microc^entrum  ")  homologous  with 
the  micro-nucleus  of  the  Infusoria,  the  centrodesmus  (p.  79)  representing  a  part  of 
the  original  achromatic  elements.  The  metazoan  nucleus  is  compared  to  the  proto- 
zoan macro-nucleus.  The  improbability  of  a  direct  derivation  of  the  Metazoa  from 
Infusoria,  urged  by  Boveri  ('95)  and  Hertwig  ("96),  has  led  Lauterborn  ("96)  to  the 
view  that  the  metazoan  centrosome  and  nucleus  are  respectively  derivatives  of  two 
equivalent  nuclei,  such  as  Schaudinn  ("95)  describes  in  Ainivba  hinucUdta,  the 
='Nebenkorper"  of  Paraifia'ba  (cf.  p.  94),  being  regarded  as  an  intermediate  step, 
and  the  micro-nucleus  of  Infusoria  a  side-branch.  R.  Hertwig  ('96),  on  the  other 
hand,  regards  the  metazoon  centrosome  as  a  derivative  of  an  intra-nuciear  botlv  such 
as  the  '' nucleolo-centrosome  "  of  Euglena  (p.  91),  which  has  itself  arisen  through 
a  condensation  of  the  general  achromatic  substance.  With  this  view  Calkins  (98), 
on  the  whole,  agrees;  but  he  regards  it  as  probable  that  the  ••  nucleoIo-ccntrDsome  " 

granules  of  which  one  or  two  remain  as  the  persistent  centrosome.  while  others  are  eonverted 
into  microsomes  or  other  cytoplasmic  structures.  It  is  probable  that  somethinj;  similar 
occurs  in  the  echinoderms. 

1  The  microsome  is  conceived,  if  I  understand  Watase  rightly,  not  as  a  permanenl  mor- 
phological body,  but  as  a  temporary  varicosity  of  the  thread,  which  may  lose  its  i.lentity  in 
the  thread  and  reappear  when  the  thread  contracts.  The  centrosome  is  in  like  manner  not 
a  permanent  organ  like  the  nucleus,  but  a  temporary  body  formed  at  the  focus  of  the  astral 
rays.  Once  formed,  however,  it  may  long  persist  even  after  disappearance  o\  the  aster,  and 
serve  as  a  centre  of  formation  for  a  new  aster. 


3i6  SOME   PROBLEMS   OF  CELL-ORGANIZATION 

of  Eitgleiia  and  Ainaba  and  the  sphere  of  N'octiluca  and  Paramooba  are  to  be  com- 
pared with  the  attraction-sphere,  while  the  centrosome  may  have  had  a  different 
origin. 

It  appears  to  me  that  none  of  these  views  rests  upon  a  very  substantial  basis,  and 
they  must  be  taken  rather  as  suggestions  for  further  work  than  as  w^ell-grounded 
conclusions. 


F.     The  Archoplasmic  Structures 
I.    Hypothesis  of  Fibrillar  Persistence 

The  asters  and  attraction-spheres  have  a  special  interest  for  the 
study  of  cell-organs ;  for  they  are  structures  that  may  divide  and 
persist  from  cell  to  cell  or  may  lose  their  identity  and  re-form  in  suc- 
cessive cell-generations,  and  we  may  here  trace  with  the  greatest 
clearness  the  origin  of  a  cell-organ  by  differentiation  out  of  the  struc- 
tural basis.  Two  sharply  opposing  views  of  these  structures  have 
been  held,  represented  among  the  earlier  observers  on  the  one  hand 
by  Boveri,  on  the  other  by  Blitschli,  Klein,  Van  Beneden,  and  Carnoy. 
The  latter  observers  held  that  the  astral  rays  and  spindle-fibres,  and 
hence  the  attraction-sphere,  arise  through  a  morphological  rearrange- 
ment of  the  preexisting  protoplasmic  meshwork,  under  the  influence  of 
the  centrosome.  This  view,  which  may  be  traced  back  to  the  early 
work  of  Fol  ('73)  and  Auerbach  ('74),  was  first  clearly  formulated 
by  Biitschli  ('76),  who  regarded  the  aster  as  the  optical  expression  of 
a  peculiar  physico-chemical  alteration  of  the  protoplasm  primarily 
caused  by  diffusion-currents  converging  to  the  central  area  of  the 
aster.^  An  essentially  similar  view  is  maintained  in  Biitschli's  recent 
great  work  on  protoplasm,^  the  astral  "  rays "  being  regarded  as 
nothing  more  than  the  meshes  of  an  alveolar  structure  arranged 
radially  about  the  centrosomes  (Fig.  10,  B).  The  fibrous  appearance 
of  the  astral  rays  is  an  optical  illusion,  for  they  are  not  fibres,  but  i^at 
lamellae  forming  the  walls  of  elongated  closed  chambers.  This  view 
has  recently  been  urged,  especially  by  Erlanger  ('97,  4,  etc.),  who 
sees  in  all  forms  of  asters  and  spindles  nothing  more  than  a  modified 
alveolar  structure. 

The  same  general  conception  of  the  aster  is  adopted  by  most  of 
those  who  accept  the  fibrillar  or  reticular  theory  of  protoplasm,  the 
astral  rays  and  spindle-fibres  being  regarded  as  actual  fibres  forming 
part  of  the  general  network.  One  of  the  first  to  frame  such  a  con- 
ception was  Klein  i^']'^\  who  regarded  the  aster  as  due  to  "a  radial 
arrangement  of  what  corresponds  to  the  cell-substance,"  the  latter 

1  For  a  veiy  careful  review  of  the  early  views  on  this  subject,  see  Mark,  Liinax,  i88i. 
2 '92,  2,  pp.  158-169. 


THE  ARCHOPLASMIC  STRUCTURES 


317 


being  described  as  having  a  fibrillar  character.^  The  same  view  is 
advocated  by  Van  Beneden  in  1883.  With  Klein,  Heitznian,  and 
Frommann  he  accepted  the  view  that  the  intra-nuclear  and  extra- 
nuclear  networks  were  organically  connected,  and  maintained  that 
the  spindle-fibres  arose  from  both.^  *'The  star-like  rays  of  the  asters 
are  nothing  but  local  differentiations  of  the  protoplasmic  network.-^ 
.  ••  .  In  my  opinion  the  appearance  of  the  attraction-spheres,  the 
polar  corpuscle  (centrosome),  and  the  rays  extending  from  it,  includ- 
ing the  achromatic  fibrils  of  the  spindle,  are  the  result  of  the  appear- 
ance in  the  egg-protoplasm  of  two  centres  of  attraction  comparable 
to  two  magnetic  poles.  This  appearance  leads  to  a  regular  arrange- 
ment of  the  reticulated  protoplasmic  fibrils  and  of  the  achromatic 
nuclear  substance  with  relation  to  the  centres,  in  the  same  way  that 
a  magnet  produces  the  stellate  arrangement  of  iron  filings."* 

This  view  is  further  developed  in  Van  Beneden's  second  paper, 
pubUshed  jointly  with  Neyt  {'^7).  "  The  spindle  is  nothing  but  a 
differentiated  portion  of  the  asters."  ^  The  aster  is  a  **  radial  structure 
of  the  cell-protoplasm,  whence  results  the  image  designated  by  the 
name  of  aster." '^  The  operations  of  cell-division  are  carried  out 
through  the  "  contractility  of  the  fibrillae  of  the  cell-protoplasm  and 
their  arrangement  in  a  kind  of  radial  muscular  system  composed  of 
antagonizing  groups."  *" 

An  essentially  similar  view  of  the  achromatic  figure  has  been 
advocated  by  many  later  workers.  Numerous  observers,  such  as 
Rabl,  Flemming,  Carnoy,  Watase,  Wilson,  Reinke,  etc.,  have  ob- 
served that  the  astral  fibres  branch  out  peripherally  into  the  general 
meshwork  and  become  perfectly  continuous  with  its  meshes,  and 
tracing  the  development  of  the  aster,  step  by  step,  have  concluded 
that  the  rays  arise  by  a  direct  progressive  modification  of  the  pre- 
existing structure.  The  most  extreme  development  of  this  view  is 
contained  in  the  works  of  Heidenhain  ('93,  '94),  Buhler  (95),  Kosta- 
necki  and  Siedlecki  ('97),  which  are,  however,  only  a  development  of 
the  ideas  suggested  by  Rabl  in  a  brief  paper  published  several  years 
before.  Rabl  ('89,  2)  suggested  that  neither  spindle-fibres  nor  astral 
rays  really  lose  their  identity  in  the  resting  cell,  being  only  modified 
in  form  to  constitute  the  mitome  or  filar  substance  (meshwork),  but 
still  beins:  centred  in  the  centrosome.  Fission  of  the  centrosome  is 
followed  by  that  of  the  latent  spindle-fibres  (forming  the  linin- 
network);  hence  each  chromosome  is  connected  by  pairs  of  daughter- 

1  It  is  interesting  to  note  that  in  the  same  place  Klein  anticipated  the  theory  of  fibrillar 
contractility,  both  the  nuclear  and  the  cytoplasmic  reticulmii  being  regarded  as  contractile 
(/.r.,  p.  417). 

2  'S3,  p.  592.  *  '53,  p.  550.  6  /.^..  p.  275. 

3  '83,  p.  576.  5  '87,  p.  263.  '  /.^.,  p.  2S0. 


3l8  SOME  PROBLEMS   OF  CELL-ORGANIZATION 

fibres  with  the  respective  centrosomes.  Heidenhain,  adopting  the 
first  of  these  assumptions,  builds  upon  it  an  elaborate  theory  of  cell- 
polarity  and  cell-division  already  considered  in  part  at  pages  103-105. 
Sometimes  the  astral  rays  ('*  organic  radii  ")  retain  their  radial  arrange- 
ment throughout  the  life  of  the  cell  (leucocytes,  Fig.  49) ;  more  com- 
monly they  are  disguised  and  lost  to  view  in  the  cytoplasmic  meshwork. 
All,  however,  are  equal  in  length  and  in  tension  —  assumptions  based 
on  the  one  hand  on  the  occurrence  of  concentric  circles  of  microsomes 
in  the  aster,  on  the  other  hand  on  the  analogy  of  the  artificial  model 
described  at  page  104.  Biihler  ('95)  and  Kostanecki  and  Siedlecki 
('97)  likewise  unreservedly  accept  the  view  that  besides  the  centro- 
some  the  entire  system  of  ''  organic  radii,"  including  astral  rays, 
mantle-fibres,  and  central  spindle-fibres,  persists  in  the  resting  cell  in 
modified  form,  and  is  centred  in  the  centrosome.  Kostanecki  finally 
('97)  takes  the  last  step,  logically  necessitated  by  the  foregoing  con- 
clusion, and  apparently  supported  also  by  the  crossing  of  the  astral 
rays  opposite  the  equator  of  the  spindle  and  the  relations  of  their 
peripheral  ends,  concluding  that  the  monocentric  astral  system  is  con- 
verted into  the  dicentric  system  (amphiaster)  by  longitudinal  fiSsion 
of  the  rays}  Thus  the  entire  mitome  of  the  mother-cell  divides  into 
equal  halves  for  daughter-cells  ;  and  since  the  radii  consist  of  micro- 
somes, each  of  these  must  likewise  divide  into  two.^ 

Could  this  tempting  hypothesis  be  established,  Roux's  interpretation 
of  nuclear  division  (p.  224)  could  be  extended  also  to  the  cytoplasm  ; 
and  the  aster-  and  amphiaster-formation,  with  the  spireme-forma- 
tion,  might  be  conceived  as  a  device  for  the  meristic  division  of  the 
entire  cell-substance  —  a  result  which  would  place  upon  a  substantial 
basis  the  general  corpuscular  theory  of  protoplasm.  Unfortunately, 
however,  the  hypothesis  rests  upon  a  very  insecure  foundation  :  first, 
because  it  is  based  solely  upon  the  fibrillar  theory  of  protoplasm  ; 
second,  because  of  the  very  incomplete  direct  evidence  of  such  a 
splitting  of  the  rays  ;  third,  because  there  is  very  strong  evidence  that 
in  many  cases  the  old  astral  rays  wholly  disappear,  to  be  replaced  by 
new  ones.^  We  may  best  consider  this  adverse  evidence  in  connec- 
tion with  a  general  account  of  the  opposing  archoplasm-hypothesis. 

2.    The  Archoplasm  Hypothesis 

Entirely  opposed  to  the  foregoing  conception  are  the  views  of 
Boveri   and   his  followers,   the  starting  point  of  which  is  given  by 

1 '97,  p.  680. 

2  This  view  had  been  definitely  stated  also  by  O.  Schultze  in  1890. 

^  There  is,  however,  no  doubt  that  the  aster  as  a  whole  does,  in  some  cases,  divide  into 
two  —  for  instance,  in  the  echinoderm-egg.  Fig.  95. 


THE  ARCHOPLASMIC  STRUCTURES  3IQ 

Boveri's  celebrated  archoplasm-hypothesis.  Boveri  has  from  the  first 
maintained  that  the  amphiastral  fibres  are  quite  distinct  from  the  Gen- 
eral cell-meshwork.  In  his  earlier  papers  he  maintained  ('88,  2)  that 
the  attraction-sphere  of  the  resting  cell  is  composed  of  a  distinct  sub- 
stance, ''  arcJioplasm,''  consisting  of  granules  or  microsomes  aggre- 
gated about  the  centrosome  as  the  result  of  an  attractive  force  exerted 
by  the  latter.  From  the  material  of  the  attraction-sphere  arises  the 
entire  achromatic  figure,  including  both  the  spindle-fibres  and  the 
astral  rays,  and  these  have  nothing  to  do  with  the  general  reticulum 
of  the  cell.  They  grow  out  from  the  attraction-sphere  into  the  reticu- 
lum as  the  roots  of  a  plant  grow  into  the  soil,  and  at  the  close  of 
mitosis  are  again  withdrawn  into  the  central  mass,  breaking  up  into 
granules  meanwhile,  so  that  each  daughter-cell  receives  one-half  of 
the  entire  archoplasmic  material  of  the  parent-cell.  Boveri  was 
further  incUned  to  believe  that  the  individual  granules  or  archoplas- 
mic microsomes  were  "  independent  structures,  not  the  nodal  points  of  a 
general  network,"  and  that  the  archoplasmic  rays  arose  by  the  arrange- 
ment of  these  granules  in  rows  without  loss  of  their  identity.^  In  a 
later  paper  on  the  sea-urchin  this  view  underwent  a  considerable 
modification  through  the  admission  that  the  archoplasm  may  not  ])re- 
exist  as  formed  material,  but  that  the  rays  and  fibres  may  be  a  new 
formation,  crystallizing,  as  it  were,  out  of  the  protoplasm  about  the 
centrosome  as  a  centre,  but  having  no  organic  relation  with  the  gen- 
eral reticulum ;  though  Boveri  still  held  open  the  possibilitv  that  the 
archoplasm  might  preexist  in  the  form  of  a  specific  homogeneous  sub- 
stance distributed  through  the  cell,  though  not  ordinarily  demonstra- 
ble by  reagents.^  In  this  form  the  archoplasm-theory  approaches 
very  nearly  that  of  Strasburger,  described  below. 

There  are  three  orders  of  facts  that  tell  in  favour  of  Boveri's  modi- 
fied theory :  first,  the  existence  of  persistent  archoplasm-masses  or 
attraction-spheres  from  which  the  amphiasters  arise  ;  second,  the 
origin  of  amphiasters  in  alveolar  protoplasm ;  and,  third,  the  increas- 
ing number  of  accounts  asserting  the  replacement  of  the  old  asters 
by  others  of  quite  new  formation.  In  at  least  one  case,  namely,  that 
of  Noctiluca,  the  entire  achromatic  figure  is  formed  from  a  permanent 
attraction-sphere  lying  outside  the  nucleus  and  perfectly  distinct  fn^n 
the  general  cell-meshwork.^  Other  cases  of  this  kind  are  very  rare, 
and  in  most  cases  the  attraction-sphere  sooner  or  later  disintegrates,^ 
but  in  the  formation  of  the  spermatozoa  we  have  many  examples  of 
archoplasmic  masses  (Nebenkern,  attraction-sphere,  idiozome),  which 
apparently  consist  of  a  specific  substance  having  a  special  relation  to 
the  achromatic  figure. 

1  '88,  2,  p.  80.  3  Ishikawa,  '94,  '98,   Calkins.  '98,  2. 

2  '95,  2,  p.  40.  ^  Cf.  p.  323. 


120 


SOME   PROBLEMS   OF  CEIJ.-OK<JAX/ZAT/O.V 


The  aniphiastral  formation  in  alveolar  protoplasm  skives  very  clear 
evidence  aj^ainst  the  theory  of  fibrillar  persistence.  Here  the  fibrillar 
rays  can  be  seen  growing  out  lhroii«;h  the  walls  of  the  alveoli  ^  cpiite 
distinct  from,  thou«;h  embeddeil  in,  them.  At  the  close  of  mitosis 
every  trace  of  the  fibrillar  formation  may  disappear,  r.i^.  in  echino- 
derm-ej;i;s  atter  formation  of  the  polar  bodies,  the  protoplasm  retain- 
ing only  a  typical  alveolar  structure. 


Fig.  155.  —  .Stages  in  tlic  first  clc.iv.ii,'!-  of  the  cpt^  in  Cer,-hratnlii<i  (.i-C.  CnF.)  and  T^i.-i/.ir^fnrd 
(D-/\  CiRIFFIN). 

A.  First  appearance  of  the  cleavage-centrosome  at  the  poles  of  the  fused  germ-nuclei ;  cleavage- 
astf-rs  forming  within  the  degenerating  sperm-asters.  B.  Final  anajihase  of  first  cleavage,  showing 
persistent  centrosomes  and  new  asters  forming.  C.  Immediately  after  division.  D-F.  Three 
stages  of  the  late  anapluise  in  Thaliisseina,  showing  formation  of  new  asters  within  the  old.  ( Cf. 
Fig.  99.) 

The  stronr^cst  evidence  a^^ainst  fibrillar  persistence  is,  however, 
given  by  recent  studies  on  mitosis,  showing  on  the  one  hand  that  the 
new  astral  centres  do  hot  coincide  with  the  old  ones,  on  the  other 
that  the  old  ravs  degenerate  /;/  situ,  to  be  replaced  by  new  ones. 
Aside  from  many  earlier  observers,  who  believed  the  entire  aster  to 
disappear  at  the  close  of  mitosis,  the  first  to  assert  the  wholly  new 

1  C/.  Reinke  r'95),  Wilson  ('99). 


THE  ARCIIOPLASMIC  STRUCTURES  32  I 

formation  of  the  rays  was  Driiner,  who  maintained  in  the  case  of  the 
mitosis  of  salamander  testis-cells,  that  "not  a  sin^de  fibre  of  the  astral 
system  of  the  mother-cell  is  carried  over  unchan<;ed  into  the  orj;anism 
of  the  daughter-cell"  ('95,  p.  309)-  The  same  conclusion  was  soon 
afterward  supported  by  Braus  ('95)  in  the  case  of  the  cleava<;e- 
mitoses  of  Triton.  The  most  convincing  evidence  of  this  fact  has 
been  given  by  studies  on  the  maturation  and  fertihzation  of  the  egg 
by  Grifhn  ('96,  '99),  MacFarland  ('97),  Lillie  ('99),  and  Coe  ('99),  all  of 
whom  find  that  the  new  astral  centres,  arising  by  division  of  the  cen- 
trosome,  move  away  from  the  old  position,  to  lu/iic/i,  hoivcvcr,  the  old 
rays  still  converge  ivhile  the  new  asters  are  independently  forming 
(Fig.  155).  This  is  shown  with  especial  clearness  in  the  i^g'-^  of  Cere- 
brattilus  (Coe),  where  the  peripheral  portions  of  the  old  asters  persist 
until  the  new  amphiaster  is  completely  formed.  This  observation 
seems  conclusively  to  overturn  Kostanecki's  hypothesis  of  the  persist- 
ence and  division  of  the  rays,  and  together  with  the  work  of  MacFar- 
land gives  a  very  strong  support  to  Boveri's  later  view. 

It  still  remains  an  open  question  whether  the  rays  actuallv  arise 
from  the  substance  of  the  centrosome,  from  a  specific  surrounding 
archoplasm,  or  by  differentiation  out  of  the  general  substance  of  the 
meshwork.  The  first  of  these  possibilities  has  been  urged  in  a  very 
interesting  way  by  Watase  ('94),  who  believes  that  the  centrosome 
"spins  out  the  cytoplasmic  filaments"^  of  the  spindle  and  aster,  and 
that  ordinary  microsomes  may  in  Hke  manner  spin  out  the  fibril  la-  of 
ordinary  cytoplasmic  networks.^  This  view  is  sustained  by  the  mode 
of  origin  of  the  axial  filament  in  the  spermatozoa  and  that  of  the  cilia 
in  plant  spermatozoids.  It  is,  on  the  other  hand,  opposed  by  the 
almost  infinitesimal  bulk  of  the  centrosome  as  compared  with  that  of 
the  aster  that  may  form  about  it,  and  by  the  formation  of  the  spindles 
in  higher  plants  in  the  apparent  absence  of  centrosomes.  On  the 
whole,  the  facts  do  not  seem  at  present  to  warrant  the  acceptance  of 
Watase's  ingenious  hypothesis,  and  the  most  probable  view  is  that 
of  Driiner  and  Boveri,  that  the  rays  are  differentiated  out  of  the  walls 
of  the  meshwork.  In  cases  where  the  protoplasm  is  reticular  or 
fibrillar  the  differentiation  of  the  ravs  mav  be  indistinguishable  from 
a  mere  rearrangement  of  the  thread-work  ;  in  alveolar  protoplasm 
they  may  be  seen  as  new  formations,  while  in  either  ca.se  the 
material  of  the  old  aster  may  be  more  or  less  directly  utilized  in  the 
building  of  the  new.  The  feature  common  to  all  is  the  periodic 
activity  either  of  the  centre  itself  or  of  the  surrounding  protoplasm, 
and  the  coincidence  or  non-coincidence  of  the  new  aster  with  the  old 
is  apparently  a  secondary  matter. 

1  I.e.,  p.  283. 

2  See  the  same  paper  for  a  suggestive  comparison  of  the  astral  librilLv  to  muscle-fihrcs 


322  SOME  PROBLEMS   OF  CELL-ORGANIZATION 

In  its  original  form  the  archoplasm  hypothesis,  as  stated  by  Boveri, 
was  developed  with  reference  only  to  the  material  of  the  spindle- 
fibres  and  astral  rays.  Later  writers  have  greatly  extended  the  con- 
ception on  the  basis  of  Boveri's  earlier  view  that  archoplasm  is  a 
specific  form  of  protoplasm,  possessing  specially  active  properties. 
Strasburger  ('92-98),  whose  views  have  already  been  considered  in 
part,  believes  the  protoplasm  to  consist  of,  or  to  show  a  tendency  to 
differentiate  itself  into,  two  distinct  substances,  namely,  a  specially 
active  fibrillar  kijioplasin  and  a  less  active  alveolar  tropJioplasin. 
The  former  gives  rise  to  the  mitotic  fibrillae,  constitutes  the  periph- 
eral cell  layer,  or  HautschicJit,  from  which  the  membrane  arises, 
forms  the  substance  of  the  centrosomes,  and  gives  origin  to  the  con- 
tractile substance  of  cilia  and  flagella.  The  kinoplasm  is  thus  mainly 
concerned  with  the  motor  phenomena  of  the  cell,  the  trophoplasm 
with  those  of  nutrition ;  and  this  physiological  difference  is  morpho- 
logically expressed  in  the  fact  that  the  former  has  in  general  a 
fibrillar  structure,  the  latter  an  alveolar.  Beyond  this  the  two  forms 
of  protoplasm  show  a  difference  of  staining-reaction,  the  kinoplasmic 
fibrillae  staining  deeply  with  gentian-violet  and  iron-haematoxylin, 
while  the  trophoplasm  is  but  slightly  stained. 

Prenant  ('98,  '99)  still  further  extends  the  hypothesis,  adopting  the 
view  that  the  "  ergastoplasmic  "  (Garnier)  fibrillae  of  gland-cells  ^  are 
equivalent  to  the  kinoplasmic  or  archoplasmic  fibrillae  of  the  mitotic 
figure,  and  to  the  fibrillae  of  nerve-  and  muscle-fibres  as  well.  He  is 
thus  led  to  the  conception  of  a  dominating  or  "  superior  "  cytoplasm 
(including  "archoplasm,"  "kinoplasm,"  "ergastoplasm"),  which  arises 
by  differentiation  out  of  the  general  cytoplasm,  plays  the  leading  role 
in  the  elaboration  of  active  cell-elements  ("  cytosomes "),  such  as 
mitotic,  neural,  and  glandular  fibrillae,  and  finally,  its  role  accom- 
plished, may  disappear.  Under  the  same  category  with  the  foregoing 
structures  are  placed  the  centrosome,  attraction-sphere,  mid-body, 
idiozome,  Nebenkern,  and  yolk-nucleus. 

Such  a  generous  expansion  of  the  archoplasm-hypothesis  brings  it 
perilously  near  to  a  rediictio  ad  absitrdmn  ;  for  the  step  is  not  a  great 
one  to  the  identification  of  the  "  superior  protoplasm  "  with  the  active 
cell-substance  in  general,  w.hich  would  render  the  whole  hypothesis 
superfluous.  Physiologically,  we  can  drav/  no  definite  line  of  demar- 
cation between  the  more  and  the  less  active  protoplasmic  elements, 
and  it  may  further  be  doubted  whether  such  a  boundary  exists  even 
between  the  latter  and  the  metaplasmic  substances.^  It  is  further 
quite  unjustifiable  to  infer  physiological  likeness  from  similarity  in 
staining-reaction^  or  in  fibrillar  structure.  For  these  reasons  the 
hypothesis  of  "  superior  protoplasm  "  seems  one  of  doubtful  utility. 

^  Cf.  the  pancreas,  p.  44.  2  cf.  p.  29.  3  q*;  p^  -^^^ 


THE  ARCHOPLASMIC  STRUCT C RES  323 

In  its  more  restricted  form,  however,  the  urchoplasm  or  kinopiasm 
hypothesis  is  of  high  interest  as  indicating  a  common  element  in  the 
origin  and  function  of  the  mitotic  fibrilkie,  the  centrosome  and  mid- 
body,  and  the  contractile  substances  of  cilia,  flagella,  and  muscle- 
fibres.  The  main  interest  of  the  hypothesis  seems  to  me  to  lie  in  the 
definite  genetic  relations  that  have  been  traced  between  the  archo- 
plasmic  structures  of  successive  cell-generations  (as  is  most  clearly 
shown  in  the  phenomena  of  maturation  and  fertilization).  It  has 
been  pointed  out  at  various  places  in  the  preceding  chapters  ^  how 
many  apparently  contradictory  phenomena  in  cell-division,  fertiliza- 
tion, and  related  processes  can  be  brought  into  relation  with  one 
another  under  the  assumption  of  a  specific  substance,  carried  by  the 
centrosome  or  less  definitely  localized,  which  gives  the  stimulus  to 
division,  which  is  concerned  in  the  formation  of  the  mitotic  figure 
and  of  contractile  elements,  and  which  may  be  transmitted  from  cell 
to  cell  without  loss  of  its  specific  character.  There  seems,  however, 
to  be  clear  evidence  that  such  substance  (or  substances),  if  it  exists, 
is  not  to  be  regarded  as  being  necessarily  a  permanent  constituent  of 
the  cell,  but  only  as  a  phase,  more  or  less  persistent,  in  the  general 
metabolic  transformation  of  the  cell-substance.^ 

3.    The  Attraction-spJiere 

As  originally  used  by  Van  Beneden  ^  the  term  attraction-spJicrc  was 
applied  (in  Ascains^  to  the  central  mass  of  the  aster  surrounding  the 
"  corpuscule  central "  and  consisting  of  medullary  and  cortical  zones, 
as  already  described  (p.  310).  The  cortical  zone  is  bounded  by  a  di.s- 
tinct  circle  of  microsomes  from  which  the  astral  rays  proceed  ;  and  at 
the  close  of  cell-division  the  rays  were  stated  to  fade  away,  leaving 
only  the  attraction-sphere,  which,  like  the  centrosome,  was  regarded 
as  a  permanent  cell-organ.  Later  researches  have  conclusively  shown 
that  the  attraction-sphere  cannot  be  regarded  as  a  permanent  organ, 
since  in  many  cases  it  disintegrates  and  disappears.  This  occurs,  for 
example,  in  the  early  prophases  of  mitosis  in  the  testis-cells  of  the  sala- 
mander,^ where  the  sphere  breaks  up  and  scatters  through  the  cell  as 
the  new  amphiaster  forms  (Fig.  27).  A  very  interesting  ca.se  of  this 
kind  occurs  in  the  cleavage  of  the  ovum  in  Crcpidiila,  as  described  by 
Conklin  ('99).  The  spheres  here  persist  for  a  considerable  period 
after  division  (Fig.  192),  but  have  no  direct  relation  to  those  of  the 
ensuing  division,  finally  disappearing  ///  situ.  The  new  spheres  are 
formed  about  the  centrosomes,  which  Conklin  believes  to  migrate 
out  of  the  old  spheres  (somewhat  as  occurs  in  the  spermatid,  p.  iC7)to 
their  new  position.     The  interesting  point  here  is  that  the  old  sphere 

1  (7:  pp.  Ill,  215.  2Qrp.  171.  8'Sj,  p.  548. 

*  Druner,  '95,  Rawitz,  '96,  Meves,  '96, 


324  SOME  PROBLEMS   OF  CELL-ORGANIZATION 

takes  up  such  a  position  as  to  pass  entirely  into  one  of  the  grand- 
daughter-cells, while  the  new  sphere-substance  is  equally  distributed 
between  them  and  in  its  turn  passes  into  one  of  the  cells  of  the  en- 
suing division.^ 

In  Crepidula,  as  in  Ascaris,  the  attraction-sphere  represents  only 
the  central  part  (centrosphere)  of  the  aster.  In  some  cases,  however, 
e.o.  in  leucocytes,  the  entire  aster  may  persist,  and  the  term  attrac- 
tion-spJiere  has  by  some  authors  been  applied  to  the  whole  structure. 
Later  workers  have  proposed  different  terminologies,  which  are  at 
present  in  a  state  of  complete  confusion.  Fol  ('91)  proposed  to  call 
the  centrosome  the  astrocentre,  and  the  spherical  mass  surrounding  it 
(attraction-sphere  of  Van  Beneden)  the  astrospJiere.  Strasburger 
accepted  the  latter  term  but  proposed  the  new  word  centrospJiere 
for  the  astrosphere  and  the  centrosome  taken  together.^  A  new 
complication  was  introduced  by  Boveri  ('95),  who  applied  the  word 
*'  astrosphere  "  to  the  entire  ^^.y/^-r  exclusive  of  the  centrosome,  in  which 
sense  the  phrase  "  astral  sphere  "  had  been  employed  by  Mark  in  1881. 
The  word  "astrosphere  "  has  therefore  a  double  meaning  and  would 
better  be  abandoned  in  favour  of  Strasburger's  convenient  term  centro- 
spJiere, which  may  be  understood  as  equivalent  to  the  ''  astrosphere  " 
of  Fol. 

Besides  these  terms  we  have  Heidenhain's  microcentruvi  (p.  311), 
equivalent  to  the  centrosome  or  group  of  centrosomes  at  the  centre  of 
the  aster,  with  its  surrounding  sphere ;  ^  Kostajiecki's  and  Siedlecki's 
microspJiere,  applied  to  the  central  region  of  the  aster  surrounding  the 
centrosome  whether  bounded  by  a  distinct  microsome-circle  or  not ;  ^ 
Erlanger's  centroplasm,  equivalent  to  microsphere;^  Ziegler's  ecto- 
spJiere  and  entosphere,  applied  to  the  cortical  and  medullary  zones 
respectively  ;  and  Meves's  idio::^ome,  applied  to  the  "attraction-sphere  " 
of  the  spermatids.^  This  profusion  of  technical  terms  has  arisen 
through  the  desire  to  avoid  ambiguity  in  the  use  of  the  term  "  attrac- 
tion-sphere," which,  like  the  word  "  Nebenkern  "  (p.  163),  has  been 
applied  to  bodies  of  quite  different  origin  and  fate.  If  we  adhere  to 
Van  Beneden's  original  use  of  the  term  it  must  be  confined  to  the  body 
surrounding  the  centrosome,  forming  a  part  of,  or  directly  derived 
from,  an  aster,  and  giving  rise  wholly  or  in  part  to  the  succeeding  aster. 
Meves  ('96),  Rawitz  ('96),  Erlanger  ('97,  2),  and  others  have,  however, 
clearly  shown  that  the  "  attraction-sphere "  surrounding  the  centro- 
some (in  testis-cells)  may  not  only  contain  other  material  derived  from 
the  cytoplasm,  e.g.  the  "  centrodeutoplasm  "  of  Erlanger,  but  may 
take  no  direct  part  in  the  succeeding  aster-formation,  disintegrating 
and  scattering  through  the  cell  as  the  new  aster  forms  (Fig.  2^).     In 

1  Cf.  p.  424.  3  '94^  p.  463.  5  '96^  3^  p.  8. 

2'92,  p.  5.  4'96,  p.  217.  6  '97^  4,  p.  313. 


THE  ARCHOPLASMIC  STRUCTURES  325 

Other  cases  a  sphere  closely  simulating  an  attraction-sphere  may 
arise  in  the  cytoplasm  without  apparent  relation  to  the  centrosomes 
or  to  the  preceding  aster,  e.g.  the  yolk-nucleus  or  the  sphere  from 
which  the  acrosome  arises  in  mammalian  spermatogenesis.^  To  call 
such  structures  ''attraction-spheres"  or  "  archoplasm-masses "  is  to 
beg  an  important  question;  and  in  all  such  doubtful  cases  the  simple 
word  sphere  should  be  used.^  In  case  of  the  aster  itself  we  may,  for 
descriptive  purposes,  employ  Strasburger's  convenient  and  non-com- 
mittal term  centrosphere,  to  designate  in  a  somewhat  vague  and  general 
way  the  central  mass  of  the  aster  surrounding  the  centrosome,  leavinc^ 
its  exact  relation  to  Van  Beneden's  attraction-sphere  to  be  determined 
in  each  individual  case.  Where  the  centrosphere  shows  two  concen- 
tric zones  (medullary  and  cortical),  they  may  be  well  designated  with 
Ziegler  as  entosphere  ("  centrosome"  of  Boveri)  and  ectosp/iere. 

As  regards  the  structure  of  the  centrosphere,  two  well-marked  types 
have  been  described.  In  one  of  these,  described  by  Van  Beneden  in 
Ascaris,  by  Heidenhain  in  leucocytes,  by  Driiner  and  Braus  in  divid- 
ing cells  of  Amphibia,  and  by  Francotte,  Van  der  Stricht,  Lillie,  Kos- 
tanecki,  and  others,  in  various  segmenting  eggs,  the  centrosphere  has 
a  radiate  structure,  being  traversed  by  rays  which  stretch  between  the 
centrosome  and  the  peripheral  microsome-circle  (Fig.  152,  D,  E,  F\ 
when  the  latter  exists.  In  the  other  form,  described  by  Vejdovsky  in 
the  eggs  of  Rhynchelmis,  by  Solger  and  Zimmermann  in  pigment-cells, 
by  myself  in  Nereis,  by  Riickert  in  Cyclops,  by  Mead  in  CJicctoptcnts, 
Griffin  in  Thalassejua,  Coe  in  Cerebratulus,  Gardiner  in  Polycliccrus, 
and  many  others,  the  centrosphere  has  a  non-radial  reticular  or  vesicu- 
lar structure,  in  which  the  centrosomes  lie  (Figs.  152,  //,  155).  Kos- 
tanecki  and  others  have  endeavoured  to  show  that  such  structures  are 
artifacts,  insisting  that  in  perfectly  fixed  material  the  astral  rays  always 
traverse  the  centrosphere  to  the  centrosome.  This  interpretation  is, 
however,  contradicted  by  the  fact  that  the  new  asters  developing  in 
the  centrospheres  during  the  anaphases  and  telophases  of  such  forms 
as  Thalassemia  or  Cerebratulus  (Figs.  99,  155)  show  perfect  fixation  of 
the  rays.  The  reticular  centrosphere  almost  certainly  arises  as  a  nor- 
mal differentiation  of  the  interior  of  the  aster,  which,  as  Griffin  ('q6) 
has  suggested,  probably  marks  the  beginning  of  the  degeneration  of 
the  whole  astral  apparatus,  to  make  way  for  the  newly  developing 
system. 

The  radial  centrosphere  is  in  Ascaris  divided  into  cortical  and  medul- 
lary zones,  as  already  described  (p.  310),  the  aster  being  bounded  by 
a  distinct  circle  of  microsomes.  The  true  interpretation  of  these  zones 
was  given  through  Heidenhain's  beautiful  studies  on  the  asters  in  leu- 
cocytes, and  the  still  more  thorough  later  work  of  Driiner  on  the  sper- 

1  Q^  p.  170.  ^  CJ.  Lenhossek,  '98. 


326 


SOME   PROBLEMS   OF  CELL-ORGAXIZATION 


matocyte-divisions  of  the  salamander.  In  leucocytes  (Fig.  49)  the 
large  persistent  aster  has  at  its  centre  a  well-marked  radial  sphere 
bounded  by  a  circle  of  microsomes,  as  described  by  Van  Beneden,  but 
without  division  into  cortical  and  medullary  zones.     The  astral  rays, 

however,  show  indications  of  other 
circles  of  microsomes  lying  outside 
the  centrosphere.  Druner  found 
that  a  whole  series  of  such  concen- 
tric circles  might  exist  (in  the  cell 
shown  in  Fig.  1 56  no  less  than  nine), 
but  that  the  innermost  two  are  often 
especially  distinct,  so  as  to  mark  off 
a  centrosphere  composed  of  a  medul- 
lary and  a  cortical  zone  precisely  as 
described  by  Van  Beneden.  These 
observations  show  conclusively  that 
the  centrosphere  of  the  radial  type 
is  merely  the  innermost  portion  of 
the  aster,  which  acquires  a  boundary 
through  the  especial  development 
of  a  ring  of  microsomes,  or  other- 
wise,    and    which     often     further 

Fig.  i'56.  —  Spermatogonium  of  salaman-  ,  .  .     . 

der.    [Druner.]  acquires  an  mtense  stammg-capac- 

The  nucleus  lies  below.  Above  is  the  enor-  ity  SO  aS  tO  appear  like  a  CCUtrOSOme 
mous  aster,  the  centrosome  at  its  centre,  its     ^^    ^  ^  ,>^_       j^^   ThysanOZOOIl  (Van  der 

Stricht)  only  a  single  ring  of  micro- 
somes exists,  and  this  lies  at  the 
boundary  between  the  medullary 
and  cortical  zones  (Fig.  152,  D),  the  latter  differing  from  the  outer 
region  only  in  the  greater  delicacy  of  the  rays  and  their  lack  of 
staining-capacity,  thus  producing  a  '*  Heller  Hof."  In  other  cases,  no 
"  microsome-circles "  exist;  but  even  here  a  clear  zone  often  surrounds 
the  centrosome  {e.g.  in  Physa,  t.  Kostanecki  and  Wierzejski),  like  that 
seen  in  the  cortical  zone  of  TJiysanozoon. 

There  are  some  observations  indicating  that  the  entosphere  (medul- 
lary zone)  may  be  directly  derived  from  the  centrosome  (central 
granule).  This  is  the  conclusion  reached  by  Lillie  in  the  case  of  Unio 
referred  to  above,  where,  during  the  prophases  of  the  second  polar 
spindle,  the  central  granule  enlarges  and  breaks  up  into  a  group  of 
granules  from  which  the  new  entosphere  is  formed.  Van  der  Stricht 
('98)  reaches  a  similar  conclusion  in  case  of  the  first  polar  spindle  of 
TJiysanozoon.  We  may  perhaps  give  the  same  interpretation  to  the 
large  pluricorpuscular  centrum  of  echinoderms  (p.  314).  This  obser- 
vation may  be  used  in  support  of  the  probability  that  the  astral  rays 


rays  showing  indications  of  nine  concentric 
circles  of  microsomes.  The  area  within  the 
second  circle  probably  represents  the  "  attrac- 
tion-sphere "  of  Van  Beneden. 


SUMMARY  AND    CONCLUSION-  327 

may  be  actually  derived  from  the  centrosome  (p.  321) ;  but  Lillie  finds 
in  some  cases  that  in  the  same  mitosis  the  entosphere  is  formed  by  a 
different  process,  arising  by  a  differentiation  of  the  cytoplasm  around 
the  central  granule.  The  former  case,  therefore,  may  be  interpreted 
to  mean  simply  that  the  centrosome  may  give  rise  to  other  cytoplasmic 
elements  (as  has  already  been  shown  in  the  formation  of  the  sperma- 
tozoon, p.  172),  the  material  of  which  may  then  contribute  either 
directly  or  indirectly  to  the  building  of  the  aster ;  and  the  facts  do 
not  come  into  collision  with  the  view  that  the  astral  rays  are  in  f^en- 
eral  formed  from  the  cytoplasmic  substance. 

G.     Summary  and  Conxlusion 

A  minute  analysis  of  the  various  parts  of  the  cell  leads  to  the  con- 
clusion that  all  cell-organs,  whether  temporary  or  **  permanent,"  are 
local  differentiations  of  a  common  structural  basis.  Temporary  organs, 
such  as  cilia  or  pseudopodia,  are  formed  out  of  this  basis,  persist  lor  a 
time,  and  finally  merge  their  identity  in  the  common  basis  again.  Per- 
manent organs,  such  as  the  nucleus  or  plastids,  are  constant  areas  in 
the  same  basis,  which  never  are  formed  de  novo,  but  arise  by  the  divi- 
sion of  preexisting  areas  of  the  same  kind.  These  two  extremes  are, 
however,  connected  by  various  intermediate  gradations,  examples  of 
which  are  the  contractile  vacuoles  of  Protozoa,  which  belong  to  the 
category  of  temporary  organs,  yet  in  many  cases  are  handed  on  from 
one  cell  to  another  by  fission,  and  the  attraction-spheres  and  asters, 
which  may  either  persist  from  cell  to  cell  or  disappear  and  re-form 
about  the  centrosome.  There  is  now  considerable  evidence  that  the 
centrosome  itself  may  in  some  cases  have  the  character  of  a  perma- 
nent organ,  in  others  may  disappear  and  re-form  like  the  asters. 

The  facts  point  toward  the  conclusion,  which  has  been  especially 
urged  by  De  Vries  and  Wiesner,  that  the  power  of  division,  not  only 
of  the  cell-organs,  but  also  of  the  cell  as  a  whole,  may  have  its  root  in 
a  like  power  on  the  part  of  more  elementary  masses  or  units  of  which 
the  structural  basis  is  itself  built,  tJie  degree  of  permaiieuee  in  the  eell- 
organs  depending  on  the  degree  of  cohesion  manifested  by  these  elemen- 
tary bodies.  If  such  bodies  exist,  they  must,  however,  in  their  primary 
form,  lie  beyond  the  present  Hmits  of  the  microscope,  the  visible  struc- 
tures arising  by  their  enlargement  or  aggregation.  The  cell,  therefore, 
cannot  be  regarded  as  a  colony  of  ''granules  "  or  other  gross  morpho- 
logical elements.  The  phenomena  of  cell-division  show,  however,  that 
the  dividing  substance  tends  to  differentiate  itself  into  several  orders 
of  visible  morphological  aggregates,  as  is  most  clearly  shown  in  the 
nuclear  substance.  Here  the  highest  term  is  the  plurivalent  chromo- 
some, the  lowest  the  smallest  visible  dividing  basichromatin-grains, 


328  SOME   PROBLEMS    OF  CELL-ORGANIZATION 

while  the  intermediate  terms  are  formed  by  the  successive  aggrega- 
tion of  these  to  form  the  chromatin-granules  of  which  the  dividing 
chromosomes  consist.  Whether  any  or  all  of  these  bodies  are  *'  indi- 
viduals "  is  a  question  of  words.  The  facts  point,  however,  to  the 
conclusion  that  at  the  bottom  of  the  series  there  must  be  masses  that 
cannot  be  further  split  up  without  loss  of  their  characteristic  proper- 
ties, and  which  form  the  elementary  morphological  units  of  the  nucleus. 
In  case  of  the  cytoplasm  the  evidence  is  far  less  satisfactory. 
Could  Rabl's  theory  of  fibrillar  persistence,  as  developed  by  Heiden- 
hain  and  Kostanecki,  be  established,  we  should  indeed  have  almost  a 
demonstration  of  panmeristic  division  in  the  cytoplasm.  At  present, 
however,  the  facts  do  not  admit  the  acceptance  of  that  theory,  and 
the  division  of  the  visible  cytoplasmic  granules  must  remain  a  quite 
open  question.  Yet  w^e  should  remember  that  the  dividing  plastids 
of  plant-cells  are  often  very  minute,  and  that  in  the  centrosome  we 
have  a  body,  no  larger  in  many  cases  than  a  **  microsome,"  which  is 
positively  known  to  be  in  some  cases  a  persistent  morphological  ele- 
ment, having  the  power  of  growth,  division,  and  persistence  in  the 
daughter-cells.  Probably  these  powers  of  the  centrosome  would 
never  have  been  discovered  were  it  not  that  its  staining-capacity  ren- 
ders it  conspicuous  and  its  position  at  the  focus  of  the  astral  rays 
isolates  it  for  observation.  When  we  consider  the  analogy  between 
the  centrosome  and  the  basichromatin-grains,  when  we  recall  the 
evidence  that  the  latter  graduate  into  the  oxychromatin-granules,  and 
these  in  turn  into  the  cytomicrosomes,  we  must  admit  that  Brlicke's 
cautious  suggestion  that  the  whole  cell  might  be  a  congeries  of  self- 
propagating  units  of  a  lower  order  is  sufficiently  supported  by  fact 
to  constitute  a  legitimate  working  hypothesis. 


LITERATURE.     VI  i 

Van  Beneden.  E.  —  (See  List  IV.) 

Van  Beneden  and  Julin.  —  La  segmentation  chez  les  Ascidiens  et  ses  rapports  avec 

Torganisation  de  la  larve  :  Arch.  Biol..  V.     1884. 
Boveri.  Th. — Zellenstudien.     (See  List  IV.) 

Briicke,  C.  —  Die  Elementarorganismen  :    lVie?ier  Silz.-Ber.,  XIAV.     1861. 
Biitschli,  0.  —  Protoplasma.     (See  List  I.) 
Delage.  Yves.  —  La  structure  du  protoplasma,  et  les  theories  sur  Theredite.     Parts, 

1895.        .. 
Hacker.  V. — Uber  den  heutigen    Stand    der  Centrosomenfrage :    Vcrh.  d.  deutsch. 

Zo'dl.  Ges.     1894. 
Heidenhain,  M.  —  (See  List  I.) 
Herla.  V.  —  Etude  des  variations  de  la  mitose  chez  Pascaride  megalocephale  :  Arch. 

Biol.yAW.     1893. 

1  See  also  Literature,  I.,  II.,  IV.,  V. 


LITERATURE  329 

Morgan,  T.  H.  —  The  Action    of  Salt-solutions  on  the  Fertilized  and  Unfertilized 

Eggs  of  Arbacia  and  Other  Animals.     Arch.  Entiu.,  VIII.  3.     1898. 
Kostanecki,  K.  —  Ueber  die  Bedeutung  der  Polstrahung  wahrend  der  Mitose.     An/i. 

inik.  Anat.,  XLIX.      1897. 
Nussbaum,  M.  —  Uber  die  Teilbarkeit  der  lebendigen  Materie  :  Arc/i.  mik     Inat 

XXVI.     1886. 
Prenant,  A.  —  Sur    le   protoplasma   superieure  (archiplasme,  kinoplasme.  ergastro- 

plasme)  :  Journ.  Anat.  et  Phys.,  XXIV.-V.     1898-99.     (Full  Literature-lists.) 
Rabl,  C.  —  iJber  Zellteilung:  iJ/(9r///./rt;/^r(^.,  X.     1885.     Anat.  AnzeigerAX .     1889. 
Ruckert,  J.  —  (See  List  IV.) 

De  Vries,  H.  —  Intracellulare  Pangenesis:  Jena..  1889. 

Watase,  S.  —  Homology  of  the  Centrosome  :  Journ.  Morph.,  VIII.  2.  1893. 
Id.  —  On  the  Nature  of  Cell-organization  :  Woods  H oil  Biol.  Lectures.  1893. 
Wiesner,  J.  —  Die  Elementarstruktur  und  das  Wachstum  der  lebenden  Substanz : 

Wien,  1892. 
Wilson.  Edm.  B.  —  Archoplasm,  Centrosome,  and  Chromatin  in  the  Sea-urchin  Egg: 

Journ.  Morph.,  Vol.  XI.     1895. 


\ 


CHAPTER   VII 

SOME   ASPECTS   OF   CELL-CHEMISTRY   AND   CELL-PHYSIOLOGY 

"  Les  phenomfenes  fonctionnels  ou  de  depense  vitale  auraient  done  leur  siege  dans  le  proto- 
flasme  cellulaire. 

"  Le  noyau  est  un  appareil  de  synthese  organique,  V instrument  de  la  production,  le  gertne  de  la 
cellule"  Claude  Bernard.i 


A.     Chemical    Relations   of    Nucleus   and   Cytoplasm 

It  is  no  part  of  the  purpose  of  this  work  to  give  even  a  sketch  of 
general  cell-chemistry.  I  shall  only  attempt  to  consider  certain  ques- 
tions that  bear  directly  upon  the  functional  relations  of  nucleus  and 
cytoplasm  and  are  of  especial  interest  in  relation  to  the  process  of 
nutrition  and  through  it  to  the  problems  of  development.  It  has 
often  been  pointed  out  that  we  know  little  or  nothing  of  the  chemical 
conditions  existing  in  living  protoplasm,  since  every  attempt  to  examine 
them  by  precise  methods  necessarily  kills  the  protoplasm.  We  must, 
therefore,  in  the  main  rest  content  with  inferences  based  upon  the 
chemical  behaviour  of  dead  cells.  But  even  here  investigation  is  be- 
set with  difficulties,  since  it  is  in  most  cases  impossible  to  isolate  the 
various  parts  of  the  cell  for  accurate  chemical  analysis,  and  we  are 
obliged  to  rely  largely  on  the  less  precise  method  of  observing  with 
the  microscope  the  visible  effects  of  dyes  and  other  reagents.  This 
difficulty  is  increased  by  the  fact  that  both  cytoplasm  and  karyoplasm 
are  not  simple  chemical  compounds,  but  mixtures  of  many  complex 
substances ;  and  both,  moreover,  undergo  periodic  changes  of  a  com- 
plicated character  which  differ  very  widely  in  different  kinds  of  cells. 
Our  knowledge  is,  therefore,  still  fragmentary,  and  we  have  as  yet 
scarcely  passed  the  threshold  of  a  subject  which  belongs  largely  to 
the  cytology  of  the  future. 

It  has  been  shown  in  the  foregoing  chapter  that  all  the  parts  of  the 
cell  arise  as  local  differentiations  of  a  general  protoplasmic  basis. 
Despite  the  difficulties  of  chemical  analysis  referred  to  above,  it  has 
been  determined  with  certainty  that  some  at  least  of  these  organs  are 
the  seat  of  specific  chemical  change ;  just  as  is  the  case  in  the  various 
organs  and  tissues  of  the  organism  at  large.     Thus,  the  nucleus  is 

1  Le(;o7ts  sur  les  phenoitiejies  de  la  vie,  I.,  1878,  p.   198. 

330 


CHEMICAL  RELATIONS  OF  NUCLEUS  AND    CYTOPLASM        331 

characterized  by  the  presence  of  nuclein  (chromatin)  which  has  been 
proved  by  chemical  analysis  to  differ  widely  from  the  cytoplasmic 
substances,!  while  the  various  forms  of  plastids  are  centres  for  the 
formation  of  chlorophyll,  starch,  or  pigment.  These  facts  give  -round 
for  the  conclusion  that  the  morphological  differentiation  of  cell-or<'-ans 
IS  m  general  accompanied  by  underlying  chemical  specializadons 
which  are  themselves  the  expression  of  differences  of  metabolic  ac- 
tivity ;  and  these  relations,  imperfectly  comprehended  as  they  are  are 
of  fundamental  importance  to  the  student  of  development. 

I.      The  Proteids  and  their  Allies 

The  most  important  chemical  compounds  found  in  the  cell  are  the 
group  oi protein  substances,  and  there  is  every  reason  to  believe  that 
these  form  the  principal  basis  of  living  protoplasm  in  all  of  its  forms. 
These  substances  are  complex  compounds  of  carbon,  hydrogen,  nitro- 
gen, and  oxygen,  often  containing  a  small  percentage  of  sul^Dhur,  and 
in  some  cases  also  phosphorus  and  iron.  They  form  a  very  extensive 
group  of  which  the  different  members  differ  considerably  in  physical 
and  chemical  properties,  though  all  have  certain  common  traits  and 
are  closely  related.  They  are  variously  classified  even  by  the  latest 
writers.  By  many  authors  (for  example  Halliburton,  '93')  the  word 
''proteids  "  is  used  in  a  broad  sense  as  synonymous  with  albuminous 
substajtces,  including  under  them  the  various  forms  of  albumin  (eq:<;- 
albumin,  cell-albumin,  muscle-albumin,  vegetable-albumins),  ,<,V^>/;////// 
(fibrinogin  vitellin,  etc.),  and  the /^//^//^j- (diffusible  hydrated  proteids). 
Another  series  of  nearly  related  substances  are  the  albuminoids 
(reckoned  by  some  chemists  among  the  ''proteids"),  examples  of 
which  are  gelatin,  mucin,  and,  according  to  some  authors  also, 
nuclein,  and  the  7iucleo-albuniins.  Some  of  the  best  authorities  how- 
ever, among  them  Kossel  and  Hammarsten,  follow  the  usage  of 
Hoppe-Seyler  in  restricting  the  woxd.  proteid  \.o  substances  of  greater 
complexity  than  the  albumins  and  globulins.  Examples  of  these 
are  the  nuclein s  and  nucleo-proteids,  which  are  comj^ounds  of  nu- 
cleinic  acid  with  albumin,  histon,  or  protamin.  The  nucleo-proteids, 
found  only  in  the  nucleus,  are  not  to  be  confounded  with  the  nucleo- 

^  It  has  long  been  known  that  a  form  of  "  nuclein  "  may  also  he  obtained  from  the  nucleo- 
albmiiins  of  the  cytoplasm,  e.g.  from  the  yolk  of  hens'  eggs  (vitellin).  Sucii  nucleins  tliffcr, 
however,  from  those  of  nuclear  origin  in  not  yielding  as  cleavage-jiroducts  the  nuclein  bases 
(adenin,  xanthin,  etc.).  The  term  "  paranuclein  "  (Kossel)  or  "  pseudo-nuclcin  "  (Ham- 
marsten) has  therefore  been  suggested  for  this  substance.  True  nucleins  containing  a  large 
percentage  of  albumin  are  distinguished  as  ntuleo-protcids.  They  may  be  split  into  albumin 
(or  albumin  radicals)  and  nucleinic  acid,  the  latter  yielding  as  cleavage-products  the  nuclein 
bases.  Pseudo-nucleins  containing  a  large  percentage  of  albumin  are  designated  as  nuclro- 
albtimins,  which,  in  like  manner  split  into  albumin  and  paranucleinic  or  pseudo-nucleinic  acid, 
which  yields  no  nuclein  bases.     (See  Hammarsten,  '94.) 


332 


CELL-CHEMISTRY  AND    CELL-PHYSIOLOGY 


albumins,  which  are  compounds  of  pseudo-nucleinic  acid  with  albumin 
and  yield  no  nuclein-bases  (xanthin,  hypoxanthin,  adenin,  guanin)  as 
decomposition  products. 

The  distribution  of  these  substances  through  the  cell  varies 
greatly  not  only  in  different  cells,  but  at  different  periods  in  the  life 
of  the  same  cell.  The  cardinal  fact  always,  however,  remains,  that 
tJiere  is  a  definite  and  constant  contrast  between  nucleus  and  cytoplasm. 
The  latter  always  contains  large  quantities  of  nucleo-albumins,  certain 
globulins,  and  sometimes  small  quantities  of  albumins  and  peptones ; 
the  former  contains,  in  addition  to  these,  nuclein  and  nucleo-proteids, 
which  form  its  main  bulk,  and  its  most  constant  and  characteristic 
feature.  It  is  the  remarkable  substance,  nuclein,  —  which  is  almost 
certainly  identical  with  chromatin,  —  that  chiefly  claims  our  attention 
here  on  account  of  the  physiological  role  of  the  nucleus. 


2.    The  Nuclein   Series 

Nuclein  was  first  isolated  and  named  by  Miescher,  in  1 871,  by 
subjecting  cells  to  artificial  gastric  digestion.  The  cytoplasm  is  thus 
digested,  leaving  only  the  nuclei ;  and  in  some  cases,  for  instance  pus- 
cells  and  spermatozoa,  it  is  possible  by  this  method  to  procure  large 
quantities  of  nuclear  substance  for  accurate  quantitative  analysis. 
The  results  of  analysis  show  it  to  be  a  complex  albuminoid  substance, 
rich  in  phosphorus,  for  which  Miescher  gave  the  chemical  formula 
C29H49N9P3022-  The  earlier  analysis  of  this  substance  gave  some- 
what discordant  results,  as  appears  in  the  following  table  of  per- 
centage-compositions :  ^  — 


These  differences  led  to  the  opinion,  first  expressed  by  Hoppe- 
Seyler,  and  confirmed  by  later  investigations,  that  there  are  several 
varieties  of  nuclein  which  form  a  group  having  certain  characters 
in  common.  Altmann  ('89)  opened  the  way  to  an  understanding  of 
the  matter  by  showing  that  **  nuclein  "  may  be  split  up  into  two  sub- 
stances ;  namely,  ( i )  an  organic  acid  rich  in  phosphorus,  to  which  he 

^From  Halliburton,  '91,  p.  203.     [The  oxygen-percentage  is  omitted  in  this  table.] 


CHEMICAL   RELATIONS  OF  NUCLEUS  AND    CYTOPLASM        333 

gave  the  name  nucleinic  acid,  and  (2)  a  form  of  albumin.  Moreover, 
the  nuclein  may  be  synthetically  formed  by  the  re-combination  of 
these  two  substances.  Pure  nucleinic  acid,  for  which  Miescher  (96) 
afterward  gave  the  formula  C^oHg^Nj^P^O.^^.^  contains  no  sulphur,  a 
high  percentage  of  phosphorus  (above  9%),  and  no  albumin.  l^y 
adding  it  to  a  solution  of  albumin  a  precipitate  is  formed  which 
contains  sulphur,  a  lower  percentage  of  phosphorus,  and  has  the 
chemical  characters  of  "  nuclein."  This  indicates  that  the  discord- 
ant results  in  the  analyses  of  nuclein,  referred  to  above,  were 
probably  due  to  varying  proportions  of  the  two  constituents ;  and 
Altmann  suggested  that  the  "nuclein  "  of  spermatozoa,  which  contains 
no  sulphur  and  a  maximum  of  phosphorus,  might  be  uncombined 
nucleinic  acid  itself.  Kossel  accordingly  drew  the  conclusion,  based 
on  his  own  work  as  well  as  that  of  Liebermann,  Altmann,  Malfatti. 
and  others,  that  "what  the  histologists  designate  as  clironiatin  con- 
sists essentially  of  combinations  of  nucleinic  acid  with  more  or  less 
albumin,  and  in  some  cases  may  even  be  free  nucleinic  acid.  The 
less  the  percentage  of  albumin  in  these  compounds,  the  nearer  do 
their  properties  approach  those  of  pure  nucleinic  acid,  and  we  may 
assume  that  the  percentage  of  albumin  in  the  chromatin  of  the  same 
nucleus  may  vary  according  to  physiological  conditions."  ^  In  the 
same  year  Halliburton,  following  in  part  Hoppe-Seyler,  stated  the 
same  view  as  follows.  The  so-called  "  nucleins  "  form  a  series  lead- 
ing downward  from  nucleinic  acid  thus  :  — 

(i)   Those  containing  no  albumin  and  a  maximum  (9-10%)  of  phos- 
phorus (pure  nucleinic  acid).     Nuclei  of  spermatozoa. 

(2)  Those  containing  little  albumin  and  rich  in  phosphorus.     Chro- 

matin of  ordinary  nuclei. 

(3)  Those  with  a  greater  proportion  of  albumin  —  a  series  of  sub- 

stances in  which  may  probably  be  included /jvr;//;/  (nucleoli) 
and  plastin  (Hnin).     These  graduate  into 

(4)  Those   containing  a  minimum  (0.5   to    1%)  of   phosphorus  — 

the  nucleo-albumins,  which  occur  both  in  the  nucleus  and  in 
the  cytoplasm  (vitellin,  caseinogen,  etc.). 

Finally,  we  reach  the  globuUns  and  albumins,  especially  character- 
istic of  the  cell-substance,  and  containing  no  nucleinic  acid.  "  We  thus 
pass  by  a  gradual  transition  (from  the  nucleo-albumins)  to  the  other 
proteid  constituents  of  the  cell,  the  cell-globulins,  which  contain  no 
phosphorus  whatever,  and  to  the  products  of  cell-activity,  such  as 
the  proteids  of  serum  and  of  egg-white,  which  are  also  principally 

1  Derived  from  analysis  of  the  salmon-sperm.  ^ '93»  P-  ^S^- 


334  CELL-CHEMISTRY  AND    CELL-PHYSIOLOGY 

phosphorus-free."  ^  Further,  "  in  the  processes  of  vital  activity  there 
are  changing  relations  between  the  phosphorized  constituents  of  the 
nucleus,  just  as  in  all  metabohc  processes  there  is  a  continual  inter- 
change, some  constituents  being  elaborated,  others  breaking  down 
into  simpler  products."  This  latter  conclusion  has  been  well  estab- 
Ushed;  the  others,  as  stated  by  Halliburton,  require  some  modification, 
on  the  one  hand,  through  the  results  of  later  analyses  of  chromatin, 
on  the  other,  because  of  the  failure  to  distinguish  between  the  nucleo- 
proteids  and  the  nucleo-albumins.  First,  it  has  been  shown  by 
Miescher  ('96),  Kossel  ('96),  and  Mathews  ('97,  2)  that  the  chromatin 
of  the  sperm-nuclei  (in  fish  and  sea-urchins)  is  not  pure  nucleinic  acid, 
as  Altmann  conjectured,  but  a  salt  of  that  acid,  with  histon,  protamin, 
or  a  related  substance.  Thus,  in  the  spermatozoa  of  the  salmon, 
Miescher's  analyses  give  60.56%  of  nucleinic  acid  and  35.56%  of 
protamin  (CigH28N902).  In  the  herring  the  chromatin  is  a  compound 
of  nucleinic  acid  (over  63%)  and  a  form  of  protamin  called  by  Kossel 
"  clupein  "  (CgoH-^Nj^Og).  In  the  '$>Q,?,-m:z\v\rv  Arbacia  Mathews  finds 
the  chromatin  to  be  a  compound  of  nucleinic  acid  and  ''  arbacin,"  a 
histon-hke  body.  Kossel  finds  also  that  chromatin  (nuclein)  derived 
from  the  thymus  gland,  and  from  leucocytes,  is  largely  a  histon 
salt  of  nucleinic  acid,  the  proportion  of  the  latter  being,  however, 
much  less  than  in  the  sperm-chromatin,  while  albumin  is  also  present. 
In  these  cases,  therefore,  the  greater  part  of  the  nucleinic  acid  is  com- 
bined not  with  albumin  but  with  a  histon  or  protamin  radical.  Second, 
the  nucleo-albumins  of  the  cytoplasm  are  in  no  sense  transitional  be- 
tween the  nucleins  and  the  albumins,  since  they  contain  no  true 
nucleinic  acid,  but  only  pseudo-nucleinic  acid.'-^  The  fact  nevertheless 
remains  that  the  nucleins  and  nucleo-proteids,  though  confined  to  the 
nucleus,  form  a  series  descending  from  such  highly  phosphorized 
bodies  as  the  sperm-chromatin  toward  bodies  such  as  the  albumins, 
which  are  especially  characteristic  of  the  cytoplasm  ;  and  that  they 
vary  in  composition  with  varying  physiological  conditions.  The  way 
is  thus  opened  for  a  more  precise  investigation  of  the  physiological 
role  of  nucleus  and  cytoplasm  in  metabolism. 

3.    Staining-reaction  of  the  Niiclcin  Series 

In  bringing  these  facts  into  relation  with  the  staining-reactions  of 
the  cell,  it  is  necessary  briefly  to  consider  the  nature  of  staining- 
reactions  in  general,  and  especially  to  warn  the  reader  that  in  the 
whole  field  of  "  micro-chemistry "  we  are  still  on  such  uncertain 
ground  that  all  general  conclusions  must  be  taken  with  reserve. 

First,  it  is  still  uncertain  how  far  staining-reactions  depend  upon 
chemical  reaction  and  how  far  upon   merely  physical  properties  of 


CHEMICAL   RELATIONS   OF  NUCLEUS  AND    CYTOPLASM        y^ 

the  bodies  stained.  The  prevalent  view  that  staining-reactiuns  are 
due  to  a  chemical  combination  of  the  dye  with  the  elements  of  the 
cell  has  been  attacked  by  Gierke  ('85),  Rawitz  ('97),  and  Fischer 
('97>  '99 )»  al^  of  whom  have  endeavoured  to  show  that  these  reactions 
are  of  no  value  as  a  chemical  test,  being  only  a  result  of  surface- 
attraction  and  absorption  due  to  purely  physical  qualities  of  the 
bodies  stained.  On  the  other  hand,  a  long  series  of  experiments, 
beginning  with  Miescher's  discovery  ('74)  that  isolated  nucleinic  acid 
forms  green  insoluble  salts  with  methyl-green,  and  continued  by 
Lilienfeld,  Heidenhain,  Paul  Mayer,  and  others,  gives  strong  reason 
to  believe  that  beyond  the  physical  imbibition  of  colour  a  true  chemical 
union  takes  place,  which,  with  due  precautions,  gives  us  at  least  a 
rough  test  of  the  chemical  conditions  existing  in  the  cell.^ 

Second,  similarity  of  staining-reactioii  is  by  no  means  always  indica- 
tive of  chemical  similarity,  as  is  shown,  for  example,  by  the  fact  that 
in  cartilage  both  nuclei  and  inter-cellular  matrix  are  intensely  stained 
by  methyl-green,  though  chemically  they  differ  very  widely. 

Third,  colour  in  itself  gives  no  evidence  of  chemical  nature ;  for  the 
nucleus  and  other  elements  of  the  same  cell  may  be  stained  red, 
green,  or  blue,  according  to  the  dye  employed,  and  to  class  them  as 
"  erythrophilous,"  "  cyanophilous,"  and  the  like,  is  therefore  absurd. 

Fourth,  tJie  character  of  the  staining-reaction  is  influenced  and  in 
some  cases  determined  by  the  fixation  or  other  prelimiiiary  treat  me  fit, 
a  principle  made  use  of  practically  in  the  operations  of  mordaunting, 
but  one  which  may  give  very  misleading  results  unless  carefullv  con- 
trolled. Thus  Rawitz  ('95)  shows  that  certain  colours  which  ordinarily 
stain  especially  the  nucleus  (saffranin,  gentian-violet),  can  be  made  to 
stain  only  the  cytoplasm  through  preliminary  treatment  of  object 
with  solutions  of  tannin,  followed  by  tartar-emetic.  In  like  manner 
Mathews  ('98)  shows  that  many  of  the  *'  nuclear"  tar-colours  (saffra- 
nin, methyl-green,  etc.)  stain  or  do  not  stain  the  cytoplasm,  according 
as  the  material  has  been  previously  treated  with  alkaline  or  with  acid 
solutions. 

The  results  with  which  we  now  have  to  deal  are  based  mainly 
upon  experiments  with  tar-colours  (*' aniline  dyes").  Ehrlich  ('79) 
long  since  characterized  these  dyes  as  **  acid  "  or  "basic,"  according 
as  the  colouring  matter  plays  the  part  of  an  acid  or  a  base  in  the  com- 
pound employed,  showing  further  that,  other  things  equal,  the  basic 
dyes  (methyl-green,  saffranin,  etc.)  are  especially  "nuclear  stains" 
and  the  acid  (rubin,  eosin,  orange,  etc.)  "plasma  stains."  Malfatti 
('91),  and  especially  Lilienfeld  ('92,  '93),  following  out  Miescher's 
earlier  work  ('74),  found  that  albumin  stains  preeminently  in  the 
acid  stains,  nucleinic  acid  only  in  the  basic  ;  and,  further,  that  artifi- 

1  Cf.  Mayer,  '91,  '92,  '97;    Lilienfeld,  '93;    Mathews,  '98. 


336  CELL-CHEMISTRY  AND    CELL-PHYSIOLOGY 

cial  nucleins,  prepared  by  combining  egg-albumin  with  nucleinic  acid 
in  various  proportions,  show  a  varying  affinity  for  basic  and  acid 
dyes  according  as  the  nucleinic  acid  is  more  or  less  completely 
saturated  with  albumin.  Lilienf eld's  starting-point  was  given  by  the 
results  of  Kossel's  researches  on  the  relations  of  the  nuclein  group, 
which  are  expressed  as  follows  :  ^  — 

Nucleo-proteid  (i%  of  P  or  less), 
by  peptic  digestion  splits  into 


Peptone  Nuclein  (3-4%  P)^ 

by  treatment  with  acid  splits  into 

I ' ^ 

Albumin  Nucleinic  acid  (9- 1 0%  P) , 

heated  with  mineral  acids  splits  into 

« 

.  Phosphoric  acid  Nuclein  bases  (A  ca?-bohj/drale.) 

(adenin.  guanin.  etc.). 

Now,  according  to  Kossel  and  Lilienfeld,  the  principal  nucleo- 
proteid  in  the  nucleus  of  leucocytes  is  nucleo-Jiiston,  containing  about 
3%  of  phosphorus,  which  may  be  split  into  a  form  of  nuclein  playing 
the  part  of  an  acid,  and  an  albuminoid  base,  the  Jiiston  of  Kossel ; 
the  nuclein  may  in  turn  be  split  into  albumin  and  nucleinic  acid. 
These  four  substances  —  albumin,  nucleo-histon,  nuclein,  nucleinic 
acid  —  thus  form  a  series  in  which  the  proportion  of  phosphorus, 
which  is  a  measure  of  the  nucleinic  acid,  successively  increases  from 
zero  to  9-10%.  If  the  members  of  this  series  be  treated  with  the 
same  mixture  of  red  acid  fuchsin  and  basic  methyl-green,  the  result 
is  as  follows.  Albumin  (egg-albumin)  is  stained  red,  nucleo-histon 
greenish  blue,  nuclein  bluish  green,  nucleinic  acid  intense  green.  "We 
see,  therefore,  that  the  principle  that  determines  the  staining  of  the 
nuclear  substances  is  always  the  nucleinic  acid.  All  the  nuclear  sub- 
stances, from  those  richest  in  albumin  to  those  poorest  in  it,  or  con- 
taining none,  assume  the  tone  of  the  nuclear  {i.e.  basic)  stain,  but  the 
combined  albumin  modifies  the  green  more  or  less  toward  blue."  ^ 
Lilienfeld  explains  the  fact  that  chromatin  in  the  cell-nucleus  seldom 
appears  pure  green  on  the  assumption,  supported  by  many  facts, 
that  the  proportions  of  nucleinic  acid  and  albumin  vary  with  different 
physiological  conditions,  and  he  suggests  further  that  the  intense 
staining-power  of  the  chromosomes  during  mitosis  is  probably  due 
to  the  fact  that  they  contain  a  maximum  of  nucleinic  acid.  Very 
interesting  is  a  comparison  of  the  foregoing  staining-reactions  with 
those  given  by  a  mixture  of  a  red  basic  dye  (saffranin)  and  a  green 
acid  one  ("  light  green  ").  With  this  combination  an  effect  is  given 
which  reverses  that  of  the  Biondi-EhrUch  mixture ;  i.e.  the  nuclein 

1  From  Lilienfeld,  after  Kossel  ('92,  p.  129).  ^  I.e.,  p.  394. 


CHEMICAL   RELATIONS   OF  NUCLEUS  AND   CYTOPLASM        337 

is  coloured  red,  the  albumin  green,  which  is  a  beautiful  demon- 
stration of  the  fact  that  staining-reagents  cannot  be  logically  classified 
according  to  colour,  but  only  according  to  their  chemical  nature, 
and  gives  additional  ground  for  the  view  that  staining-reactions  of 
this  type  are  the  result  of  a  chemical  rather  than  a  merely  physical 
combination. 

These  results  must  be  taken  with  some  reserve  for  the  following'- 
reasons  :  Mathews  ('98)  has  shown  that  methyl-green  and  other  basic 
dyes  will  energetically  stain  albumose,  coagulated  egg-albumin,  and 
the  cell-cytoplasm  in  or  after  treatment  by  alkaline  fluids;  while  con- 
versely the  acid  dyes  do  not  stain,  or  only  slightly  stain,  these  sub- 
stances under  the  same  conditions.  This  probably  does  not  affect 
the  validity  of  Heidenhain's  results,^  since  he  worked  with  acid  solu- 
tions. What  is  more  to  the  point  is  the  fact  that  hyaline  cartilage 
and  mucin,  though  containing  no  nucleinic  acid,  stain  intenselv  with 
basic  dyes.  Mathews  probably  gives  the  clue  to  this  reaction,  in 
the  suggestion  that  it  is  here  probably  due  to  the  presence  of  other 
acids  (in  the  case  of  cartilage  a  salt  of  chondroitin-sulphuric  acid, 
according  to  Schmiedeberg);  from  which  Mathews  concludes  that 
the  basic  dyes  will,  in  acid  or  neutral  solutions,  stain  any  element  of 
the  tissues  that  contains  an  organic  acid  in  a  salt  combination  with  a 
strong  base.^  Accepting  this  conclusion,  we  must  therefore  recognize 
that,  as  far  as  the  cytoplasm  is  concerned,  the  basic  or  **  nuclear  " 
stains  are  in  no  sense  a  test  for  nuclein,  but  only  for  salts  of  organic 
acids  in  general.  In  case  of  the  nucleus,  however,  we  know  from 
direct  analysis  that  we  are  dealing  with  varying  combinations  of 
nucleinic  acid,  and  hence,  with  the  precautions  indicated  above,  may 
draw  provisional  conditions  from  the  staining-reactions. 

Thus  regarded,  the  changes  of  staining-reaction  in  the  chromatin 
are  of  high  interest.  Heidenhain  ('93,  '94),  in  his  beautiful  studies 
on  leucocytes,  has  correlated  some  of  the  foregoing  results  with  the 
staining-reactions  of  the  cell  as  follows.  Leucocytes  stained  with 
the  Biondi-Ehrlich  mixture  of  acid  fuchsin  and  methyl-green  sl-.ow 
the  following  reactions.  Cytoplasm,  centrosome,  attraction-sj)here, 
astral  rays,  and  spindle-fibres  are  stained  pure  red.  The  nuclear  sub- 
stance shows  a  very  sharp  differentiation.  The  chromatic  network 
and  the  chromosomes  of  the  mitotic  figure  are  green.  The  linin- 
substance  and  the  true  nucleoli  or  plasmosomes  aj)pear  red,  like  the 
cytoplasm.  The  Hnin-network  of  leucocytes  is  stated  by  Heidenhain 
to  consist  of  two  elements,  namely,  of  red  granules  or  microsomes 
suspended  in  a  colourless  network.  The  latter  alone  is  called  **  linin  " 
by  Heidenhain.  To  the  red  granules  is  applied  the  term  *'  ox-ychro- 
matin,"  while  the  green  substance  of  the  ordinary  chromatic  network, 

1  See  below.  -  '98.  PP-  451-452. 


338  CELL-CHEMISTRY  AND    CELL-PHYSIOLOGY 

forming  the  "  chromatin  "  of  Flemming,  is  called  *'  basichromatin."  ^ 
Morphologically,  the  granules  of  both  kinds  are  exactly  alike,-  and 
in  many  cases  the  oxychromatin-granules  are  found  not  only  in  the 
"  achromatic  "  nuclear  network,  but  also  intermingled  with  the  basi- 
chromatin-granules  of  the  chromatic  network.  Collating  these  results 
with  those  of  the  physiological  chemists,  Heidenhain  concludes  that 
basichromatin  is  a  substance  rich  in  phosphorus  (/.^.  nucleinic  acid), 
oxychromatin  a  substance  poor  in  phosphorus,  and  that,  further, 
'*  basichromatin  and  oxychromatin  are  by  no  means  to  be  regarded 
as  permanent  unchangeable  bodies  but  may  change  their  colour- 
reactions  by  combining  with  or  giving  off  phosphorus."  In  other 
words,  **  the  affinity  of  the  chromatophilous  microsomes  of  the  nuclear 
network  for  basic  and  acid  aniline  dyes  is  regulated  by  certain  physio- 
logical conditions  of  the  nucleus  or  of  the  cell."'^ 

This  conclusion,  which  is  entirely  in  harmony  with  the  statements 
of  Kossel  and  Halliburton  quoted  above,  opens  up  the  most  interest- 
ing questions  regarding  the  periodic  changes  in  the  nucleus.  The 
staining-power  of  chromatin  is  at  a  maximum  when  in  the  preparatory 
stages  of  mitosis  (spireme-thread,  chromosomes).  During  the  ensuing 
growth  of  the  nucleus  it  always  diminishes,  suggesting  that  a  com- 
bination with  albumin  has  taken  place.  This  is  illustrated  in  a  very 
striking  way  by  the  history  of  the  egg-nucleus  or  germinal  vesicle, 
which  exhibits  the  nuclear  changes  on  a  large  scale.  It  has  long 
been  known  that  the  chromatin  of  this  nucleus  undergoes  great 
changes  during  the  growth  of  the  ^gg,  and  several  observers  have 
maintained  its  entire  disappearance  at  one  period.  Riickert  first 
carefully  traced  out  the  history  of  the  chromatin  in  detail  in  the 
eggs  of  sharks,  and  his  general  results  have  since  been  confirmed  by 
Born  in  the  eggs  of  Triton.  In  the  shark  Pristiiirns,  Riickert  ('92,  i) 
finds  that  the  chromosomes,  which  persist  throughout  the  entire 
growth-period  of  the  ^gg,  undergo  the  following  changes  (Fig.  157): 
At  a  very  early  stage  they  are  small,  and  stain  intensely  with  nuclear 
dyes.  During  the  growth  of  the  ^gg  they  undergo  a  great  increase 
in  size,  and  progressively  lose  tJieir  staining-capacity .  At  the  same 
time  their  surface  is  enormously  increased  by  the  development  of 
long  threads  which  grow  out  in  every  direction  from  the  central  axis 
(Fig.  157,  A).  As  the  ^gg  approaches  its  full  size,  the  chromosomes 
rapidly  diminish  in  size,  the  radiating  threads  disappear,  and  the  stain- 
ing-capacity increases  (Fig.  157,^).  They  are  finally  again  reduced  to 
minute,  intensely  staining  bodies  which  enter  into  the  equatorial  plate 
of  the  first  polar,  mitotic  figure  (Fig.  157,  C).  How  great  the  change 
of  volume  is  may  be  seen  from  the  following  figures.  At  the  beginning 
the  chromosomes  measure,  at  most,  12  yi  (about  o-qVo^  ^^O  ^'^  length  and 

^'94,  P-  543-  ^^•^•.  P-  547-  ^^•^•'  P-  548- 


CHEMICAL   RELATIONS   OF  NUCLEUS  AND    CYTOPLASM        339 

1  /z  in  diameter.  At  the  lieight  of  their  development  they  are  ahnost 
eight  times  their  original  length  and  twenty  times  their  original 
diameter.  In  the  final  period  they  are  but  2  /x  in  length  and  i  //  in  di- 
ameter. These  measurements  show  a  change  of  volume  so  enormous, 
even  after  making  due  allowance  for  the  loose  structure  of  the  large 
chromosomes,  that  it  cannot  be  accounted  for  by  mere  swelling  or 
shrinkage.      The  chromosomes  evidently  absorb  a  large  amount  of 


Fig.  157.  _  Chromosomes  of  the  germinal  vesicle  in  the  shark  Pnstiurus,  at  different  periods, 
drawn  to  the  same  scale.     [RiJCKERT.] 

A.   At  the  period  of  maximal  size    and  minimal  staining-capacity  {tgg  3  mm.  in  diameter) 
B.   Later  period  (egg  13  mm.  in  diameter).     C.   At  the  close  of  ovarian  life,  of  nunmial  size  and 
maximal  staining-power. 


matter,  combine  with  it  to  form  a  substance  of  diminished  stammg- 
capacity,  and  finally  give  off  matter,  leaving  an  intensely  stammg 
substance  behind.  As  Riickert  points  out,  the  great  mcrease  ot  sur- 
face in  the  chromosomes  is  adapted  to  facilitate  an  exchange  of  mate- 
rial between  the  chromatin  and  the  surrounding  substance ;  and  he 
concludes  that  the  coincidence  between  the  growth  ot  the  chromo- 
somes and  that  of  the  ^gg  points  to  an  intimate  connection  between 
the  nuclear  activity  and  the  formative  energy  of  the  cytoplasm. 


340  CELL-CHEMISTRY  AND    CELL-PHYSIOLOGY 

If  these  facts  are  considered  in  the  Hght  of  the  known  staining- 
reaction  of  the  nuclein  series,  we  must  admit  that  the  following  con- 
clusions are  something  more  than  mere  possibilities.  We  may  infer 
that  the  original  chromosomes  contain  a  high  percentage  of  nucleinic 
acid ;  that  their  growth  and  loss  of  staining-power  is  due  to  a  combi- 
nation with  a  large  amount  of  albuminous  substance  to  form  a 
lower  member  of  the  nuclein  series,  probably  a  nucleo-proteid ;  that 
their  final  diminution  in  size  and  resumption  of  staining-power  is 
caused  by  a  giving  up  of  the  albumin  constituent,  restoring  the 
nuclein  to  its  original  state  as  a  preparation  for  di\'ision.  The  growth 
and  diminished  staining-capacity  of  the  chromatin  occurs  during  a 
period  of  intense  constructive  activity  in  the  cytoplasm  ;  its  diminu- 
tion in  bulk  and  resumption  of  staining-capacity  coincides  with  the 
cessation  of  this  activity.  This  result  is  in  harmony  with  the  obser- 
vations of  Schwarz  and  Zacharias  on  growing  plant-cells,  the  per- 
centage of  nuclein  in  the  nuclei  of  embryonic  cells  (meristem)  being 
at  first  relatively  large  and  diminishing  as  the  cells  increase  in  size. 
It  agrees  further  with  the  fact  that  of  all  forms  of  nuclei  those  of  the 
spermatozoa,  in  which  growth  is  suspended,  are  richest  in  nucleinic 
acid,  and  in  this  respect  stand  at  the  opposite  extreme  from  the  nuclei 
of  the  rapidly  growing  egg-cell. 

Accurately  determined  facts  in  this  direction  are  still  too  scanty  to 
admit  of  a  safe  generalization.  They  are,  however,  enough  to  indi- 
cate the  probability  that  chromatin  passes  through  a  certain  cycle 
in  the  life  of  the  cell,  the  percentage  of  albumin  or  of  albumin-radicals 
increasing  during  the  vegetative  activity  of  the  nucleus,  decreasing  in 
its  reproductive  phase.  In  other  words,  a  combination  of  albumin 
with  nuclein  or  nucleinic  acid  is  an  accompaniment  of  constructive 
metabolism.  As  the  cell  prepares  for  division,  the  combination  is 
dissolved  and  the  nuclein-radicle  or  nucleinic  acid  is  handed  on  by 
division  to  the  daughter-cells.  A  tempting  hypothesis,  suggested 
by  Mathews  on  the  basis  of  Kossel's  work,  is  that  nuclein,  or  one  of 
its  constituent  molecular  groups,  may  in  a  chemical  sense  be  regarded 
as  the  formative  centre  of  the  cell  which  is  directly  involved  in  the 
process  by  which  food-matters  are  built  up  into  the  cell-substance. 
Could  this  be  established,  we  should  have  not  only  a  clear  light  on 
the  changes  of  staining-reactions  during  the  cycle  of  cell-life,  but  also 
a  clue  to  the  nuclear  "  control "  of  the  cell  through  the  process  of 
synthetic  metabolism.  This  hypothesis  fits  well  with  the  conclusions 
of  other  physiological  chemists  that  the  nucleus  is  especially  con- 
cerned in  synthetic  metabolism.  Kossel  concludes  that  the  formation 
of  new  organic  matter  is  dependent  on  the  nucleus,^  and  that  nuclein 
in  some  manner  plays  a  leading  role  in  this  process ;  and  he  makes 

1  Schiefferdecker  and  Kossel,  Gewebelehre,  p.  57. 


PHYSIOLOGICAL   RELATLONS   OF  NUCLEUS  AND    CYTOPLASM      341 

some  interesting  suggestions  regarding  the  synthesis  of  complex 
organic  matters  in  the  living  cell  with  nuclein  as  a  starting-point. 
Chittenden,  too,  in  a  review  of  recent  chemico-physiological  dis- 
coveries regarding  the  cell,  concludes  :  "  The  cell-nucleus  may  be 
looked  upon  as  in  some  manner  standing  in  close  relation  to  those 
processes  which  have  to  do  with  the  formation  of  organic  substances. 
Whatever  other  functions  it  may  possess,  it  evidently,  through  the 
inherent  qualities  of  the  bodies  entering  into  its  composition,  has  a 
controlling  power  over  the  metaboUc  processes  in  the  cell,  modifying 
and  regulating  the  nutritional  changes  "  ('94). 

These  conclusions,  in  their  turn,  are  in  harmony  with  the  hypothesis 
advanced  twenty  years  ago  by  Claude  Bernard  ('78),  who  maintained 
that  the  cytoplasm  is  the  seat  of  destructive  metabolism,  the  nucleus 
the  organ  of  constructive  metabolism  and  organic  synthesis,  and 
insisted  that  the  role  of  the  nucleus  in  nutrition  gives  the  key  to  its 
significance  as  the  organ  of  development,  regeneration,  and  inheri- 
tance.^ 

B.     Physiological  Relations  of  Nucleus  and  Cytoplasm 

How  nearly  the  foregoing  facts  bear  on  the  problem  of  the  mor- 
phological formative  power  of  the  cell  is  obvious ;  and  they  have  in  a 
measure  anticipated  certain  conclusions  regarding  the  role  of  nucleus 
and  cytoplasm,  which  we  may  now  examine  from  a  somewhat  differ- 
ent point  of  view. 

Briicke  long  ago  drew  a  clear  distinction  between  the  chemical  and 
molecular  composition  of  organic  substances,  on  the  one  hand,  and, 
on  the  other  hand,  their  definite  grouping  in  the  cell  b\'  which  arises 
organization  in  a  morphological  sense.  Claude  Bernard,  in  like  man- 
ner, distinguished  between  eheniical  synthesis,  through  which  organic 
matters  are  formed,  and  morphological  synthesis,  by  which  the)'  are 
built  into  a  specifically  organized  fabric ;  but  he  insisted  that  these 
two  processes  are  but  different  phases  or  degrees  of  the  same  phe- 
nomenon, and  that  both  are  expressions  of  the  nuclear  activity.  We 
have  now  to  consider  some  of  the  evidence  that  the  power  of  mor- 
phological, as  well  as  of  chemical,  synthesis  centres  in  the  nucleus, 
and  that  this  is  therefore  to  be  regarded  as  the  especial  organ  of 
inheritance.  This  evidence  is  mainly  derived  from  the  comparison 
of  nucleated  and  non-nucleated  masses  of  protoplasm  ;  from  the  form, 

1  "  II  semble  done  que  la  cellule  qui  a  perdu  son  noyau  soit  sterilisee  au  point  de  vue  de 
la  generation,  c'est  a  dire  de  la  synthese  morphologique,  et  qu'elle  le  soit  aussi  au  point  de 
vue  de  la  synthese  chimique,  car  elle  cesse  de  produire  des  principes  immediats,  et  ne  peut 
guere  qu'oxyder  et  detruire  ceux  qui  s'y  etaient  accumules  par  une  elaliDiation  anterieure  du 
noyau.  II  semble  done  que  le  noyau  soit  \&  gernie  de  nutrition  de  la  cellule  :  il  attire  autour 
de  lui  et  elabore  les  materiaux  nutritifs  "  ('78,  p.  523). 


342 


CELL-CHEMISTRY  AND    CELL-PHYSIOLOGY 


position,  and  movements  of  the  nucleus  in  actively  growing  or  metab- 
olizing cells ;  and  from  the  history  of  the  nucleus  in  mitotic  cell- 
division,  in  fertilization,  and  in  maturation. 


I.    Experiments  on  Unicellnlar  Organisms 

Brandt  i^yj)  long  since  observed  that  enucleated  fragments  of  Acti- 
nosphcerium  soon  die,  while  nucleated  fragments  heal  their  wounds 

and  continue  to  live.  The 
first  decisive  comparison  be- 
tween nucleated  and  non-nu- 
cleated masses  of  protoplasm 
was,  however,  made  by  Moritz 
Nussbaum  in  1884  in  the  case 
of  an  infusorian,  OxytricJia. 
If  one  of  these  animals  be 
cut  into  two  pieces,  the  sub- 
sequent behaviour  of  the  two 
fragments  depends  on  the 
presence  or  absence  of  the 
nucleus  or  a  nuclear  frag- 
ment. The  nucleated  frag- 
ments quickly  heal  the  wound, 
regenerate  the  missing  por- 
tions, and  thus  produce  a 
perfect  animal.  On  the  other 
hand,  enucleated  fragments, 
consisting  of  cytoplasm  only, 
quickly  perish.  Nussbaum 
therefore  drew  the  conclusion 
that  the  nucleus  is  indispens- 
able for  the  formative  energy 
of  the  cell.  The  experiment 
was  soon  after  repeated  by  Gruber('85)in  the  case  of  Stentor^  another 
infusorian,  and  with  the  same  result  (Fig.  159).  Fragments  possess- 
ing a  large  fragment  of  the  nucleus  completely  regenerated  within 
twenty-four  hours.  If  the  nuclear  fragment  were  smaller,  the  re- 
generation proceeded  more  slowly.  If  no  nuclear  substance  were 
present,  no  regeneration  took  place,  though  the  wound  closed  and 
the  fragment  lived  for  a  considerable  time.  The  only  exception  — 
but  it  is  a  very  significant  one  —  was  the  case  of  individuals  in  which 
the  process  of  normal  fission  had  begun  ;  in  these  a  non-nucleated 
fragment  in  which  the  formation  of  a  new  peristome  had  already  been 
initiated  healed  the  wound  and  completed  the  formation  of  the  peri- 


Fig.  158.  —  Stylonychia,  and  enucleated  irag- 
ments.     [Verworn.] 

At  the  left  an  entire  animal,  showing  planes  of 
section.  The  middle  piece,  containing  two  nuclei, 
regenerates  a  perfect  animal.  The  enucleated  pieces, 
shown  at  the  right,  swim  about  for  a  time,  but  finally 
perish. 


PHYSIOLOGICAL   RELATIONS   OF  NUCLEUS  AND    CYTOPLASM      343 

stome.  Lillie  ('96)  has  recently  found  that  Steiitor  may  by  shaking 
be  broken  into  fragments  of  all  sizes,  and  that  nucleated  fragments 
as  small  as  2V  the  volume  of  the  entire  animal  are  still  capable  of 
complete  regeneration.     All  non-nucleated  fragments  perish. 

These  studies  of  Nussbaum  and  Gruber  formed  a  prelude  to  more 
extended  investigations  in  the  same  direction  by  Gruber,  Balbiani, 
Hofer,  and  especially  Verworn  Verworn  {''^^)  proved  that  in  /V/r- 
stomclla,  one  of  the  Foraminifera,  nucleated  fragments  are  able  to 


B 


C 


Fig.  159.  —  Regeneration  in  the  unicellular  animal  Stentor.       [From  GRUBER  after  BALBl.\Nr.] 

A.  Animal  divided  into  three  pieces,  each  containing  a  fragment  of  the  nucleus.  B.  The  three 
fragments  shortly  afterward.  C.  The  three  fragments  after  twenty-four  hours,  each  regenerated 
to  a  perfect  animal. 

repair  the  shell,  while  non-nucleated  fragments  lack  this  power. 
Balbiani  ('89)  showed  that  although  non-nucleated  fragments  of 
Infusoria  had  no  power  of  regeneration,  they  might  nevertheless 
continue  to  live  and  swim  actively  about  for  many  days  after  the 
operation,  the  contractile  vacuole  pulsating  as  usual.  Hofer  ('89), 
experimenting  on  Auuvba,  found  that  non-nucleated  fragments  might 
live  as  long  as  fourteen  days  after  the  operation  (Fig.  160).  Their 
movements  continued,  but  were  somewhat  modified,  and  little  by 
little  ceased,  but  the  pulsations  of  the  contractile  vacuole  were  but 
slightly  affected;  they  lost  more  or  less  completely  the  capacity  to 


344 


CELL-CHEMISTRY  AND   CELL-PHYSIOLOGY 


digest  food,  and  the  power  of  adhering  to  the  substratum.  Nearly 
at  the  same  time  Verworn  ('89)  pubUshed  the  results  of  an  extended 
comparative  investigation  of  various  Protozoa  that  placed  the  whole 
matter  in  a  very  clear  light.  His  experiments,  while  fully  confirming 
the  accounts  of  his  predecessors  in  regard  to  regeneration,  added 
many  extremely  important  and  significant  results.  Non-nucleated 
fragments  both  of  Infusoria  {e.g.  Lachrymaria)  and  rhizopods  {Poly^ 


/  .>  i  •■•'  »1" "' '  ''•*•'*  ^' ' 

!.■■■'::•':•!■'''-■::■/ 


c 


N.T'X  V."..\ 


% 

SC? 


^^\ 


\>.;.\ 


"."N 


■  >i''.  v^  " 


/:■:> — v''v>^vV  •.••/■':N..-'.'"^...v>it  / 


\. 


;..i:; 


Fig.  160.  —  Nucleated  and  non-nucleated  fragments  of  A?na;ba.     [HOFER.] 
A.  B.  An  Amceba  divided  into  nucleated  and  non-nucleated  halves,  five  minutes  after  the  opera- 
tion.    C.  D.  The  two  halves  after  eight  davs,  each  containing  a  contractile  vacuole. 


stomcUa,  TJialassicolla)  not  only  live  for  a  considerable  period,  but 
perform  perfectly  normal  and  characteristic  movements,  show  the 
same  susceptibility  to  stimulus,  and  have  the  same  power  of  ingulf- 
ing food,  as  the  nucleated  fragments.  TJicy  lack,  hoivever,  the  power 
of  digestion  and  secretion.  Ingested  food-matters  may  be  slightly 
altered,  but  are  never  completely  digested.  The  non-nucleated  frag- 
ments are  unable  to  secrete  the   material  for  a  new  shell  {Polysto- 


PHYSIOLOGICAL  RELATIONS  OF  NUCLEUS  AND    CYTOPLASM 


345 


mella)  or  the  slime  by  which  the  animals  adhere  to  the  substratum 
{^Amoeba,  Difflugia,  Polystomclla).  Beside  these  results  should  be 
placed  the  well-known  fact  that  dissevered  nerve-fibres  in  the  higher 
animals  are  only  regenerated  from  that  end  which  remains  in  con- 
nection with  the  nerve-cell,  while  the  remaining  portion  invariably 
degenerates. 


A 


u 


C 


D 


Fig.  i6i.  —  Formation  of  membranes  by  protoplasmic  fragments  of  plasmolyzed  cells.  [Town- 
send.] 

A.  Plasmolvzed  cell,  leaf-hair  of  Cucitrbita,  showing  protoplasmic  balls  connected  by  strands. 

B.  Calyx-hair  of   Gaillardia ;    nucleated  fragment   with   membrane,   non-nucleated    one    naked. 

C.  Root-hair  of  Marchantia  ;  all  the  fragments,  connected  by  protoplasmic  strands,  have  formed 
membranes.  D.  Leaf-hair  of  Cijcurbita ;  non-nucleated  fragment,  with  membrane,  connected 
with  nucleated  fragment  of  adjoining  cell. 

These  beautiful  observations  prove  that  destructive  metabolism,  as 
manifested  by  coordinated  forms  of  protoplasmic  contractility,  may 
go  on  for  some  time  undisturbed  in  a  mass  of  cytoplasm  deprived  of 
a  nucleus.  On  the  other  hand,  the  building  up  of  new  chemical  or 
morphological  products  by  the  cytoplasm  is  only  initiated  in  the 
presence  of  a  nucleus  and  soon  ceases  in  its  absence.  These  facts 
form  a  complete  demonstration  that  the  nucleus  plays  an  essential 


346  CELL-CHEMISTRY  AND    CELL-PHYSIOLOGY 

part  not  only  in  the  operations  of  synthetic  metaboUsm  or  chemical 
synthesis,  but  also  in  the  viorpJiological  determinatioji  of  these  opera- 
tions, i.e.  the  morphological  synthesis  of  Bernard  —  a  point  of  capital 
importance  for  the  theory  of  inheritance,  as  will  appear  beyond. 

Convincing  experiments  of  the  same  character  and  leading  to  the 
same  result  have  been  made  on  the  cells  of  plants.  Francis  Darwin 
(^Ty)  observed  more  than  twenty  years  ago  that  movements  actively 
continued  in  protoplasmic  filaments,  extruded  from  the  leaf-hairs  of 
Dipsaciis,  that  were  completely  severed  from  the  body  of  the  cell. 
Conversely,  Klebs  ('79)  soon  afterward  showed  that  naked  proto- 
plasmic fragments  of  Vaucheria  and  other  algae  were  incapable  of 
forming  a  new  cellulose  membrane  if  devoid  of  a  nucleus ;  and  he 
afterward  showed  i^^y^  that  the  same  is  true  of  Zygnema  and  CEdo- 
gonium.  By  plasmolysis  the  cells  of  these  forms  may  be  broken  up 
into  fragments,  both  nucleated  and  non-nucleated.  The  former  sur- 
round themselves  with  a  new  wall,  grow,  and  develop  into  complete 
plants ;  the  latter,  while  able  to  form  starch  by  means  of  the  chloro- 
phyll they  contain,  are  incapable  of  utilizing  it,  and  are  devoid  of  the 
power  of  forming  a  new  membrane,  and  of  growth  and  regeneration. 
A  beautiful  confirmation  of  this  is  given  by  Townsend  ('97),  who  finds 
in  the  case  of  root-hairs  and  pollen-tubes,  that  when  the  protoplasm  is 
thus  broken  up,  a  membrane  may  be  formed  by  both  nucleated  and 
non-nucleated  fragments,  by  the  latter  however  only  ivJien  they  remaiji 
cofinected  with  the  nucleated  masses  by  protoplasmic  strands,  however 
fine.  If  these  strands  be  broken,  the  membrane-forming  power  is 
lost.  Of  very  great  interest  is  the  further  observation  (made  on  leaf- 
hairs  in  Qicnrbita)  that  the  influence  of  the  nucleus  may  thus  extend 
from  cell  to  cell,  an  enucleated  fragment  of  one  cell  having  the  power 
to  form  a  membrane  if  connected  by  intercellular  bridges  with  a 
nucleated  fragment  of  an  adjoining  cell  (Fig.  161). 

2.    Position  and  Movements  of  the  Nuclens 

Many  observers  have  approached  the  same  problem  from  a  dif- 
ferent direction  by  considering  the  position,  movements,  and  changes 
of  form  in  the  nucleus  with  regard  to  the  formative  activities  in  the 
cytoplasm.  To  review  these  researches  in  full  would  be  impossible, 
and  we  must  be  content  to  consider  only  the  well-known  researches 
of  Haberlandt  i^'J'j)  and  Korschelt  ('89),  both  of  whom  have  given 
extensive  reviews  of  the  entire  subject  in  this  regard.  Haberlandt's 
studies  related  to  the  position  of  the  nucleus  in  plant-cells  with 
especial  regard  to  the  growth  of  the  cellulose  membrane.  He  deter- 
mined the  very  significant  fact  that  local  growth  of  the  cell-wall  is 
always  preceded  by  a  movement  of  the  nucleus  to  the  point  of  growth. 
Thus,  in  the  formation  of  epidermal  cells,  the  nucleus  lies  at  first  near 


PHYSIOLOGICAL   RELATLONS   OF  NUCLEUS  AND    CYTOPLASM 


347 


the  centre,  but  as  the  outer  wall  thickens,  the  nucleus  moves  toward 
it,  and  remains  closely  applied  to  it  throughout  its  growth,  after  which 
the  nucleus  often  moves  into  another  part  of  the  cell  (Fig.  162,  A,  B). 
That  this  is  not  due  simply  to  a  movement  of  the  nucleus  toward  the 
air  and  light  is  beautifully  shown  in  the  coats  of  certain  seeds,  where 
the  nucleus  moves,  not  to  the  outer,  but  to  the  inner  wall  of  the  cell 
and  here  the  thickening  takes  place  (Fig.  162,  C).     The  same  position 


\    k 


\  \  A 


A 


B 


C 

Fig.  162.  —  Position  of  the  nuclei  in  growing  plant-cells.     [Haherlandt.] 
A.  Young  epidermal  cell  oi  Luzula  with  central  nucleus,  before  thickening  of  the  membrane. 
B.  Three  epidermal  cells  of  Monstera,  during  the  thickening  of  the  outer  wall.     C.  Cell  from  the 
seed-coat  of  Scopulina,  during  the  thickening  of  the  inner  wall.     D.  E.  Position  of  the  nuclei  dur- 

ing  the  formation  of  branches  in  the  root-hairs  of  the  pea. 


of  the  nucleus  is  shown  in  the  thickening  of  the  walls  of  the  guard- 
cells  of  stomata,  in  the  formation  of  the  peristome  of  mosses,  and  in 
many  other  cases.  In  the  formation  of  root-hairs  in  the  pea.  the  pri- 
mary outgrowth  always  takes  place  from  the  immediate  neighbourhood 
of  the  nucleus,  which  is  carried  outward  and  remains  near  the  tip  of 
the  growing  hair  (Fig.  162,  D,  E).  The  same  is  true  of  the  rhizoids 
of  fern-prothallia  and  liverworts.       In  the  hairs  of  aerial   plants  this 


348  CELL-CHEMISTRY  AND    CELL-PHYSIOLOGY 

rule  is  reversed,  the  nucleus  lying  near  the  base  of  the  hair ;  but  this 
apparent  exception  proves  the  rule,  for  both  Hunter  and  Haberlandt 
show  that  in  this  case  growth  of  the  hair  is  not  apical,  but  proceeds 
from  the  base !  V^ery  interesting  is  Haberlandt's  observation  that  in 
the  regeneration  of  fragments  of  VaiicJieria  the  growing  region,  where 
a  new  membrane  is  formed,  contains  no  chlorophyll,  but  numerous 
nuclei.  The  general  result,  based  on  the  study  of  a  large  number  of 
cases,  is,  in  Haberlandt's  words,  that  **the  nucleus  is  in  most  cases 
placed  in  the  neighbourhood,  more  or  less  immediate,  of  the  points  at 
which  growth  is  most  active  and  continues  longest."  This  fact  points 
to  the  conclusion  that  ''  its  function  is  especially  connected  with  the 
developmental  processes  of  the  cell,"  ^  and  that  "in  the  growth  of  the 
cell,  more  especially  in  the  growth  of  the  cell-wall,  the  nucleus  plays 
a  definite  part." 

Korschelt's  work  deals  especially  with  the  correlation  between  form 
and  position  of  the  nucleus  and  the  nutrition  of  the  cell,  and  since  it 
bears  more  directly  on  chemical  than  on  morphological  synthesis,  may 
be  only  briefly  reviewed  at  this  point.  His  general  conclusion  is  that 
there  is  a  definite  correlation,  on  the  one  hand,  between  the  position  of 
the  nucleus  and  the  source  of  food-supply,  on  the  other  hand,  between 
the  size  of  the  nucleus  and  the  extent  of  its  surface  and  the  elabora- 
tion of  material  by  the  cell.  In  support  of  the  latter  conclusion  many 
cases  are  brought  forward  of  secreting  cells  in  which  the  nucleus  is  of 
enormous  size  and  has  a  complex  branching  form.  Such  nuclei  occur, 
for  example,  in  the  silk-glands  of  various  lepidopterous  larvae  (Meckel, 
Zaddach,  etc.),  which  are  characterized  by  an  intense  secretory  activity 
concentrated  into  a  very  short  period.  Here  the  nucleus  forms  a 
labyrinthine  network  (Fig.  14,  E),  by  which  its  surface  is  brought  to  a 
maximum,  pointing  to  an  active  exchange  of  material  between  nucleus 
and  cytoplasm.  The  same  type  of  nucleus  occurs  in  the  Malpighian 
tubules  of  insects  (Leydig,  R.  Hertwig),  in  the  spinning-glands  of 
amphipods  (Mayer),  and  especially  in  the  nutritive  cells  of  the  insect 
ovary  already  referred  to  at  page  151.  Here  the  developing  ovum  is 
accompanied  and  surrounded  by  cells,  w^hich  there  is  good  reason  to 
believe  are  concerned  with  the  elaboration  of  food  for  the  egg-cell. 
In  the  earwig  Forficiila  each  ^gg  is  accompanied  by  a  single  large 
nutritive  cell  (Fig.  163),  which  has  a  very  large  nucleus  rich  in  chro- 
matin (Korschelt).  This  cell  increases  in  size  as  the  ovum  grows,  and 
its  nucleus  assumes  the  complex  branching  form  shown  in  the  figure. 
In  the  butterfly  Vanessa  there  is  a  group  of  such  cells  at  one  pole 
of  the  ^g,g,  from  which  the  latter  is  believed  to  draw  its  nutriment 
(Fig.  jf).  A  very  interesting  case  is  that  of  the  annelid  OpJuyotrocJia, 
referred  to  at  page  151.     Here,  as  described  by  Korschelt,  the  ^gg  floats 

1  I.e.,  p.   99. 


PHYSIOLOGICAL  RELATIONS   OF  NUCLEUS  AND    CYTOPLASM      349 


in  the  perivisceral  fluid,  accompanied  by  a  nurse-cell  having  a  very 
large  chromatic  nucleus,  while  that  of  the  ^gg  is  smaller  and  poorer 
in  chromatin.  Astheecro: 
completes  its  growth,  the 
nurse-cell  dwindles  away 
and  finally  perishes  (Fig. 
jG).  In  all  these  cases 
it  is  scarcely  possible  to 
doubt  that  the  ^gg  is  in  a 
measure  relieved  of  the 
task  of  elaborating  cyto- 
plasmic products  by  the 
nurse-cell,  and  that  the 
great  development  of 
the  nucleus  in  the  latter 
is  correlated  with  this 
function. 

Regarding  the  posi- 
tion and  movements  of 
the  nucleus,  Korschelt 
reviews  many  facts 
pointing  toward  the 
same  conclusion.  Per- 
haps the  most  sugges- 
tive of  these  relate  to 
the  nucleus  of  the  e^s: 
during  its  ovarian  his- 
tory. In  many  of  the 
insects,  as  in  both  the 
cases  referred  to  above, 
the  egg-nucleus  at  first 
occupies  a  central  posi- 


g 


n 


Fig.  163. —  Upper  portion  of  the  ovary  in  the  earw  ig /l?/-- 
Jiciila,  showing  eggs  and  nurse-cells.     [KoRSCHELT.] 

Below,  a  portion  of  the  nearly  ripe  egg  {e),  showing  deuto- 
plasm-spheres  and  germinal  vesicle  {g.v.).  Above  it  lies  the 
nurse-cell  («)  with  its  enormous  branching  nucleus.  Two  suc- 
cessively younger  stages  of  egg  and  nurse  are  shown  above. 


tion,  but  as  the  Qgg  be- 
gins to  grow,  it  moves  to  the  periphery  on  the  side  turned  toward 
the  nutritive  cells.  The  same  is  true  in  the  ovarian  eggs  of  some  other 
animals,  good  examples  of  which  are  afforded  by  various  coelenterates, 
^.^.  in  medusae  (Claus,  Hertwig)  and  actinians  ( Korschelt,  Hertwig), 
where  the  germinal  vesicle  is  always  near  the  point  of  attachment  of 
the  ^gg.  Most  suggestive  of  all  is  the  case  of  the  water-beetle  Dytis- 
ciis,  in  which  Korschelt  was  able  to  observe  the  movements  and  changes 
of  form  in  the  living  object.  The  eggs  here  lie  in  a  single  series  alter- 
nating with  chambers  of  nutritive  cells.  The  latter  contain  granules 
which  are  believed  by  Korschelt  to  pass  into  the  ^gg,  perhaps  bodily, 
perhaps  by  dissolving  and  entering  in  a  liquid  form.     At  all  events, 


350 


CELL-CHEMISTRY  AND    CELL-PHYSIOLOGY 


the  tg^  contains  accumulations  of  similar  granules,  which  extend 
inward  in  dense  masses  from  the  nutritive  cells  to  the  germinal  vesi- 
cle, which  they  may  more  or  less  completely  surround.  The  latter 
meanwhile  becomes  amoeboid,  sending  out  long  pseudopodia,  which 
are  always  directed  toward  the  principal  mass  of  granules  (Fig.  jj). 
The  granules  could  not  be  traced  into  the  nucleus,  but  the  latter  grows 
rapidly  during  these  changes,  proving  that  matter  must  be  absorbed 
by  it,  probably  in  a  liquid  form.^ 

Among  other  facts  pointing  in  the  same  direction  may  be  mentioned 
Miss  Huie's  ('97)  observations  on  the  gland-cells  of  Drosera,  and  those 
of  Mathews  ('99)  on  the  changes  of  the  pancreas-cell  in  Nect7irus. 
Stimulus  of  the  gland-cells  in  the  leaf  of  Drosera  causes  a  rapid 
exhaustion  and  change  of  staining-capacity  in  the  cytoplasm.  During 
the  ensuing  repose  the  cytoplasm  is  rebuilt  out  of  material  laid  down 
immediately  around  the  nucleus,  and  agreeing  closely  in  appearance 
and  staining-reaction  with  the  achromatic  nuclear  constituents.  The 
chromatin  increases  in  bulk  during  a  period  preceding  the  constructive 
phase,  but  decreases  (while  the  nucleolar  material  increases)  as  the 
cytoplasm  is  restored.  In  the  pancreas-cell,  as  has  long  been  known, 
the  "loaded"  cell  (before  secretion)  is  filled  with  metaplasmic  zymo- 
gen-granules,  which  disappear  during  secretion,  the  cell  meanwhile 
becoming  filled  with  protoplasmic  fibrils  (Fig.  18).  During  the  ensu- 
ing period  of  "rest"  the  zymogen-granules  are  re-formed  at  the 
expense  of  the  fibrillar  material,  which  is  finally  found  only  at  the 
base  of  the  cell  near  the  nucleus.  Upon  discharge  of  the  secretion 
(granule-material)  the  fibrillse  again  advance  from  the  nucleus  toward 
the  periphery.  Mathews  shows  that  many  if  not  all  of  them  may  be 
traced  at  one  end  actually  into  the  nuclear  wall,  and  concludes  that 
they  are  directly  formed  by  the  nucleus. 

Beside  the  foregoing  facts  may  be  placed  the  strong  evidence 
reviewed  at  pages  156-158,  indicating  the  formation  of  the  yolk-nu- 
cleus, and  indirectly  of  the  yolk-material,  by  the  nucleus.  All  of  these 
and  a  lars^e  number  of  other  observations  in  the  same  direction  lead  to 
the  conclusion  that  the  cell-nucleus  plays  an  active  part  in  nutrition, 
and  that  it  is  especially  active  during  the  constructive  phases.  On  the 
whole,  therefore,  the  behaviour  of  the  nucleus  in  this  regard  is  in  har- 
mony with  the  result  reached  by  experiment  on  the  one-celled  forms, 
though  it  gives  in  itself  a  far  less  certain  and  convincing  result. ^ 

1  Mention  may  conveniently  here  be  made  of  Richard  Hertwig's  interesting  observation 
that  in  starved  individuals  of  Actinospluvriwu  the  chromatin  condenses  into  a  single  mass, 
while  in  richly  fed  animals  it  is  divided  into  fine  granules  scattered  through  the  nucleus 

('98,  p.  8). 

-  Loeb  ('98,  '99)  makes  the  interesting  suggestion  that  the  nucleus  is  especially  con- 
cerned in  the  oxydative  processes  of  the  cell,  and  that  this  is  the  key  to  its  7-dle  in  the  syn- 
thetic process.      It  has   been   shown  that   oxydations    in  the   living    tissues   are    probably 


PHYSIOLOGICAL  RELATIONS   OF  NUCLEUS  AND    CYTOPLASM      -^  ti 

We  now  turn  to  evidence  which,  though  less  direct  than  the  above, 
is  scarcely  less  convincing.  This  evidence,  which  has  been  exhaus- 
tively discussed  by  Hertwig,  Weismann,  and  Strasburger,  is  drawn 
from  the  history  of  the  nucleus  in  mitosis,  fertilization,  and  matura- 
tion. It  calls  for  only  a  brief  review  here,  since  the  facts  have  been 
fully  described  in  earlier  chapters. 

3.     TJie  Nucleus  in  Mitosis 

To  Wilhelm  Roux  i^^i)  we  owe  the  first  clear  recognition  of  the 
fact  that  the  transformation  of  the  chromatic  substance  during  mitotic 
division  is  manifestly  designed  to  effect  a  precise  division  of  all  its 
parts,  —  i.e.  a  panmeristic  division  as  opposed  to  a  mere  mass-division, 
—  and  their  definite  distribution  to  the  daughter-cells.  "The  essential 
operation  of  nuclear  division  is  the  division  of  the  mother-granules  " 
{i.e.  the  individual  chromatin-grains) ;  "all  the  other  phenomena  are 
for  the  purpose  of  transporting  the  daughter-granules  derived  from 
the  division  of  a  mother-granule,  one  to  the  centre  of  one  of  the 
daughter-cells,  the  other. to  the  centre  of  the  other."  In  this  respect 
the  nucleus  stands  in  marked  contrast  to  the  cytoplasm,  which  under- 
goes on  the  whole  a  mass-division,  although  certain  of  its  elements, 
such  as  the  plastids  and  the  centrosome,  may  separately  divide,  like 
the  elements  of  the  nucleus.  From  this  fact  Roux  argued,  first,  that 
different  regions  of  the  nuclear  substance  must  represent  different 
qualities,  and  second,  that  the  apparatus  of  mitosis  is  designed  to 
distribute  these  qualities,  according  to  a  definite  law,  to  the  daughter- 
cells.  The  particular  form  in  which  Roux  and  Weismann  developed 
this  conception  has  now  been  generally  rejected,  and  in  any  form  it 
has  some  serious  difficulties  in  its  way.  We  cannot  assume  a  precise 
localization  of  chromatin-elements  in  all  parts  of  the  nucleus ;  for  on 
the  one  hand  a  large  part  of  the  chromatin  may  degenerate  or  be  cast 
out  (as  in  the  maturation  of  the  ^ZZ),  and  on  the  other  hand  in  the 
Protozoa  a  small  fragment  of  the  nucleus  is  able  to  regenerate  the 
whole.  Nevertheless,  the  essential  fact  remains,  as  Hertwig,  Kolliker, 
Strasburger,  De  Vries,  and  many  others  have  insisted,  that  in  mitotic 
cell-division  the  chromatin  of  the  mother-cell  is  distributed  with  the 
most  scrupulous  equality  to  the  nuclei  of  the  daughter-cells,  and  that 
in  this  regard  there  is  a  most  remarkable  contrast  between  nucleus 
and    cytoplasm.     This    holds    true   with    such    wonderful    constancy 

dependent  upon  certain  substances  (oxydation  ferments)  that  in  some  manner,  not  vet 
clearly  understood,  facilitate  the  process;  and  the  work  of  Spitzer  ('97)  has  shown  that 
these  substances  (obtained  from  tissue-extracts)  belong  to  the  group  of  nucleo-proteids, 
which  are  characteristic  nuclear  substances.  The  view  thus  suggested  opens  a  further  way 
toward  more  exact  inquiry  into  the  nuclear  functions,  though  it  is  not  to  be  supposed  that 
the  nucleus  is  the  sole  oxydative  centre  of  the  cell,  as  is  obvious  from  the  prolonged  activity 
of  non-nucleaied  protoplasmic  masses. 


352 


CELL-CHEMISTRY  AND    CELL-PHYSIOLOGY 


throughout  the  series  of  living  forms,  from  the  lowest  to  the  highest, 
that  it  must  have  a  deep  significance.  And  while  we  are  not  yet  in 
a  position  to  grasp  its  full  meaning,  this  contrast  points  unmistakably 
to  the  conclusion  that  the  most  essential  material  handed  on  by  the 
mother-cell  to  its  progeny  is  the  chromatin,  and  that  this  substance 
therefore  has  a  special  significance  in  inheritance. 

4.    The  Ahicleus  in  Fertilization 

The  foregoing  argument  receives  an  overwhelming  reenforcement 
from  the  facts  of  fertilization.     Although  the  ovum  supplies  nearly 

all  the  cytoplasm  for  the  embry- 
onic body,  and  the  spermatozoon 
at  most  only  a  trace,  the  latter  is 
nevertheless  as  potent  in  its  effect 
on  the  offspring  as  the  former.  On 
the  other  hand,  the  nuclei  con- 
tributed by  the  germ-cells,  though 
apparently  different,  become  in 
the  end  exactly  equivalent  in  every 
visible  respect  —  in  structure,  in 
staining-reactions,  and  in  the  num- 
ber and  form  of  the  chromosomes 
to  which  each  gives  rise.  But 
furthermore  the  substance  of  the 
two  germ-nuclei  is  distributed  with 
absolute  equality,  certainly  to  the 
first  two  cells  of  the  embrvo,  and 
probably  to  all  later-formed  cells. 
The  latter  conclusion,  which  long 
remained  a  mere  surmise,  has  been 
rendered  nearly  a  certainty  by 
the  remarkable  observations  of 
Ruckert,  Zoja,  and  Hacker,  de- 
scribed in  Chapters  IV.  and  VI. 
We  must  therefore  accept  the  high 
probability  of  the  conclusion  that 
the  specific  character  of  the  cell  is 
in  the  last  analysis  determined  by 
that  of  the  nucleus,  that  is  by  the 
chromatin,  and  that  in  the  equal 
distribution  of  paternal  and  ma- 
egg-fragment  of  Sphcer  echinus  granular  is,  fertil-  tcmal  chromatin  tO  all  the  CClls  of 
ized  with  spermatozoon  of  Echinus  microtuber-  ^j^^  offsprinP"  We  find  the  physio- 
culatus,  ana  showing  purely  paternal  characters.  .       ,1  •  r    ■,       r  \ 

B.  Normal  Pluteus  of  Echinus  microtuberculatus.    loglCal  explanation  of  the  fact  that 


Fig.  164.  —  Normal  and  dwarf  larvae  of  the 
sea-urchin.     [BOVERI.] 

A.  Dwarf  Pluteus  arising  from  an  enucleated 


PHYSIOLOGICAL   RELATIONS   OF  NUCLEUS  AND    CYTOPLASM      353 

every  part  of  the  latter  may  show  the  characteristics  of  cither  or  both 
parents. 

Boveri  ('89,  '95,  i)  has  attempted  to  test  this  conckision  by  a  most 
ingenious  and  beautiful  experiment ;  and  although  his  conclusions  do 
not  rest  on  absolutely  certain  ground,  they  at  least  open  the  way  to 
a  decisive  test.  The  Hertwig  brothers  showed  that  the  eggs  of  sea- 
urchins  might  be  broken  into  pieces  by  shaking,  and  that  spermatozoa 
would  enter  the  enucleated  fragments  and  cause  them  to  segment. 
Boveri  proved  that  such  a  fragment,  if  fertilized  by  a  spermatozoon, 
would  even  give  rise  to  a  dwarf  larva,  indistinguishable  from  the  nor- 
mal in  general  appearance  except  in  size.  The  nuclei  of  such  larvae 
are  considerably  smaller  than  those  of  the  normal  larvae,  and  were 
shown  by  Morgan  ('95,  4)  to  contain  only  half  the  nuDibcr  of  cJiromo- 
somes,  thus  demonstrating  their  origin  from  a  single  sperm-nucleus. 
Now,  by  fertilizing  enucleated  egg-fragments  of  one  species  ( SphcB- 
recJiiniis  granulans)  with  the  spermatozoa  of  another  {Echinus  niicro- 
tuberculatus),  Boveri  obtained  in  a  few  instances  dwarf  Plutei  show- 
ing except  in  size  the  pure  paternal  cJiaracters  {i.e.  those  of  Echinus, 
Fig.  164).  From  this  he  concluded  that  the  maternal  cytoplasm  has 
no  determining  effect  on  the  offspring,  but  supplies  only  the  material 
in  which  the  sperm-nucleus  operates.  Inheritance  is,  therefore,  ef- 
fected by  the  nucleus  alone. 

The  later  studies  of  Seehger  ('94),  Morgan  ('95,  4),  and  Drisch 
('98,  3)  showed  that  this  result  is  not  entirely  conclusive,  since  hybrid 
larvae  arising  by  the  fertilization  of  an  entire  ovum  of  one  species  bv 
a  spermatozoon  of  the  other  show  a  very  considerable  range  of  varia- 
tion ;  and  while  most  such  hybrids  are  intermediate  in  character 
between  the  two  species,  some  individuals  may  nearly  approximate 
to  the  characters  of  the  father  or  the  mother.  Despite  this  fact 
Boveri  ('95,  i)  has  strongly  defended  his  conclusion,  though  admitting 
that  only  further  research  can  definitely  decide  the  question.  It  is 
to  be  hoped  that  this  highly  ingenious  experiment  may  be  repeated 
on  other  forms  which  may  afford  a  decisive  result. 

5.      The  Nucleus  in  Maturation 

Scarcely  less  convincing,  finally,  is  the  contrast  between  nucleus 
and  cytoplasm  in  the  maturation  of  the  germ-cells.  It  is  scarcely 
an  exaggeration  to  say  that  the  whole  process  of  maturation,  in  its 
broadest  sense,  renders  the  cytoplasm  of  the  germ-cells  as  unlike, 
the  nuclei  as  like,  as  possible.  The  latter  undergo  a  scries  of  com- 
plicated changes  which  result  in  a  perfect  equivalence  between  them 
at  the  time  of  their  union,  and,  more  remotely,  a  perfect  equality  of 
distribution  to  the  embryonic   cells.     The   cytoplasm,   on   the   other 

2  A 


354  CELL-CHEMISTRY  AND    CELL-PHYSIOLOGY 

hand,  undergoes  a  special  differentiation  in  each  to  effect  a  second- 
ary division  of  labour  between  the  germ-cells.  When  this  is  corre- 
lated with  the  fact  that  the  germ-cells,  on  the  whole,  have  an  equal 
effect  on  the  specific  character  of  the  embryo,  we  are  again  forced 
to  the  conclusion  that  this  effect  must  primarily  be  sought  in  the 
nucleus,  and  that  the  cytoplasm  is  in  a  sense  only  its  agent. 

C.     The  Centrosome 

Existing  views  regarding  the  functions  of  the  centrosome  may  con- 
veniently be  arranged  in  two  general  groups,  the  first  including  those 
which  regard  this  structure  as  a  relatively  passive  body,  the  second 
those  which  assume  it  to  be  an  active  organ.  To  the  first  belongs  the 
hypothesis  of  Heidenhain  ('94),  accepted  by  Kostanecki  ('97,  i)  and 
some  others,  that  the  centrosome  serves  essentiall}^  as  an  insertion- 
point  for  the  astral  rays  (''organic  radii"),  and  plays  a  relatively 
passive  part  in  the  phenomena  of  mitosis,  the  active  functions  being 
mainly  performed  by  the  surrounding  structures.  To  the  same 
category  belongs  the  view  of  Miss  Foot  that  the  formation  of  the 
centrosome  is,  as  it  were,  incidental  to  that  of  the  aster  —  ''the 
expression,  rather  than  the  cause,  of  cell-activity  "  ('97,  p.  810).  To 
the  second  group  belong  the  views  of  Van  Beneden,  Boveri,  Biitschli, 
Carnoy,  and  others  who  regard  the  centrosome  as  playing  a  more 
active  role  in  the  life  of  the  cell.  Both  of  the  former  authors  have 
assumed  the  centrosomes  to  be  active  centres  by  the  action  of  which 
the  astral  systems  are  organized ;  and  they  are  thus  led  to  the  conclu- 
sion that  the  centrosome  is  essentially  an  organ  for  cell-division  and 
fertilization  (Boveri),  and  in  this  sense  is  the  "dynamic  centre"  of 
the  cell.^  To  Carnoy  and  Biitschli  is  due  the  interesting  suggestion  ^ 
that  the  centrosomes  are  to  be  regarded  further  as  centres  of  cJiemical 
action  to  which  their  remarkable  effect  on  the  cytoplasm  is  due. 
That  the  centrosome  is  an  active  centre,  rather  than  a  passive  body 
or  one  created  by  the  aster-formation,  is  strongly  indicated  by  its 
behaviour  both  in  mitosis  and  in  fertilization.  Griffin  ('96,  '99)  points 
out  that  at  the  close  of  division  in  TJialassema  the  daughter-centro- 
somes  migrate  away  from  the  old  astral  centre  and  incite  about 
themselves  in  a  different  region  the  new  astral  systems  for  the 
ensuing  mitosis  (Figs.  99,  155);  and  similar  conditions  are  described 
by  Coe  in  Cerebratnlns  ('98).  In  fertilization  the  aster-formation  can- 
not be  regarded  as  a  general  action  of  the  cytoplasm,  but  as  a  local 
one  due  to  a  local  stimulus  given  by  something  in  the  spermatozoon ; 
for  in  polyspermy  a  sperm-aster  is  formed  for  every  spermatozoon 
(p.    198).     This    stimulus    is    given    by    something    in    the    middle- 

1    Cf.  pp.   76,   192.  2   Qr  p_   jjQ^ 


THE    CENTROSOME 


355 


piece  (p.  212),  which  is  itself  genetically  related  to  the  centrosome  of 
the  last  cell-generation  (p.  170).  These  facts  seem  explicable  only 
under  the  assumption  that  in  these  cases  the  centrosome,  or  a  sub- 
stance which  it  carries,  gives  an  active  stimulus  to  the  cytoplasm 
which  incites  the  aster-formation  about  itself,  and  in  the  words  of 
Grifhn  ''  disengages  the  forces  at  work  in  mitosis  "  ('96,  p.  174).  For 
these  reasons  I  incline  to  the  view  that  in  the  artificial  aster-formation 
described  by  Morgan  ^  the  centrosomes  there  observed  should  not  be 
regarded  as  the  creations  of  the  asters,  but  rather  as  local  deposits 
of  material  which  incite  the  aster-formation  around  them.  That  the 
centrosomes  or  astral  centres  are  centres  of  division  (whether  active 
or  passive)  is  beautifully  shown  by  Boveri's  interesting  observations 
on  ''partial  fertilization"  referred  to  at  page  194. 

Again,  Boveri  has  observed  that  the  segmenting  ovum  of  Ascaris 
sometimes  contains  a  supernumerary  centrosome  that  does  not  enter 


Fig.  165.  —  Kggs  oi  Ascaris  with  supernumerary  centrosome.     [buVERl.] 
A.  First  cleavage-spindle  above,  isolated  centrosome  below.     B.  Result  of  the  ensuing  division. 


into  connection  with  the  chromosomes,  but  lies  alone  in  the  cytoplasm 
(Fig.  165).  Such  a  centrosome  forms  an  independent  centre  of  divi- 
sion, the  cell  dividing  into  three  parts,  two  of  which  are  normal 
blastomeres,  while  the  third  contains  only  the  centrosome  and  attrac- 
tion-sphere. The  fate  of  such  eggs  was  not  determined,  but  they 
form  a  complete  demonstration  that  it  is  in  this  case  the  centrosome 
and  not  the  nucleus  that  determines  the  centres  of  division  in  the 
cell-body.  Scarcely  less  conclusive  is  the  case  of  dispermic  eggs  in 
sea-urchins.  In  such  eggs  both  sperm-nuclei  conjugate  with  the  egg- 
nucleus,  and  both  sperm-centrosomes  divide  (Fig.  166).  The 
cleavage-nucleus,  therefore,  arises  by  the  union  of  three  nuclei  and 
four  centrosomes.  Such  eggs  divide  at  the  first  cleavage  into  four 
equal  blastomeres,  each  of  which  receives  one  of  the  centrosomes. 

1  Cf.  p.  307. 


356 


CELL-CIIEMJSTRY  AXD    CELL-PHYSIOLOGY 


The  latter  must,  therefore,  be  the  centres  of  division  ;  ^  though  it 
must  not  be  forgotten  that,  in  some  cases  at  any  rate,  normal  division 
requires  the  presence  of  nuclear  matter  (p.  io8). 

The  centrosome  must,  however,  be  something  more  than  a  mere 
division-centre ;  for,  on  the  one  hand,  in  leucocytes  and  pigment-cells 
the  astral  system  formed  about  it  is  devoted,  as  there  is  good  reason 
to  believe,  not  to  cell-division,  but  to  movements  of  the  cell-body  as  a 
whole;  and,  on  the  other  hand,  as  we  have  seen  (pp.  165,  172),  it  is 
concerned  in  the  formation  of  the  flagella  of  the  spermatozoa  and 
spermatozoids,  and  probably  also  in  that  of  cilia  in  epithelial  cells. 
Strasburger  ('97)  was  thus  led  to  the  conclusion  that  the  centrosome 
is  essentially  a  mass  of  kinoplasm,  i.e.  the  active  motor  plasm, ^  and 
a  nearly  similar  view  has  been  adopted  by  several  recent  zoologists. 


Fig.  166.  —  Cleavage  of  dispermic  egg  of  Toxopneustes. 

A.  One  sperm-nucleus  has  united  with  the  egg-nucleus,  shown  at  a.  b. ;  the  other  Hes  above. 
Both  sperm-asters  have  divided  to  form  amphiasters  (a.  b.  and  c.  d.).  B.  The  cleavage-nucleus, 
formed  by  union  of  the  three  germ-nuclei,  is  surrounded  by  the  four  asters.  C.  Result  of  the  first 
cleavage,  the  four  blastomeres  lettered  to  correspond  with  the  four  asters. 


Hennecfuv  concludes  that  the  centrosomes  are  "  motor  centres  of 
the  kinoplasm  "  both  for  external  and  for  internal  manifestations.'^ 
Lenhossek  regards  them  as  "  motors  "  for  the  control  of  ciliary  action 
as  well  as  for  that  of  the  spermatozoon,"*  and  perhaps  also  for  that  of 
muscle-fibrillDe.^  Zimmerman  concludes  that  *'  the  microcentrum  is 
the  motor  centre  of  the  cell,  that  is,  the  *  kinocentrum  '  opposed  to 
the  nucleus  as  the  *  chemocentrum.'"  ^  Regarding  their  control  of 
ciliary  action,  he  makes  the  same  suggestion  as  that  of  Henneguy  and 
Lenhossek  cited  above.  He  adds  the  further  very  interesting  sug- 
gestions that  the  centrosomes  may  be  concerned  with  the  pseudopodial 
movements  in  the  epithelial  cells  of  the  intestine,  and  that  they  may 

1  This  phenomenon  was  first  observed  by  Ilertwig,  and   afterward  by  Driesch.     I  have 
repeatedly  observed  the  internal  changes  in  the  living  eggs  of  Toxopneustes. 

2  cf.  p.  221.  ^  '98.  p- 107-  ^  '98.  p-  697- 


3  '98,  p.  495. 


5  ' 


99.  P-  342. 


THE    CENTROSOME 


357 


also  be  concerned  in  the  protoplasmic  contraction  of  gland-cells  by 
which  the  excretion  is  expelled.  [This  is  based  on  the  fact  that 
the  centrosomes  are  found  in  the  free  (pseudopodia-forming)  ends  of 
the  epitheUal  cells,  and  on  the  position  of  the  centrosomes  in  goblet- 
cells  (Fig.  23)  and  in  those  of  the  lachrymal  gland.]  Peter  ('99)  has 
attempted  to  test  these  conclusions  experimentally  by  cuttino-  or  tear- 
ing off  cilia  from  the  cell-body  (gut-epitheUum  of  Aiiodonta)  and  also 
by  isolating  the  tails  of  spermatozoa.  In  groups  consisting  of  only  a 
few  cilia,  separated  from  the  nucleus,  the  movements  actively  con- 
tinue, while  those  that  are  separated  from  the  basal  bodies  cease  to 
beat.     Spermatozoon  tails  separated  from  the  head  also  continue  to 


Fig.  167.  —  Centrosomes  and  cilia  in  spermatocytes  of  a  butterfly.    [Henneguy.] 


move,  but  only  if  they  remain  connected  with  the  middle-piece. 
Peter,  therefore,  supports  the  above  conclusions  of  Henneguy  and 
Lenhossek.  On  the  other  hand,  Meves  ('99)  finds  that  movements 
of  the  undulating  membrane  in  the  tails  of  salamander-spermatozoa 
continue  if  the  middle-piece  be  entirely  removed;  while  a  number 
of  earher  observers^  have  observed  in  flagellates  that  a  flagellum 
separated  from  the  body  may  actively  continue  its  movements  for  a 
considerable  time. 

Further  research  is  therefore  required  to  test  these  suggestions. 
The  intimate  connection  of  the  centrosomes  with  the  formation,  on  the 
one  hand,  of  the  astral  rays,  on  the  other  of  contractile  organs,  such 

1  See  Klebs,  '83,  Biitschli,  '85,  Fischer,  '94,  2. 


358  CELL-CHEMISTRY  AND    CELL-PHYSIOLOGY 

as  cilia,  flagella,  and  pseudopodia,^  the  centrosomes  in  ciliated  cells 
and  spermatozoa,  and  in  the  swarm-spores  of  Noctiluca,  is,  however,  a 
most  striking  fact,  and  is  one  of  the  strongest  indirect  arguments  in 
favour  of  the  general  theory  of  fibrillar  contractility  in  mitosis. 

D.     Summary  and  Conxlusion 

The  facts  reviewed  in  the  foregoing  pages  converge  to  the  conclu- 
sion that  the  differentiation  of  the  cell-substance  into  nucleus  and 
cytoplasm  is  the  expression  of  a  fundamental  physiological  division 
of  labour  in  the  cell.  Experiments  upon  unicellular  forms  demonstrate 
that,  in  the  entire  absence  of  a  nucleus,  protoplasm  is  able  for  a 
considerable  time  to  liberate  energy  and  to  manifest  coordinated 
activities  dependent  on  destructive  metabolism.  There  is  here  sub- 
stantial ground  for  the  view  that  the  cytoplasm  is  the  principal 
seat  of  these  activities  in  the  normal  cell.  On  the  other  hand, 
there  is  strong  cumulative  evidence  that  the  nucleus  is  intimately 
concerned  in  the  constructive  or  synthetic  processes,  whether  chemical 
or  morphological. 

That  the  nucleus  has  such  a  significance  in  synthetic  metaboHsm 
is  proved  by  the  fact  that  digestion  and  absorption  of  food  and 
growth  soon  cease  with  its  removal  from  the  cytoplasm,  while  destruc- 
tive metabolism  may  long  continue  as  manifested  by  the  phenomena 
of  irritability  and  contractiHty.  It  is  indicated  by  the  position  and 
movements  of  the  nucleus  in  relation  to  the  food-supply  and  to  the 
formation  of  specific  cytoplasmic  products.  It  harmonizes  with  the 
fact,  now  universally  admitted,  that  active  exchanges  of  material 
go  on  between  nucleus  and  cytoplasm.  The  periodic  changes  of 
staining-capacity  undergone  by  the  chromatin  during  the  cycle  of  cell- 
life,  taken  in  connection  with  the  researches  of  physiological  chemists 
on  the  chemical  composition  and  staining-reactions  of  the  nuclein 
series,  indicate  that  the  phosphorus-rich  substance  known  as  nucleinic 
acid  plays  a  leading  part  in  the  constructive  process.  During  the 
vegetative  phases  of  the  cell  this  substance  is  combined  with  a  large 
amount  of  the  albumin  radicles  histon,  protamin,  and  related  sub- 
stances, and  probably  in  part  with  albumin  itself,  to  form  nuclein. 
During  the  mitotic  or  reproductive  processes  this  combination  appears 
to  be  dissolved,  the  albuminous  elements  being  in  large  part  split 
off,  leaving  the  substance  of  the  chromosomes  with  a  high  percentage 
of  nucleinic  acid,  as  is  shown  by  direct  analysis  of  the  sperm-nucleus 
and  is  indicated  by  the  staining-reactions  of  the  chromosomes.  There 
is,  therefore,  considerable  ground  for  the  hypothesis  that  in  a  chemi- 
cal sense  this  substance  is  the  most  essential  nuclear  element  handed 

^  ^f'  PP-  92)  1 02,  on  the  central  granule  of  the  Heliozoa. 


LITERATURE  3^q 

on  from  cell  to  cell,  whether  by  cell-division  or  by  fertilization  ;  and 
that  it  may  be  a  primary  factor  in  the  constructive  processes  of  the 
nucleus  and  through  these  be  indirectly  concerned  with  those  of  the 
cytoplasm. 

The  role  of  the  nucleus  in  constructive  metabolism  is  intimately 
related  with  its  role  in  morphological  synthesis,  and  thus  in  inheri- 
tance ;  for  the  recurrence  of  similar  morphological  characters  must  in 
the  last  analysis  be  due  to  the  recurrence  of  corresponding  forms  of 
metabohc  action  of  which  they  are  the  outward  expression.  That  the 
nucleus  is  in  fact  a  primary  factor  in  morphological  as  well  as  chemi- 
cal synthesis  is  demonstrated  by  experiments  on  unicellular  plants  and 
animals,  which  prove  that  the  power  of  regenerating  lost  parts  disap- 
pears with  its  removal,  though  the  enucleated  fragment  may  continue 
to  live  and  move  for  a  considerable  period.  That  the  nuclear  sub- 
stance, and  especially  the  chromatin,  is  a  leading  factor  in  inheritance 
is  powerfully  supported  by  the  facts  of  maturation,  fertilization,  and 
cell-division.  In  maturation  the  germ-nuclei  are  by  an  elaborate 
process  prepared  for  the  subsequent  union  of  equivalent  chromatic 
elements  from  the  two  sexes.  By  fertilization  these  elements  are 
brought  together,  and  by  mitotic  division  distributed  with  exact  equal- 
ity to  the  embryonic  cells.  The  result,  which  is  especially  striking  in 
the  case  of  hybrid-fertilization,  proves  that  the  spermatozoon  is  as 
potent  in  inheritance  as  the  ovum,  though  the  latter  contributes  an 
amount  of  cytoplasm  which  is  but  an  infinitesimal  fraction  of  that 
supplied  by  the  ovum. 

It  remains  to  be  seen  whether  the  chromatin  can  actually  be  re- 
garded as  the  idioplasm  or  physical  basis  of  inheritance,  as  maintained 
by  Hertwig  and  Strasburger.  Verworn  has  justly  urged  that  the 
nucleus  cannot  be  regarded  as  the  sole  vehicle  of  inheritance,  since 
the  cooperation  of  both  nucleus  and  cytoplasm  is  essential  to  com- 
plete cell-life;  and,  as  will  be  shown  in  Chapter  IX.,  the  cytoplasmic 
organization  plays  an  important  role  in  shaping  the  course  of  devel- 
opment. Considered  in  all  their  bearings,  however,  the  facts  seem 
to  accord  best  with  the  hypothesis  that  the  cytoplasmic  organization 
is  itself  determined,  in  the  last  analysis,  by  the  nucleus  ;  ^  and  the 
principle  for  which  Hertwig  and  Strasburger  contended  is  thus  sus- 
tained. 

LITERATURE.     VII 

Bernard,  Claude.  —  Lepons  sur  les  Phenomenes  de  la  \'ie  :    ist  ed.   1878:    2d    ed. 

1885.     Paris. 
Chittenden,  R.  H.  —  Some  Recent  Chemico-phvsiological  Discoveries  regarding  tlie 

Cell :  Am.  Nat..  XXVIIL,  Feb.,  1894. 

1  C/p.  431- 


36o  CELL-CHEMISTRY  AND    CELL-PHYSIOLOGY 

Fischer,  A.  —  See  Literature  I. 

Gruber.  A.  —  Mikroskopische  VMvisekton  :  Ber.  d.  Naturf.  Ges.  Freiburg,  \\\.,  1893. 

Haberlandt,  G.  —  tjber  die  Beziehungen  zwischen  Funktion  und  Lage  des  Zellkerns. 

Fi seller,  1887. 
Id.  —  Physiologische  Pflanzenatomie.     Leipzig,  1896. 
Halliburton,  W.  D.  —  A  Text-book  of  Cliemical  Physiology  and  Pathology.      London^ 

1891. 
Id.  —  The  Chemical  Physiology  of  the  Cell  (^Gouldstonian  Lectures):  B?'it.  Med. 

Joitrn.     1893. 
Hammarsten,  0.  — Lehrbuch  der  physiologische  Chemie.     3d  ed.      Wiesbaden,  1895. 
Hertwig,  0.  and  R.  —  Uber  den  Befruchtungs-  und  Teilungsvorgang  des  tierischen 

Eies  unter  dem  Einfluss  ausserer  Agentien.    Jena,  1887. 
Kolliker,  A.  —  Das  Karyoplasma  und  die  \'ererbung,  eine  Kritik  der  Weismann'schen 

Theorie  von  der  Kontinuitat  des  Keimplasmas  :  Zeitschr.  iviss.  ZooL,  XLIV. 

1886. 
Korschelt,  E.  —  Beitrage  sur  Morphologic  und    Physiologic  des  Zellkernes  :    Zool. 

JaJirb.  Atiat.  it.  Oniog.,  IV.     1889. 
Kossel,  A.  —  Uber  die  chemische  Zusammensetzung  der  Zelle  :  Arch.  Anat.  u.  P/iys. 

1891. 
Id.  —  Uber  die  basischen  Stoffe  des  Zellkernes:  ZeiL  P/iys.  C/ieni.,  XXII.,  1896. 
Lilienfeld,  L.  —  Uber   die    Wahlverwandtschaft    der  Zellelemente   zu    Farbstoffen : 

Arch.  Anat.  n.  Phys.     1893. 
Malfatti,  H.  —  Beitrage  zur  Kenntniss  der  Nucleine  :  Zeitschr.  Phys.   Cheju.,  XVI. 

1891. 
Mathews,  A.  P.  —  The  Metabolism  of  the  Pancreas  Cell :  Journ.  Morph.,  XV.  SuppL 

1899. 
Miescher,  F.  —  Physiologisch-chemische  Untersuchungen  liber  die  Lachsmilch  :  Arch. 

Exp.  Path.  u'.  Pharm.,  XXXVII.,  1896. 
Prenant,  A.  —  See  Literature  VI. 
Riickert,  J.  —  Zur  Entwicklungsgeschichte  des  Ovarialeies  bei  Selachiern  :  An.  Anz., 

VII.     1892. 
Sachs,  J.  — Vorlesungen  liber  Pflanzen-physiologie.     Leipzig,  1882. 
Id.  —  Stofif  und  Form  der  Pflanzen-organe  :   Gesanimelte  Abhandlnngen,  II.     1893. 
Strasburger.  —  See  footnote,  p.  269. 
Verworn,  M.  —  Die  Physiologische  Bedeutung  des  Zellkerns  :    Arch,  fur  die  Ges. 

Phys.,X\A.     1892. 
Id.  —  Allgemeine  Physiologic.    Jena,  1895. 
Whitman,  C.  0.  —  The  Seat  of  Formative  and  Regenerative  Energy  :  Journ.  Morph., 

II.     1888. 
Zacharias,  E.  —  Uber  des  Verhalten  des  Zellkerns  in  wachsenden  Zellen  :  Flora,  81. 

1895. 


CHAPTER   VIII 

CELL-DIVISION   AND   DEVELOPMENT 

"  Wir  konnen  demnach  endlich  den  Satz  aufstellen,  dass  samnitliche  im  cntwickelten 
Zustande  vorhandenen  Zellen  oder  Aequivalente  von  Zellen  durch  eine  f(jrtschreitende 
Gliederung  der  Eizelle  in  morphologisch  ahnliche  Elemente  entstehen,  und  dass  die  in  einer 
embryonischen  Organ-Anlage  enthaltenden  Zellen,  so  gering  auch  ihre  Zahl  scin  mag, 
dennoch  die  ausschliessliche  ungegliederte  Anlage  fiir  sammtliche  Formbestandtheile  der 
spateren  Organe  enthalten,"  Remak.^ 

Since  the  early  work  of  Kolliker  and  Remak  it  has  been  recog- 
nized that  the  cleavage  or  segmentation  of  the  ovum,  with  which 
the  development  of  all  higher  animals  begins,  is  nothing  other  than 
a  rapid  series  of  mitotic  cell-divisions  by  which  the  Qgg  splits  up 
into  the  elements  of  the  tissues.  This  process  is  merely  a  contin- 
uation of  that  by  which  the  germ-cell  arose  in  the  parental  body. 
A  long  pause,  however,  intervenes  during  the  latter  period  of  its 
ovarian  life,  during  which  no  divisions  take  place.  Throughout  this 
period  the  egg  leads,  on  the  whole,  a  somewhat  passive  existence, 
devoting  itself  especially  to  the  storage  of  potential  energy  to  be  used 
during  the  intense  activity  that  is  to  come.  Its  power  of  division 
remains  dormant  until  the  period  of  full  maturity  approaches.  The 
entrance  of  the  spermatozoon  arouses  in  the  egg  a  new  phase  of 
activity.  Its  power  of  division,  which  may  have  lain  dormant  for 
months  or  years,  is  suddenly  raised  to  the  highest  pitch  of  intensity, 
and  in  a  very  short  time  it  gives  rise  by  division  to  a  myriad  of  de- 
scendants which  are  ultimately  differentiated  into  the  elements  of 
the  tissues. 

The  divisions  of  the  egg  during  cleavage  are  exactly  comparable 
with  those  of  tissue-cells,  and  all  of  the  essential  phenomena  of 
mitosis  are  of  the  same  general  character  in  both.  But  for  two 
reasons  the  cleavage  of  the  egg  possesses  a  higher  interest  than 
any  other  case  of  cell-division.  First,  the  egg-cell  gives  rise  by  divi- 
sion not  only  to  cells  like  itself,  as  is  the  case  with  most  tissue-cells, 
but  also  to  many  other  kinds  of  cells.  The  operation  of  cleavage  is 
therefore  immediately  connected  with  the  process  of  differentiation, 
which  is  the  most  fundamental  phenomenon  in  development.  Second, 
definite  relations  may  often  be  traced  between  the  planes  of  division 
and  the  structural   axes  of  the  adult  body,  and  these  relations  are 

1  Untersuchungen,  1855,  p.  140. 
361 


362  CELL-DIVISION  AND  DEVELOPMENT 

sometimes  so  clearly  marked  and  appear  so  early  that  with  the  very 
first  cleavage  the  position  in  which  the  embryo  will  finally  appear  in 
the  ^g%  may  be  exactly  predicted.  Such  *'  promorphological "  rela- 
tions of  the  segmenting  ^^^  possess  a  very  high  interest  in  their 
bearing  on  the  theory  of  germinal  localization  and  on  account  of  the 
light  which  they  throw  on  the  conditions  of  the  formative  process. 

The  present  chapter  is  in  the  main  a  prelude  to  that  which 
follows,  its  purpose  being  to  sketch  some  of  the  external  features 
of  early  development  regarded  as  particular  expressions  of  the  gen- 
eral rules  of  cell-division.  For  this  purpose  we  may  consider  the 
cleavage  of  the  ovum  under  two  heads,  namely  :  — - 

1.  TJie  Geometrical  Relations  of  Cleavage-forms,  with  reference 
to  the  general  rules  of  cell-division. 

2.  TJie  Promoi'p  ho  logical  Relations  of  the  blastomeres  and  cleav- 
age-planes to  the  parts  of  the  adult  body  to  which  they  give  rise. 

A.     Geometrical  Relations  of  Cleavage-forms 

The  geometrical  relations  of  the  cleavage-planes  and  the  relative 
size  and  position  of  the  cells  vary  endlessly  in  detail,  being  modified 
by  innumerable  mechanical  and  other  conditions,  such  as  the  amount 
and  distribution  of  the  inert  yolk  or  deutoplasm,  the  shape  of  the 
ovum  as  a  whole,  and  the  like.  Yet  all  the  forms  of  cleavage  can 
be  referred  to  a  single  type  which  has  been  moulded  this  way  or  that 
by  special  conditions,  and  which  is  itself  an  expression  of  two  general 
rules  of  cell-division,  first  formulated  by  Sachs  in  the  case  of  plant- 
cells.     These  are  :  — 

1 .  The  cell  typically  tends  to  divide  into  eqnal pai'ts. 

2.  Each  new  plane  of  division  tends  to  intersect  the  pi'cce ding  plane 
at  a  right  angle. 

In  the  simplest  and  least  modified  forms  the  direction  of  the 
cleavage-planes,  and  hence  the  general  configuration  of  the  cell- 
system,  depends  on  the  general  form  of  the  dividing  mass ;  for,  as 
Sachs  has  shown,  the  cleavage-planes  tend  to  be  either  vertical  to  the 
surface  {anticlines)  or  parallel  to  it  {periclines).  Ideal  schemes  of 
division  may  thus  be  constructed  for  various  geometrical  figures.  In 
a  flat  circular  disc,  for  example,  the  anticlinal  planes  pass  through 
the  radii;  the  periclines  are  circles  concentric  with  the  periphery.  If 
the  disc  be  elongated  to  form  an  ellipse,  the  periclines  also  become 
ellipses,  while  the  anticlines  are  converted  into  hyperbolas  confocal 
with  the  periclines.  If  it  have  the  form  of  a  parabola,  the  periclines 
and  anticlines  form  two  systems  of  confocal  parabolas  intersecting  at 
right  angles.  All  these  schemes  are  mutatis  mutandis,  directly  con- 
vertible into  the  corresponding  solid  forms  in  three  dimensions. 


GEOMETRICAL   RELATIONS    OF  CLEAVAGE-FORMS 


l^l 


Sachs  has  shown  in  the  most  beautiful  manner  that  all  the  above 
ideal  types  are  closely  approximated  in  nature,  and  Rauber  has  applied 
the  same  principle  to  the  cleavage  of  animal-cells.  The  discoid  or 
spheroid  form  is  more  or  less  nearly  realized  in  the  thalloid  growths  of 


Fig.  i68.  —  Geometrical  relations  of  cleavage-planes  in  growing  plant-tissues.  [From  f^AClls. 
after  various  authors.] 

A.  Flat  ellipsoidal  germ-disc  of  Melobesia  (Rosanoff) ;  nearly  typical  relation  of  eiiiptic 
periclines  and  hyperbolic  anticlines.  B.  C.  Apical  view  of  terminal  knob  on  epidermal  liair  of 
Pinguicola.  B.  shows  the  ellipsoid  type,  C.  the  circular  (spherical  type),  somewhat  modified 
(only  anticlines  present).  D.  Growing  point  of  Salvhiia  (Pringsheim),  typical  ellipsoid  type; 
the  single  pericline  is,  however,  incomplete.  E.  Growing  point  of  Azolla  (Strasburger)  ;  circular 
or  spheroidal  type  transitional  to  ellipsoidal.  F.  Root-cap  of  E(]uisrtinn  (Xageli  and  Leitgeb)  ; 
modified  circular  type.  G.  Cross-section  of  leaf-vein,  Trichowanes  (Prantl)  ;  ellipsoidal  type  with 
incomplete  periclines.  H.  Embryo  oi  Alisma ;  typical  elliyisoid  type,  pericline  incomplete  only 
at  lower  side.  /.  Growing  point  of  bud  of  the  pine  {AHes)  ;  typical  paraboloid  type,  both  anti- 
clines and  periclines  having  the  form  of  parabolas  (Sachs). 


various  lower  plants,  in  the  embryos  of  flowering  plants,  and  else- 
where (Fig.  1 68).  The  paraboloid  form  is  according  to  Sachs  charac- 
teristic of  the  growing  points  of  many  higher  plants  ;  and  here,  too, 
the  actual  form  is  remarkably  similar  to  the  ideal  scheme  (Fig.  i68,  /). 


364  CELL-DIVISION  AND  DEVELOPMENT 

For  our  purpose  the  most  important  form  is  the  sphere,  which  is 
the  typical  shape  of  the  egg-cell ;  and  all  forms  of  cleavage  may 
be  related  to  the  typical  division  of  a  sphere  in  accordance  with  Sachs's 
rules.  The  ideal  form  of  cleavage  would  here  be  a  succession  of 
rectangular  cleavages  in  the  three  dimensions  of  space,  the  anticlines 
passing  through  the  centre  so  as  to  split  the  ^gg  in  the  initial  stages 
successively  into  halves,  quadrants,  and  octants,  the  periclines  being 
parallel  to  the  surface  so  as  to  separate  the  inner  ends  of  these  cells 
from  the  outer.  No  case  is  known  in  which  this  order  is  accurately 
followed  throughout,  and  the  periclinal  cleavages  are  of  compara- 
tively rare  occurrence,  being  found  as  a  regular  feature  of  the  early 
cleavage  only  in  those  cases  where  the  primary  germ-layers  are  sepa- 
rated by  delamination.  The  simplest  and  clearest  form  of  egg- 
cleavage  occurs  in  eggs  like  those  of  echinoderms,  which  are  of 
spherical  form,  and  in  which  the  deutoplasm  is  small  in  amount  and 
equally  distributed  through  its  substance.  Such  a  cleavage  is  beauti- 
fully displayed  in  the  ^gg  of  the  holothurian  Synapta,  as  shown  in 
the  diagrams,  Fig.  169,  constructed  from  Selenka's  drawings.  The 
first  cleavage  is  vertical,  or  meridional,  passing  through  the  egg-axis 
and  dividing  the  ^gg  into  equal  halves.  The  second,  which  is  also 
meridional,  cuts  the  first  plane  at  right  angles  and  divides  the  ^gg 
into  quadrants.  The  third  is  horizontal,  or  equatorial ,  dividing  the 
^gg  into  equal  octants.  The  order  of  division  is  thus  far  exactly 
that  demanded  by  Sachs's  rule  and  agrees  precisely  with  the  cleavage 
of  various  kinds  of  spherical  plant-cells.  The  later  cleavages  depart 
from  the  ideal  type  in  the  absence  of  periclinal  divisions,  the  embryo 
becoming  hollow,  and  its  walls  consisting  of  a  single  layer  of  cells  in 
which  anticHnal  cleavages  occur  in  regular  rectangular  succession. 
The  fourth  cleavage  is  again  meridional,  giving  two  tiers  of  eight 
cells  each  ;  the  fifth  is  horizontal,  dividing  each  tier  into  an  upper 
and  a  lower  layer.  The  regular  alternation  is  continued  up  to  the 
ninth  division  (giving  512  cells),  when  the  divisions  pause  while  the 
gastrulation  begins.      In  later  stages  the  regularity  is  lost. 

Hertwig  s  Development  of  Sachs's  Rules.  —  Beside  Sachs's  rules 
may  be  placed  two  others  formulated  by  Oscar  Hertwig  in  1884, 
which  bear  directly  on  the  facts  just  outHned  and  which  lie  behind 
Sachs's  principle  of  the  rectangular  intersection  of  successive  division- 
planes.     These  are :  — 

1 .  The  nucleus  tends  to  take  np  a  position  at  the  centre  of  its  sphere 
of  influence,  i.e.  of  the  protoplasmic  mass  in  zvhich  it  lies. 

2.  The  axis  of  the  7nitotic  figtires  typically  lies  in  the  longest  axis 
of  the  protoplasmic  mass,  and  division  therefore  tends  to  cut  this  axis 
at  a  right  a^tgle. 

The  second  rule  explains  the  normal  succession   of   the   division- 


GEOMETRICAL   RELATIONS   OF  CLEAVAGE-FORMS  365 

planes  according  to  Sachs's  second  rule.  The  first  division  of  a  homo- 
geneous spherical  ^^^^,  for  example,  is  followed  by  a  second  division 
at  right  angles  to  it,  since  each  hemisphere  is  twice  as  Ion-  in  the 
plane  of  division  as  in  any  plane  vertical  to  it.  The  mitotic  fi-ure 
of  the  second  division  lies  therefore  parallel  to  the  first  plane  which 
forms  the  base  of  the  hemisphere,  and  the  ensuing  division  is  vertical 
to  It.  The  same  applies  to  the  third  division,  since  each  quadrant  is 
as  long  as  the  entire  ^gg  while  at  most  only  half  its  diameter.  Divi- 
sion is  therefore  transverse  to  the  long  axis  and  vertical  to  the  first 
two  planes. 

Taken   together  the  rules  of  Sachs  and  Hertwig,  applied   to  the 
^gg,  give  us  a  kind  of  ideal  type  or  model,  well  illustrated  by  the 


Fig.  169.  —  Cleavage  of  the  ovum  in  the  \\o\o\h\xx\^Xi  Synapta  (slightly  schematized)       f  After 
Selenka.]  '■ 

A-E.  Successive  cleavages  to  the  32-cell  stage.     F.  Blastula  of  128  cells. 

cleavage  of  Synapta,  described  above,  to  which  all  the  forms  of  cleav- 
age may  conveniently  be  referred  as  a  basis  of  comparison.  Numer- 
ous exceptions  to  all  four  of  these  rules  are,  however,  known,  and  they 
are  of  little  value  save  as  a  starting-point  for  a  closer  study  of  the 
facts.  Cleavage  of  such  schematic  regularity  as  that  of  Synapta  is 
extremely  rare,  both  the  form  and  the  order  of  division  being  end- 
lessly varied  and  in  extreme  cases  showing  scarcely  a  discoverable 
connection  with  the  ''type."  We  may  conveniently  consider  these 
modifications  under  the  following  three  heads  :  — 


366  CELL-DIVISION  AND  DEVELOPMENT 

1.  Variation  in  the  I'JiytJini  of  division. 

2.  Displacement  of  the  cells  {iticlnding  variations  in  the  direction 
of  cleavage). 

3.  Unequal  division  of  the  cells. 

Nothing  is  more  common  than  a  departure  from  the  regular 
rhythm  of  division.  The  variations  are  sometimes  quite  irregular, 
sometimes  follow  a  definite  rule,  as,  for  instance,  in  the  annelid  N'eiris 
(Fig.  171),  where  the  typical  succession  in  the  number  of  cells  is  with 
great  constancy  2,  4,  8,  16,  20,  23,  29,  32,  37,  38,  41,  42,  after  which 
the  order  is  more  or  less  variable.  The  factors  that  determine  such 
variations  in  the  rhythm  of  division  are  very  little  understood.  Bal- 
four, one  of  the  first  to  consider  the  subject,  sought  an  explanation  in 
the  varying  distribution  of  metaplasmic  substances,  maintaining  ('75, 
'80)  that  the  rapidity  of  division  in  any  part  of  the  ovum  is  in  general 
inversely  proportional  to  the  amount  of  deutoplasm  that  it  contains. 
The  entire  inadequacy  of  this  view  has  been  demonstrated  by  a  long 
series  of  precise  studies  on  cell-lineage,  which  show  that  while  the 
large  deutoplasm-bearing  cells  often  do  divide  more  slowly  than 
the  smaller  protoplasmic  ones  the  reverse  is  often  the  case,  while 
remarkable  differences  in  the  rhythm  of  division  are  often  observed 
in  cells  which  do  not  perceptibly  differ  in  metaplasmic  content.^  All 
the  evidence  indicates  that  the  rhythm  of  division  is  at  bottom  deter- 
mined by  factors  of  a  very  complex  character  which  cannot  be 
disentangled  from  those  which  control  growth  in  general.  LilHe 
('95,  '99)  points  out  the  very  interesting  fact,  determined  through  an 
analysis  of  the  cell-lineage  of  mollusks  and  annelids,  that  the  rate 
of  cleavage  shows  a  direct  relation  to  the  period  at  which  the  prod- 
ucts become  functional.  Thus  in  Unio  the  more  rapid  cleavage  of  a 
certain  large  cell  ("  d.  2"),  formed  at  the  fourth  cleavage,  is  obviously 
correlated  with  the  early  formation  of  the  shell-gland  to  which  it  gives 
rise,  while  the  relatively  slow  rate  of  division  in  the  first  ectomere- 
quartet  is  correlated  with  reduction  of  the  prae-trochal  region.  The 
prospective  character  shown  here  w^ill  be  found  to  apply  also  to  other 
characters  of  cleavage,  as  described  beyond. 

When  we  turn  to  the  factors  that  determine  the  direction  of  cleav- 
age or  the  displacement  of  cells  subsequent  to  division,  we  find,  as 
in  the  case  of  the  division-rhythm,  obvious  mechanical  factors  com- 
bined with  others  far  more  complex.  The  arrangement  of  tissue-cells 
usually  tends  toward  that  of  least  resistance  or  greatest  economy  of 
space ;  and  in  this  regard  they  have  been  shown  to  conform,  broadly 
speaking,  with  the  behaviour  of  elastic  spheres,  such  as  soap-bubbles 
when  massed  together  and  free  to  move.     Such  bodies,  as  Plateau 

1  Cf.  Wilson,  '92,  Kofoid,  '94,  Lillie,  '95,  Zur  Strassen,  '95,  Ziegler,  '95,  and  especially 
Jennings,  '97. 


GEOMEmiCAL   RELATIONS   OF  CLEAVAGE-FORMS  36; 

and  Lamarle  have  shown,  assume  a  pol^^hedral  form  and  tend  toward 
such  an  arrangement  that  the  area  of  surfacccontact  /v  '-.  // 
a  nun^uucn^.  Spheres  in  a  mass  thus  tend  o  aTsume  The  £^  Z 
mterlockm.  polyhedrons  so  arranged  that  three  pCsLterc  in 
a  hne,  whde  four  Imes  and  six  planes  meet  at  a  point  If  a  ran  .cd 
in  a  smgle  layer  on  an  extended  surface,  they  a'sume  the "."'0' 


A 


C 


D 


Fig.  170.  —  Cleavage  ol  Polygordius,  from  life. 

fhpl  ^°7-^^"  ^*^g^-  f'«'"  above.     B.  Corresponding  view  of  eight-cell  stage.      C  Side  v.ew  of 
the  same  (contrast  Fig.  169,  C) .     D.  Si.xteen-cell  stage  from  the  s^de. 

hexagonal  prisms,  three  planes  meeting  along  a  line  as  before.  I^oth 
these  forms  are  commonly  shown  in  the  arrangement  of  the  cells  of 
plant  and  animal  tissues;  and  Berthold  {'^^6)  and  Errcra  {'^6,  '%j) 
carefully  analyzing  the  phenomena,  have  endeavoured  to  show  that 
not  only  the  form  and  relative  position  of  cells,  but  also  the  direction 
of  cell-division,  is,  partially  at  least,  thus  determined. 

It  is  through  displacements  of  the  cells  of  this  type  that  many  of 


368  CELL-DIVISION  AND   DEVELOPMENT 

the  most  frequent  modifications  of  cleavage  arise.  Sometimes,  as  in 
Synapta,  the  alternation  of  the  cells  is  effected  through  displacement 
of  the  blastomeres  after  their  formation.  More  commonly  it  arises 
during  the  division  of  the  cells,  and  may  even  be  predetermined  by 
the  position  of  the  mitotic  figures  before  the  slightest  external  sign 
of  division.  Thus  arises  that  form  of  cleavage  known  as  the  spiral, 
oblique,  or  alternating  type,  where  the  blastomeres  interlock  during 
their  formation  and  lie  in  the  position  of  least  resistance  from  the 
beginning.  This  form  of  cleavage,  especially  characteristic  of  many 
worms  and  mollusks,  is  typically  shown  by  the  ^^^  of  Polygordius 
(Fig.  170).  The  four-celled  stage  is  nearly  like  that  of  Syiiapta, 
though  even  here  the  cells  slightly  interlock.  The  third  division  is, 
however,  oblique,  the  four  upper  cells  being  virtually  rotated  to  the 
right  (with  the  hands  of  a  watch)  so  as  to  alternate  with  the  four 
lower  ones.  The  fourth  cleavage  is  likewise  oblique,  but  at  right 
angles  to  the  third,  so  that  all  of  the  cells  interlock  as  shown  in 
Fig.  170,  D.  This  alternation  regularly  recurs  for  a  considerable 
period. 

In  many  worms  and  mollusks  the  obliquity  of  cleavage  appears 
still  earlier,  at  the  second  cleavage,  the  four  cells  being  so  arranged 
that  two  of  them  meet  along  a  ''cross-furrow"  at  the  lower  pole  of 
the  ^gg,  while  the  other  two  meet  at  the  upper  pole  along  a  similar, 
though  often  shorter,  cross-furrow  at  right  angles  to  the  lower  {e.g.  in 
Nereis,  Fig.  171).  It  is  a  curious  fact  that  the  direction  of  the  dis- 
placement is  quite  constant,  the  first  or  upper  quartet  in  the  eight- 
cell  stage  being  rotated  to  the  right,  or  with  the  hands  of  a  watch, 
the  second  quartet  to  the  left,  the  third  to  the  right,  and  so  on. 
Crampton  ('94)  has  discovered  the  remarkable  fact  that  in  P/iysa,  a 
gasteropod  having  a  reversed  or  sinistral  shell,  the  whole  order  of 
displacement  is  likewise  reversed,  and  the  same  has  recently  been 
shown  by  Holmes  ('99)  to  be  true  of  Aiicylus. 

The  spiral  or  alternating  type  of  cleavage  beautifully  illustrates 
Sachs's  second  rule  as  affected  by  modifying  conditions  ;  for,  as  may 
be  seen  by  an  inspection  of  Figs.  170,  171,  each  division-plane  is 
approximately  at  right  angles  to  the  preceding  and  succeeding 
(whence  the  "  alternation  .  of  the  spirals"  described  by  students  of 
cell-lineage),  while  they  are  so  directed  that  each  cell  as  it  is  formed 
is  placed  at  once  in  the  position  of  least  resistance  in  the  mass,  i.e.  in 
the  position  of  minimal  surface-contact.  It  is  impossible  to  resist 
the  conclusion  that  one  of  the  factors  by  which  the  position  of  the 
cells  (and  hence  the  direction  of  cell-division)  is  determined  is  a 
purely  mechanical  one,  identical  with  that  which  determines  the 
arrangement  of  soap-bubbles  and  the  like. 

Very  little  acquaintance  with  the  facts  of  development  is  however 


GEOMETRICAL  RELATIOXS   OF  CLEAVAGE-EORMS 


369 


required  to  show  that  this  purely  mechanical  factor,  though  doubtless 
real,  must  be  subordinate  to  some  other.  This  is  strikingly  shown, 
for  example,  in  the  development  of  annelids  and  mollusks,  where  the 
spiral  cleavage,  strictly  maintained  during  the  earlier  stages,  finally 
gives  way  more  or  less  completely  to  a  bilateral  type  of  division  in 
which  the  rule  of  minimal  surface-contact  is  often  violated.  We  see 
here  a  tendency  operating  directly  against,  and  finally  overcoming, 


D  E  F 

Fig.  171.  —  Cleavage  oi  Nereis.  An  example  of  a  spiral  cleavage,  unequal  from  tlie  beginning 
and  of  a  marked  determinate  character. 

^.Two-cell  stage  (the  circles  are  oil-drops).  B.  Four-cell  stage ;  the  second  cleavage-plane 
passes  through  the  future  median  plane.  C.  The  same  from  the  right  side.  D.  Eight-cell  stage. 
E.  Sixteen  cells;  from  the  cells  marked  /  arises  the  prototroch  or  larval  ciliated  belt,  from  X  the 
ventral  nerve-cord  and  other  structures,  from  D  the  mcsoblast-bands,  the  germ-cells,  and  a  part  of 
the  alimentary  canal.  F.  Twenty-nine-cell  stage,  from  the  right  side ;  /.  girdle  of  prototrochal  cells 
which  give  rise  to  the  ciliated  belt. 

the  mechanical  factor  which  predominates  in  the  earlier  stages ;  and 
in  some  cases,  e.g.  in  the  ^g%  of  Clavclina  (Fig.  177)  and  other  tuni- 
cates,  this  tendency  predominates  from  the  beginning.  In  both 
these  cases  this  *'  tendency  "  is  obviously  related  to  the  growth-jirocess 
to  which  the  future  bilateral  embryo  will  owe  its  form  ;  ^  and  every 
attempt  to  explain  the  position  of  the  cells  and  the  direction  of  cleav- 
age must  reckon  with  the  morphogenic  process  taken  as  a  whole. 
The  blastomere  is  not  merely  a  cell  dividing  under  the  stress  of  rude 

^  Cf.   Wilson  ('92,  p.  444). 
2  B 


370 


CELL-DIVISION  AND  DEVELOPMENT 


mechanical  conditions ;  it  is  beyond  this  "  a  builder  which  lays  one 
stone  here,  another  there,  each  of  which  is  placed  with  reference  to 
future  development."^ 

The  third  class  of  modifications,  due  to  unequal  division  of  the  cells, 
not  only  leads  to  the  most  extreme  types  of  cleavage  but  also  to  its 


Fig.  172.  —  The  eight-cell  stage  of  four  different  animals  showing  gradations  in  the  inequality  of 
the  third  cleavage. 

A.  The  leech  Clepsine  (Whitman).      B.  The  oh^Xo'^o^  Rhynchelmis  (Vejdovsky).       C.  The 
lamellibranch  Unto  (Lillie).     D.  Amphioxus. 


most  difficult  problems.  Unequal  divisions  appear  sooner  or  later 
in  all  forms  of  cleavage,  the  perfect  equality  so  long  maintained 
in  Synapta  being  a  rare  phenomenon.  The  period  at  which  the  in- 
equality first  appears  varies  greatly  in  different  forms.  In  Polygordius 
(Fig.  170)  the  first  marked  inequality  appears  at  the  fifth  cleavage; 

1  Lillie,  '95,  p.  46. 


GEOMETRICAL  RELATIONS   OF  CLEAVAGE-FORMS  371 

in  sea-urchins  it  appears  at  the  fourth  (Fig.  3);  in  AmpJiioxus  at  the 
third  (Fig.  172);  in  the  tunicate  Clavclina  at  the  second  ( Fig.  177); 
in  Nereis  at  the  first  division  (Figs.  60,  171).  The  extent  of  the  in- 
equality varies  in  like  manner.  Taking  the  third  cleavage  as  a  type, 
we  may  trace  every  transition  from  an  equal  division  (echinoderms, 
Polygordiiis),  through  forms  in  which  it  is  but  slightly  marked  {Avi- 
phioxns,  frog),  those  in  which  it  is  conspicuous  {Nereis,  Lymniea,  poly- 
clades,  Petroinyzon,  etc.),  to  forms  such  as  Clepsine,  where  the  cells  of 
the  upper  quartet  are  so  minute  as  to  appear  Hke  mere  buds  from  the 
four  large  lower  cells  (Fig.  172).  At  the  extreme  of  the  series  we 
reach  the  partial  or  meroblastic  cleavage,  such  as  occurs  in  the  ceph- 
alopods,  in  many  fishes,  and  in  birds  and  reptiles.  Here  the  lower 
hemisphere  of  the  ^gg  does  not  divide  at  all,  or  only  at  a  late  period, 
segmentation  being  confined  to  a  disc-hke  region  or  blastoderm  at  one 
pole  of  the  ^gg  (Fig.  173). 

Very  interesting  is  the  case  of  the  teloblasts  or  pole-cells  character- 
istic of  the  development  of  many  annelids  and  mollusks  and  found  in 
some  arthropods.  These  remarkable  cells  are  large  blastomeres,  set 
aside  early  in  the  development,  which  bud  forth  smaller  cells  in  reg- 
ular succession  at  a  fixed  point,  thus  giving  rise  to  long  cords  of  cells 
(Fig.  175).  The  teloblasts  are  especially  characteristic  of  apical 
growth,  such  as  occurs  in  the  elongation  of  the  body  in  annelids,  and 
they  are  closely  analogous  to  the  apical  cells  situated  at  the  growing 
point  in  many  plants,  such  as  the  ferns  and  stoneworts. 

Still  more  suggestive  is  the  formation  of  rudinientaiy  cells,  arising 
as  minute  buds  from  the  larger  blastomeres,  and,  in  some  cases,  appar- 
ently taking  no  part  in  the  formation  of  the  embryo  (Fig.  174).^ 

We  are  as  far  removed  from  an  explanation  of  unequal  division  as 
from  that  of  the  rhythm  and  direction  of  division.  Inequality  of  divi- 
sion, like  difference  of  rhythm,  is  often  correlated  with  inequalities  in 
the  distribution  of  metaplasmic  substances  —  a  fact  generalized  by 
Balfour  in  the  statement  ('80)  that  the  size  of  the  cells  formed  in 
cleavage  varies  inversely  to  the  relative  amount  of  protoplasm  in  the 
region  of  the  ^gg  from  which  they  arise.  Thus,  in  all  telolecithal 
ova,  where  the  deutoplasm  is  mainly  stored  in  the  lower  or  vegetative 
hemisphere,  as  in  many  worms,  mollusks,  and  vertebrates,  the  cells  of 
the  upper  or  protoplasmic  hemisphere  are  smaller  than  those  of  the 
lower,  and  may  be  distinguished  as  micrGineres  from  the  larger  viacro- 
meres  of  the  lower  hemisphere.  The  size-ratio  between  micromeres 
and  macromeres  is  on  the  whole  directly  proportional  to  the  ratio 
between  protoplasm  and  deutoplasm.  Partial  or  discoidal  cleavage 
occurs  when  the  mass  of  deutoplasm  is  so  great  as  entirely  to  prevent 
cleavage  in  the  lower  hemisphere.     This  has  been  beautifully  con- 

1  See  Wilson,  '98,  '99,  2. 


:>/ 


CELL-DIVISION  AND  DEVELOPMENT 


firmed  by  O.  Hertwig  ('98),  who,  by  placing  frogs'  eggs  in  a  centrifu- 
gal machine,  has  caused  them  to  undergo  a  meroblastic  cleavage 
through  the  artificial  accumulation  of  yolk  at  the  lower  ^^'ole,  due  to 
the  centrifugal  force. 

While  doubtless  containing  an  element  of  truth,  this  explanation  is, 
however,  no  more  adequate  than  Balfour's  rule  regarding  the  relation 
between  deutoplasm  and  rhythm  (p.  366);  for  innumerable  cases  are 
known  in  which  no  correlation  can  be  made  out  between  the  distribu- 
tion of  inert  substance  and  the  inequality  of  division.  This  is  the 
case,  for  example,  with  the  teloblasts  mentioned  above,  which  contain 
no  deutoplasm,  yet  regularly  divide  unequally.      It  seems  to  be  inap- 


K  ^^ 


r 


B 


Fig.  173.  —  Partial  or  meroblastic  cleavage  in  the  squid  LoUgo.     [Watase.] 


plicable  to  the  inequalities  of  the  first  two  divisions  in  annelids  and 
gasteropods.  It  is  conspicuously  inadequate  in  the  history  of  indi- 
vidual blastomeres,  where  the  history  of  division  has  been  accurately 
determined.  In  Nereis,  for  example,  a  large  cell  known  as  the  first 
somatoblast,  formed  at  the  fourth  cleavage  (X,  Fig.  171,  E\  under- 
goes an  invariable  order  of  division,  three  unequal  divisions  being  fol- 
lowed by  an  equal  one,  then  by  three  other  unequal  divisions,  and 
again  by  an  equal.  This  cell  contains  little  or  no  deutoplasm  and 
undergoes  no  perceptible  changes  of  substance. 

The  collapse  of  the  rule  is  most  complete  in  case  of  the  rudi- 
mentary cells  referred  to  above.  In  some  of  the  annelids,  e.g.  in 
Aricia,  where  they  were  first  observed,^  these  cells  are  derived  from 
the  very  large  primary  mesoblast-cell,  which  first  divides  into  equal 
halves.  Each  of  these  then  buds  forth  a  cell  so  small  as  to  be  no 
larger  than  a  polar  body,  and  then  immediately  proceeds  to  give  rise 

1  Cf.  Wilson,  '92,  '98. 


GEOMETRICAL  KELATIONS   OF  CLEAVAGE-IORMS 


171 


to  the  mesoblast-bands  by  continued  divisions,  always  in  the  same 
plane  at  right  angles  to  that  in  which  the  rudimentary  cells  are 
formed  (Fig.  174).  The  cause  of  the  definite  succession  of  equal  and 
unequal  divisions  is  here  wholly  unexplained.  No  less  difficult  is  the 
extreme  inequality  of  division  involved  in  the  formation  of  the  polar 
bodies.  We  cannot  explain  this  through  the  fact  that  deutoplasm  is 
collected  in  the  lower  hemisphere  ;  for,  on  the  one  hand,  the  succeed- 
ing divisions  (first  cleavages)  are  often  equal,  while,  on  the  other 
hand,  the  inequality  is  no  less  pronounced  in  eggs  having  equally 


A 


B 


Fig,  174. —  Rudimentary  blastomeres  in  the  embryo  of  an  aiuK-liu.  Aruta. 
A.  From  lower  pole ;    rudimentary  cells  at^.  <?;  the  heavy  outline  is  the  lip  of  tli 
D.  The  same  in  sagittal  optical  section,  showing  rudimentary  cell  (f).  piimnv  im 
and  mesoblast-band   (w), 

distributed  deutoplasm,  or  in  those,  like  echinoderm-eggs,  which  are 
"alecithal." 

Such  cases  prove  that  Balfour's  law  is  only  a  partial  exi>lanati<m. 
being  probably  the  expression  of  a  more  deeply  lying  cause,  and 
:here  is  reason  to  believe  that  this  cause  lies  outside  the  immediate 
mechanism  of  mitosis,  Conklin  (94)  has  called  attention  to  the 
facti  that  the  immediate  cause  of  the  inequality  j^robably  does  not 
lie  either  in  the  nucleus  or  in  the  amphiaster  ;  tor  not  only  the 
chromatin-halves,  but  also  tlic  asters,  are  exactly  ecpial  in  the  early 
prophases,  and  the  inequality  of  the  a.sters  only  appears  as  the 
division  proceeds.  Probably,  therefore,  the  cause  lies  in  some  rela- 
tion between  the  mitotic  figure  and  the  cell-body   in  which   it   lies. 

1  In  the  cleavage  of  gasteropod  eggs. 


374 


CELL-DIVISION  AND  DEVELOPMENT 


I  believe  there  is  reason  to  accept  the  conclusion  that  this  relation 
is  one  of  position,  however  caused.     A  central  position  of  the  mitotic 


Fig    175  -  Embryos  of  the  earthworm  Allolobophora  fc.tida,  showing  teloblasts  or  apical  cells. 

A.  Gastrulafrom  the  ventral  side.     B.  The  same  ^^--\''--'-^^\i^^'^l':f^^^^^^ 
blasts  or /r.;«a.;^.«...^/a./..  which  bud  forth  the  mesoblast-bands.  cell  by  cell ,  ^- 1;^^^^^^ 
comprising  a  nuroblast,  nb,  from  which  the  ventral  nerve-cord  anses   and  two  -{f  ^^^^^^^  ^^^ 
^mewhat^doubtful  nature,  but  probably  concerned  in  the  formation  of  the  -Phnd^-      <='  Patera 
group  of  teloblasts,  more  enlarged,  the  neuroblast,  nb,  m  division;  u.  the  nephioblasts. 
primary  mesoblasts  enlarged;  one  in  division. 

fio-ure  results  in  an  equal  division  ;  an  eccentric  position  caused  by  a 
radial  movement  of  the  mitotic  figure,  in  the  direction  of  its  axis 
toward  the  periphery,  leads  to  unequal  division,  and  the  greater  the 


GEOMETRICAL   RELATIONS   OF  CLEAVAGE-FORMS  375 

eccentricity,  the  greater  the  inequality,  an  extreme  form  being  beauti- 
fully shown  m  the  formation  of  the  polar  bodies.  Here  the  orjcrinal 
amphiaster  is  perfectly  symmetrical,  with  the  asters  of  equal^'size 
(Fig.  97,  A).  As  the  spindle  rotates  into  its  radial  position  and 
approaches  the  periphery,  the  development  of  the  outer  aster  be- 
comes, as  it  were,  suppressed,  while  the  central  aster  becomes  enor- 
mously large.  The  str.e  of  the  aster,  in  other  ivords,  depends  upon  the 
extent  of  the  cytoplasmic  area  that  falls  zvithin  the  sphere  of  influence 
.  of  the  centrosome ;  and  this  area  depends  upon  the  position  of  the 
centrosome.  If,  therefore,  the  polar  amphiaster  could  be  artificially 
prevented  from  moving  to  its  peripheral  position,  the  <t^^g  would 
probably  divide  equally. ^ 

This  leads  us  to  a  further  consideration  of  the  attempts  that  have 
been  made  to  explain  the  movements  of  the  mitotic  figure  through 
mechanical  or  other  causes.^  Highly  interesting  experiments  have 
been  made  by  Pflliger  ('84),  Roux  ('85),  Driesch  ('92),  and  a  number 
of  later  investigators  which  show  that  the  direction  of  cleavage  may 
be  determined,  or  at  least  modified,  by  such  a  purely  meclianical 
cause  as  pressure,  through  which  the  form  of  the  dividing  mass  is 
changed. 

Thus,  Driesch  has  shown  that  when  the  eggs  of  sea-urchins  are 
flattened  by  pressure,  the  amphiasters  all  assume  the  position  of  least 
resistance,  i.e.  parallel  to  the  flattened  sides,  so  that  the  cleavages 
are  all  vertical,  and  the  ^gg  segments  as  a  flat  plate  of  eight,  sixteen, 
or  thirty-two  cells  (Fig.  186).  This  is  totally  different  from  the  nor- 
mal form  of  cleavage ;  yet  such  eggs,  when  released  from  pressure, 
are  capable  of  development  and  give  rise  to  normal  embryos.  This 
interesting  experiment  makes  it  highly  probable  that  the  disc-like 
cleavage  of  meroblastic  eggs,  like  that  of  the  squid  or  bird,  is  in  some 
degree  a  mechanical  result  of  the  accumulation  of  yolk  by  which  the 
formative  protoplasmic  region  of  the  ovum  is  reduced  to  a  thin  layer 
at  the  upper  pole;  and  it  indicates,  further,  that  the  unequal  cleavage 
of  less  modified  telolecithal  eggs,  like  those  of  the  frog  or  snail,  are 
in  like  manner  due  to  the  displacement  of  the  mitotic  figures  toward 
the  upper  pole. 

The  results  of  Pfliiger's  and  Driesch's  pressure  experiments  obvi- 
ously harmonize  with  Hertw^ig's  second  rule,  for  the  position  of  least 
resistance  for  the  spindle  is  obviously  in  the  long  axis  of  the  proto- 
plasmic mass  which  is  here  artificially  modified  ;  and  it  harmonizes 
further  with  Driiner's  hypothesis  of  the  active  elongation  of  the 
spindle  in  mitosis  (p.  105).  There  are,  however,  a  large  number  of 
facts  which  show  that  neither  the  form  of  the  protoplasmic  mass  nor 

^  Cf.  P>ancotte  on  the  polar  l)odies  of  Turhellaria,  p.  235. 
^  For  a  good  review  and  critique,  see  Jennings.  '97. 


376 


CELL-DIVISION  AXD  DEVELOPMEXT 


the  distribution  of  metaplasmic  materials  is  sufficient  to  explain  the 
position  of  the  spindle,  whether  with  reference  to  the  direction  or  the 
inequality  of  the  cleavage. 

As  regards  the  direction  of  the  spindle,  Berthold  i^'^^)  long  since 
clearly  pointed  out  that  prismatic  or  cylindrical  vegetable  cells,  for 
instance,  those  of  the  cambium,  often  divide  lengthwise ;  and  numer- 
ous contradictions  of  Hertwig's  ''  law  "  have  since  been  observed  by 
students  of  cell-lineage  with  such  accuracy  that  all  attempts  to  explain 
them  away  have  failed.^  In  some  of  these  cases  the  position  of  the 
spindle  is  not  that  of  least  but  of  greatest  resistance,"^  the  spindle  ac- 


A 


B 


Fig.  176.  —  Segmenting  eggs  of  ^j-car/j.     [KOSTANECKI  and  SlEDLECKl,] 

A.  Early  prophase  of  second  division,  showing  double  centrosomes.     B.  Second  cleavage  in 
progress;  upper  blastomere  dividing  parallel  to  long  axis  of  the  cell. 


tually  pushing  away  the  adjoining  cell  to  make  way  for  itself.  Simi- 
lar difficulties,  some  of  which  have  been  already  considered  (p.  372), 
stand  in  the  way  of  the  attempt  to  explain  the  eccentricity  of  the 
spindle  in  unequal  division.  All  these  considerations  drive  us  to  the 
view  that  the  simpler  mechanical  factors,  such  as  pressure,  form,  and 
the  like,  are  subordinate  to  far  more  subtle  and  complex  operations 
involved  in  the  general  development  of  the  organism,  a  conclusion 
strikingly  illustrated  by  the  phenomena  of  teloblastic  division  (p.  371), 
where  the  constant  succession  of   unequal  divisions,  always   in   the 

1  Cf.  Watase  ('91),  Mead  ('94,  '97,  2),  Heidenhain   ('95),  Wheeler  ('95),  Castle  ('96), 
Jennings  ('97). 

2  See  especially  the  case  observed  by  Mead  ('94,  '97,  2),  in  the  egg  oi  Amphiij-ite. 


GEOMETRICAL   RELATIONS   OF  CLEAVAGE-FORMS  377 

same  plane,  is  correlated  with  a  deeply  lying  law  of  growth  affecting 
the  entire  formation  of  the  body.  We  cannot  comprcJiend  the  forms 
of  cleavage  witJiont  reference  to  the  end-resnlt ;  and  thus  these  phe- 
nomena acquire  a  certain  teleological  character  so  happily  expressed 
by  Lillie  (p.  370).  This  has  been  clearly  recognized  in  various  ways 
by  a  number  of  recent  writers.  Roux  ('94),  while  seeking  to  explain 
many  of  the  operations  of  mitosis  on  a  mechanical  basis,  holds  that 
the  position  of  the  spindle  is  partly  determined  by  "immanent" 
nuclear  tendencies.  Braem  ('94)  recognizes  that  the  position  of  the 
spindle  is  determined  not  merely  as  that  of  least  resistance  for  the 
mitotic  figure,  but  also  for  that  of  the  resulting  products.  I  pointed 
out  ('92)  that  the  bilateral  form  of  cleavage  in  annelids  must  be 
regarded  as  a  "forerunner"  of  the  adult  bilaterality.  Jennings  ('97) 
concludes  that  the  form  and  direction  of  cleavage  are  related  to  the 
later  morphogenetic  processes ;  and  many  similar  expressions  occur 
in  the  works  of  recent  students  of  cell-lineage.^ 

The  clearest  and  best  expression  of  this  view  is,  however,  given  bv 
Lillie  ('95,  '99),  who  not  only  correlates  the  direction  and  rate  of 
cleavage,  but  also  the  size-relations  of  the  cleavage-cells  with  the 
arrangement  of  the  adult  parts,  pointing  out  that  in  general  the  size, 
as  well  as  the  position,  of  the  blastomeres  is  directly  correlated  with 
that  of  the  parts  to  which  they  give  rise,  and  showing  that  on  this 
basis  "  one  can  thus  go  over  every  detail  of  the  cleavage,  and  know- 
ing the  fate  of  the  cells,  can  explain  all  the  irregularities  and  pecuH- 
arities  exhibited."^  Of  the  justice  of  this  conclusion  I  think  any  one 
must  be  thoroughly  convinced  who  carefully  examines  the  recent 
literature  of  cell-lineage.  It  gives  no  real  explanation  of  the  phenom- 
ena, and  is  hardly  more  than  a  restatement  of  fact.  Neither  does  it 
in  any  way  lessen  the  importance  of  studying  fully  the  mechanical 
conditions  of  cell-division.  It  does,  however,  show  how  inadequate 
have  been  most  of  the  attempts  thus  far  to  formulate  the  "  laws  "  of 
cell-division,  and  how^  superficially  the  subject  has  been  considered  by 
some  of  those  who  have  sought  for  such  "laws." 

We  now  pass  naturally  to  the  second  or  promorphological  aspect 
of  cleavage,  to  a  study  of  which  we  are  driven  by  the  foregoing  con- 
siderations. 

1  Conklin  ('99)  believes  that  many  of  the  pecuHarities  of  cleavage  may  he  explained  by 
the  assumption  of  protoplasmic  currents  which  "  carry  the  centrosomes  where  they  will,  and 
control  the  direction  of  division  and  the  relative  size  and  quality  of  the  daughter-cells,"' 
I.e.,  p.  90.  He  suggests  that  such  currents  are  of  a  chemotropic  character,  but  recognizes 
that  their  causation  and  direction  remain  unexplained. 

-  cf.  ('95),  p.  39. 


378  CELL-DIVISION  AND  DEVELOPMENT 


B.     Promorphological  Relations  of  Cleavage 

The  cleavage  of  the  ovum  has  thus  far  been  considered  in  the 
main  as  a  problem  of  cell-division.  We  have  now  to  regard  it  in  an 
even  more  interesting  and  suggestive  aspect ;  namely,  in  its  morpho- 
logical relations  to  the  body  to  which  it  gives  rise.  From  what  has 
been  said  above  it  is  evident  that  cleavage  is  not  merely  a  process  by 
which  the  ^^g  simply  splits  up  into  indifferent  cells  which,  to  use  the 
phrase  of  Pfliiger,  have  no  more  definite  relation  to  the  structure  of 
the  adult  body  than  have  snowflakes  to  the  avalanche  to  which  they 
contribute. 1  It  is  a  remarkable  fact  that  in  a  very  large  number  of 
cases  a  precise  relation  exists  between  the  cleavage-products  and  the 
adult  parts  to  which  they  give  rise ;  and  this  relation  may  often  be 
traced  back  to  the  beginning  of  development,  so  that  from  the  first 
division  onward  we  are  able  to  predict  the  exact  future  of  every  indi- 
vidual cell.  In  this  regard  the  cleavage  of  the  ovum  often  goes  for- 
ward with  a  wonderful  clocklike  precision,  giving  the  impression  of  a 
strictly  ordered  series  in  which  every  division  plays  a  definite  role  and 
has  a  fixed  relation  to  all  that  precedes  and  follows  it. 

But  more  than  this,  the  apparent  predetermination  of  the  embryo 
may  often  be  traced  still  farther  back  to  the  regions  of  the  undivided 
and  even  unfertilized  ovum.  The  Qgg,  therefore,  may  exhibit  a 
distinct  promorphology ;  and  the  morphological  aspect  of  cleavage 
must  be  considered  in  relation  to  the  promorphology  of  the  ovum  of 
which  it  is  an  expression. 

I .    PromorpJiology  of  the  Ovum 

{a)  Polarity  and  the  Egg-axis.  —  It  was  long  ago  recognized  by 
von  Baer  ('34)  that  the  unsegmented  ^gg  of  the  frog  has  a  definite 
egg-axis  connecting  two  differentiated  poles,  and  that  the  position 
of  the  embryo  is  definitely  related  to  it.  The  great  embryo] ogist 
pointed  out,  further,  that  the  early  cleavage-planes  also  are  definitely 
related  to  it,  the  first  two  passing  through  it  in  two  meridians  inter- 
secting each  other  at  a  right  angle,  while  the  third  is  transverse  to  it, 
and  is  hence  equatorial.^  Remak  afterward  recognized  the  fact  ^  that 
the  larger  cells  of  the  lower  hemisphere  represent,  broadly  speaking, 
the  ''vegetative  layer"  of  von  Baer,  i.e.  the  inner  germ-layer  or  ento- 
blast,  from  which  the  ahmentary  organs  arise ;  while  the  smaller  cells 

1  ('83),  p.  64. 

2  The  third  plane  is  in  this  case  not  precisely  at  the  equator,  but  considerably  above  it, 
forming  a  "  parallel  "  cleavage. 

3  '55,  p.  130.     Among  others  who  early  laid  stress  on  the  importance  of  the  egg-polarity 
maybe  mentioned  Auerbach  ('74),  Hatschek  ('77),  Whitman  ('78),  and  Van  Beneden  (^"^l). 


PROMORPHOLOGICAL  RELATIONS   OF  CLEAVAGE  379 

of  the  upper  hemisphere  represent  the  'f  animal  layer,"  outer  germ- 
layer  or  ectoblast  from  which  arise  the  epidermis,  the  nervous  s)^tcm, 
and  the  sense-organs.  This  fact,  afterward  confirmed  in  a  very  large 
number  of  animals,  led  to  the  designation  of  the  two  poles  as  aniimil 
and  vegetative,  formative  and  nutritive,  or  protoplasmic  and  dcuto- 
plasmic,  the  latter  terms  referring  to  the  fact  that  the  nutritive  deuto- 
plasm  is  mainly  stored  in  the  lower  hemisphere,  and  that  development 
is  therefore  more  active  in  the  upper.  The  polarity  of  the  ovum  is 
accentuated  by  other  correlated  phenomena.  In  every  case  where 
an  egg-axis  can  be  determined  by  the  accumulation  of  deutoplasm  in 
the  lower  hemisphere  the  egg-nucleus  sooner  or  later  lies  eccentri- 
cally in  the  upper  hemisphere,  and  the  polar  bodies  are  formed  at  the 
upper  pole.  Even  in  cases  where  the  deutoplasm  is  equally  distrib- 
uted or  is  wanting  —  if  there  really  be  such  cases  — an  egg-axis  is 
still  determined  by  the  eccentricity  of  the  nucleus  and  the  corre- 
sponding point  at  which  the  polar  bodies  are  formed. 

In  vastly  the  greater  number  of  cases  the  polarity  of  the  ovum  has 
a  definite  promorphological  significance  ;  for  the  egg-axis  shows  a 
definite  and  constant  relation  to  the  axes  of  the  adult  body.  It 
is  a  very  general  rule  that  the  upper  or  ectodermic  pole,  as  marked 
by  the  position  of  the  polar  bodies,  lies  in  the  median  plane  at  a  point 
which  is  afterward  found  to  lie  at  or  near  the  anterior  end.  Through- 
out the  annelids  and  mollusks,  for  example,  the  upper  pole  is  the  point 
at  which  the  cerebral  ganglia  are  afterward  formed ;  and  these 
organs  lie  in  the  adult  on  the  dorsal  side  near  the  anterior  extremity. 
This  relation  holds  true  for  many  of  the  Bilateralia,  though  the 
primitive  relation  is  often  disguised  by  asymmetrical  growth  in  the 
later  stages,  such  as  occur  in  echinoderms.  There  is,  however,  some 
reason  to  believe  that  it  is  not  a  universal  rule.  The  recent  observa- 
tions of  Castle  ('96),  which  are  in  accordance  with  the  earlier  work  of 
Seeliger,  show  that  in  the  tunicate  Ciona  the  usual  relation  is  reversed, 
the  polar  bodies  being  formed  at  the  vegetative  {i.e.  deutoplasmic  or 
entodermic)  pole,  which  afterward  becomes  the  dorsal  side  of  the 
larva.  My  own  observations  ('95)  on  the  echinoderm-egg  indicate 
that  here  the  primitive  egg-axis  has  an  entirely  inconstant  and  casual 
relation  to  the  gastrula-axis.  It  may,  however,  still  be  possi]:)lc  to 
show  that  these  exceptions  are  only  apparent,  and  the  principle  in- 
volved is  too  important  to  be  accepted  without  further  jDroof. 

{b)  Axial  Relations  of  the  Primary  Cleavage-planes.  —  Since  the 
egg-axis  is  definitely  related  to  the  embryonic  axes,  and  since  the 
first  two  cleavage-planes  pass  through  it,  we  may  naturally  look  for  a 
definite  relation  between  these  planes  and  the  embryonic  axes ;  and 
if  such  a  relation  exists,  then  the  first  two  or  four  bkistomeres  must 
likewise  have  a  definite  prospective  value  in  the  development.     Such 


38o 


CELL-DIVISION  AND   DEVELOPMENT 


relations  have,  in  fact,  been  accurately  determined  in  a  large  number 
of  cases.  The  first  to  call  attention  to  such  a  relation  seems  to  have 
been  Newport  ('54),  who  discovered  the  remarkable  fact  that  tJie  first 
cleavage-plane  in  the  frog  s  egg  coincides  with  tJie  niediaji  plane  of  the 
adult  body ;  that,  in  other  words,  one  of  the  first  two  blastomeres 
gives  rise  to  the  left  side  of  the  body,  the  other  to  the  right.  This 
discovery,  though  long  overlooked  and,  indeed,  forgotten,  was  con- 
firmed more  than  thirty  years  later  by  Pfliiger  and  Roux  {'^7).     It 


Fig.  177.  —  Bilateral  cleavage  of  the  tunicate  egg. 

A.  Four-celled  stage  of  Clavellna,  viewed  from  the  ventral  side.  B.  Sixteen-cell  stage  (VAN 
Beneden  and  JULIN).  C.  Cross-section  through  the  gastrula  stage  (Castle)  ;  a.  anterior; 
p.  posterior  end ;  /.  left,  /-.  right  side.     [Orientation  according  to  Castle.] 


was  placed  beyond  all  question  by  a  remarkable  experiment  by  Roux 
('88),  who  succeeded  in  killing  one  of  the  blastomeres  by  puncture 
with  a  heated  needle,  whereupon  the  uninjured  cell  gave  rise  to  a 
half-body  as  if  the  embryo  had  been  bisected  down  the  middle  line 
(Fig.  182). 

A  similar  result  has  been  reached  in  a  number  of  other  animals  by 
following  out  the  cell-lineage ;    e.g.  by  Van  Beneden  and  Julin  ('84) 


PROMORPHOLOGICAL   RELATIOXS  OF  CLEAVAGE 


3S1 


in  the  ^g^  of  the  tunicate  Claveliiia  (Fig.  177),  and  by  W^atase  ('91) 
in  the  eggs  of  cephalopods  (Fig.  178).  In  both  these  cases  all  the 
early  stages  of  cleavage  show  a  beautiful  bilateral  symmetry,  and  not 
only  can  the  right  and  left  halves  of  the  segmenting  o^^^^  be  di.'^tin- 
guished  with  the  greatest  clearness,  but  also  the  anterior  and  jjoste- 
rior  regions,  and  the  dorsal  and  ventral  aspects.  These  discoveries 
seemed,  at  first,  to  justify  the  hope  that  a  fundamental  law  of  develop- 
ment had  been  discovered,  and  Van  Beneden  was  thus  led,  as  early 
as  1883,  to  express  the  view  that  the  development  of  all  bilateral 
animals  would  probably  be  found  to  agree  with  the  frog  and  ascidian 
in  respect  to  the  relations  of  the  first  cleavage. 

This  cleavage  was  soon  proved  to  have  been  premature.      In  one 
series  of  forms,  not  the  first  but  the  second  cleavage-plane  was  found 


/ 


—  '  V 


Fig.  178.  —  Bilateral  cleavage  of  the  squid's  egg.     [\V.\T.\SE.] 

A.  Eight-cell   stage.     B.  The  fifth  cleavage  in  progress.     The  first  cleavage  {a-p)  coincides 
with  the  future  median  plane;  the  second  {l-r)  is  transverse. 

to  coincide  with  the  future  long  axis  (7\^^;r2>,  and  some  other  annelids  ; 
Crepidula,  UiJibrclla,  and  other  gasteropods).  In  another  series  (A 
forms  neither  of  the  first  cleavages  passes  through  the  median  plane, 
but  both  form  an  angle  of  about  45°  to  it  {Clcpsinc  and  other  leeches  ; 
RJiynchelniis  and  other  annelids  ;  Planorbis,  Xassa,  L  'nio,  and  other 
mollusks  ;  Discococlis  and  other  platodes).  In  a  few  cases  the  first 
cleavage  departs  entirely  from  the  rule,  and  is  equatorial,  as  in  Ascans 
and  some  other  nematodes.  The  whole  subject  was  finally  thrown 
into  apparent  confusion,  first  by  the  discovery  of  Clapp  ('91 ),  Jordan, 
and  Eycleshymer  ('94)  that  in  some  cases  there  seems  to  be  no  con- 
stant relation  whatever  between  the  early  cleavage-planes  and  the 
adult  axes,  even  in  the  same  species  (teleosts,  urodeles);  and  even  in 


382 


CELL-DIVISION  AND  DEVELOPMENT 


the  frog  Hertwig  showed  that  the  relation  described  by  Newport  and 
Roux  is  not  invariable.  Driesch  finally  demonstrated  that  the  direc- 
tion of  the  early  cleavage-planes  might  be  artificially  modified  by 
pressure  without  perceptibly  affecting  the  end-result  {cf.  p.  375). 

These  facts  prove  that  the  promorphology  of  the  early  cleavage- 
forms  can  have  no  fundamental  significance.  Nevertheless,  they  are 
of  the  highest  interest  and  importance ;  for  the  fact  that  the  forma- 
tive forces  by  which  development  is  determined  may  or  may  not 
coincide  with  those  controlling  the  cleavage,  gives  us  some  hope  of 

a 


1}  V 

Fig.  179.  —  Outline  of  unsegmented  squid's  egg,  to  show  bilaterality.     [Watase.] 

A.  From  right  side.     B.  From  posterior  aspect. 

a-p.  antero-posterior  axis  ;  d-v.  dorso-ventral  axis  ;  /.  left  side  ;  r.  right  side. 


disentangling  the  complicated  factors  of  development  through  a  com- 
parative study  of  the  different  forms. 

(^)  OtJier  Promo7f  ho  logical  CJiaracters  of  the  Ovnm.  —  Besides  the 
polarity  of  the  ovum,  which  is  the  most  constant  and  clearly  marked 
of  its  promorphological  features,  we  are  often  able  to  discover  other 
characters  that  more  or  less  clearly  foreshadow  the  later  develop- 
ment. One  of  the  most  interesting  and  clearly  marked  of  these  is 
the  bilateral  symmetry  of  the  ovum  in  bilateral  animals,  which  is 
sometimes  so  clearly  marked  that  the  exact  position  of  the  embryo 
may  be  predicted  in  the  unfertilized  ^gg,  sometimes  even  before  it  is 
laid.  This  is  the  case,  for  example,  in  the  cephalopod  ^gg,  as  shown 
by  Watase  (Fig.  179).  Here  the  form  of  the  new-laid  ^gg,  before 
cleavage  begins,  distinctly  foreshadows  that  of  the  embryonic  body, 
and  forms  as  it  were  a  mould  in  which  the  whole  development  is  cast. 
Its  general  shape  is  that  of  a  hen's  ^gg  sHghtly  flattened  on  one  side, 


PROMORPIWLOGICAL   RELATIOXS   OF  CLEAVAGE 


3S 


the  narrow  end,  according  to  Watase,  representing  the  dorsal  aspect 
the   broad  end   the  ventral   aspect,  the    flattened   side   the   posterior 
region,  and  the  more  convex  side  the  anterior  region.     A//  the  early 
cleavage-furroivs  are  bilaterally  arranged  ivith  respect  to  the  plane  of 


a 


-1^ 

Fig.  i8o.  — Eggs  of  the  insect  Corixa.     [Metschnikoff.] 
A.  Early  stage  before  formation  of  the  embryo,  from  one  side.     />'.    The  same  viewed  in  the 
plane  of  symmetry.     C.  The  embryo  in  its  final  position. 

a.  anterior  end;  p.  posterior;  /.  left  side,  r.  right;  f.  ventral,  d.  dorsal  aspect.  (These  letters 
refer  to  theyf«a/ position  of  the  embryo,  which  is  nearly  diametrically  opposite  to  that  in  which  it 
first  develops)  ;  m.  micropyle ;  near/  is  the  pedicle  by  which  the  egg  is  attached. 


symmetry  in  the  nndivided  egg ;  and  the  same  is   true  of   the  later 
development  of  all  the  bilateral  parts. 

Scarcely  less  striking  is  the  case  of  the  insect  ^^^.  as  has  been 
pointed  out  especially  by  Hallez,  Blochmann,  and  Wheeler  (F'igs. 
62,    180).     In  a  large   number  of    cases   the  ^^g  is  elongated   and 


384  CELL-DIVISION  AND  DEVELOPMENT 

bilaterally  symmetrical,  and,  according  to  Blochmann  and  Wheeler, 
may  even  show  a  bilateral  distribution  of  the  yolk  corresponding 
with  the  bilaterality  of  the  ovum.  Hallez  asserts  as  the  results  of  a 
study  of  the  cockroach  {Pcriplancta\  the  water-beetle  {Hydrophilus), 
and  the  locust  {Locnsta)  that  ''  the  egg-cell  possesses  the  same  orien- 
tation as  the  maternal  organism  that  produces  it ;  it  has  a  cephalic 
pole  and  a  caudal  pole,  a  right  side  and  a  left,  a  dorsal  aspect  and  a 
ventral ;  and  these  different  aspects  of  the  egg-cell  coincide  with  the 
corresponding  aspects  of  the  embryo."  ^  Wheeler  ('93),  after  ex- 
amining some  thirty  different  species  of  insects,  reached  the  same 
result,  and  concluded  that  even  when  the  ^gg  approaches  the 
spherical  form  the  symmetry  still  exists,  though  obscured.  More- 
over, according  to  Hallez  ('86)  and  later  writers,  the  Qgg  always  lies 
in  the  same  position  in  the  oviduct,  its  cephalic  end  being  turned 
forwards  toward  the  upper  end  of  the  oviduct,  and  hence  toward 
the  head-end  of  the  mother.^ 

2.    Meaning  of  the  PromorpJiology  of  the  Ovum 

The  interpretation  of  the  promorphology  of  the  ovum  cannot  be 
adequately  treated  apart  from  the  general  discussion  of  development 
given  in  the  following  chapter;  nevertheless  it  may  briefly  be 
considered  at  this  point.  Two  widely  different  interpretations  of 
the  facts  have  been  given.  On  the  one  hand,  it  has  been  sug- 
gested by  Flemming  and  Van  Beneden,^  and  urged  especially  by 
Whitman,*  that  the  cytoplasm  of  the  ovum  possesses  a  definite 
primordial  organization  which  exists  from  the  beginning  of  its  exist- 
ence even  though  invisible,  and  is  revealed  to  observation  thiough 
polar  differentiation,  bilateral  symmetry,  and  other  obvious  characters 
in  the  unsegmented  Qgg.  On  the  other  hand,  it  has  been  maintained 
by  Pfliiger,  Mark,  Oscar  Hertwig,  Driesch,  Watase,  and  the  writer 
that  all  the  promorphological  features  of  the  ovum  are  of  secondary 
origin;  that  the  egg-cytoplasm  is  at  the  beginning  isotropous  —  i.e. 
indifferent  or  homaxial  —  and  gradually  acquires  its  promorphological 
features  during  its  preembryonic  history.  Thus  the  ^gg  of  a  bilateral 
animal  is  at  the  beginning  not  actually,  but  only  potentially,  bilateral. 
Bilaterality  once  established,  however,  it  forms  as  it  were  the  mould 
in  which  the  cleavage  and  other  operations  of  development  are  cast, 

I  believe  that  the  evidence  at  our  command  weighs  heavily  on 
the  side  of   the  second  view,  and  that  the  first  hypothesis  fails  to 

1  See  Wheeler,  '93,  p.  67. 

2  The  micropyle  usually  lies  at  or  near  the  anterior  end,  but  may  be  at  the  posterior. 
It  is  a  very  important  fact  that  the  position  of  the  polar  bodies  varies,  being  sometimes  at 
the  anterior  end,  sometimes  on  the  side,  either  dorsal  or  lateral  (Heider,  Blochmann). 

3- See  p.  298.  *  Cf.  pp.  299,  300. 


PROMORPHOLOGICAL   RELATIONS  OF  CLEAVAGE 


385 


take  sufficient  account  of  the  fact  that  development  docs  not  nec- 
essarily begin  with  fertilization  or  cleavage,  but  may  begin  at  a  far 
earlier  period  during  ovarian  life.  As  far  as  the  visible  promorpho- 
logical  features  of  the  ovum  are  concerned,  this  conclusion  is  beyond 
question.  The  only  question  that  has  any  meaning  is  whether  these 
visible  characters  are   merely  the  expression  of  a  more   subtle  pre- 


Fig.  181.  —  Variations  in  the  axial  relations  of  the  eggs  of  Cyclops.  From  sections  of  the  eggs 
as  they  lie  in  the  oviduct.     [HaCKER.] 

A.  Group  of  eggs  showing  variations  in  relative  position  of  the  polar  spindles  and  the  sperm- 
nucleus  (the  latter  black)  ;  in  a  the  sperm-nucleus  is  opposite  to  the  polar  spindle,  in  b,  near  it  or 
at  the  side.  B.  Group  showing  variations  in  the  axis  of  first  cleavage  with  reference  to  the  polar 
bodies  (the  latter  black)  ;  a,  b,  and  ^  show  three  different  positions. 


existing  invisible  organization  of  the  same  kind.  I  do  not  believe 
that  this  question  can  be  answered  in  the  affirmative  save  by  the 
trite  and,  from  this  point  of  view,  barren  statement  that  every  effect 
must  have  its  preexisting  cause.  That  the  ^gg  possesses  no  fixed 
and  predetermined  cytoplasmic  localization  with  reference  to  the 
adult  parts,  has,  I  think,  been  demonstrated  through  the  remarkable 


2C 


386  CELL-DIVISION  AND  DEVELOPMENT 

experiments  of  Driesch,  Roux,  and  Boveri,  which  show  that  a  frag- 
ment of  the  ^^g  may  give  rise  to  a  complete  larva  (p.  353).  There 
is  strong  evidence,  moreover,  that  the  egg-axis  is  not  primordial  but 
is  established  at  a  particular  period ;  and  even  after  its  establishment 
it  may  be  entirely  altered  by  new  conditions.  This  is  proved,  for 
example,  by  the  case  of  the  frog's  egg,  in  which,  as  Pfluger  ('84), 
Born  ('85),  and  Schultze  ('94)  have  shown,  the  cytoplasmic  materials 
may  be  entirely  rearranged  under  the  influence  of  gravity,  and  a 
new  axis  established.  In  sea-urchins,  my  own  observations  ('95) 
render  it  probable  that  the  egg-axis  is  not  finally  established  until 
after  fertilization.  These  and  other  facts,  to  be  more  fully  considered 
in  the  following  chapter,  give  strong  ground  for  the  conclusion  that 
the  promorphological  features  of  the  ^gg  are  as  truly  a  result  of 
development  as  the  characters  coming  into  view  at  later  stages.  They 
are  gradually  established  during  the  preembryonic  stages,  and  the 
^gg,  when  ready  for  fertilization,  has  already  accomplished  part  of 
its  task  by  laying  the  basis  for  what  is  to  come. 

Mark,  who  was  one  of  the  first  to  examine  this  subject  carefully, 
concluded  that  the  ovum  is  at  first  an  indifferent  or  homaxial  cell 
{i.e.  isotropic),  which  afterward  acquires  polarity  and  other  promor- 
phological features.^  The  same  view  was  very  precisely  formulated 
by  Watase  in  1891,  in  the  following  statement,  which  I  believe  to 
express  accurately  the  truth  :  "  It  appears  to  me  admissible  to  say 
at  present  that  the  ovum,  w^hich  may  start  out  without  any  definite 
axis  at  first,  may  acquire  it  later,  and  at  the  moment  ready  for  its 
cleavage  the  distribution  of  its  protoplasmic  substances  may  be  such 
as  to  exhibit  a  perfect  symmetry,  and  the  furrows  of  cleavage  may 
have  a  certain  definite  relation  to  the  inherent  arrangement  of  the 
protoplasmic  substances  which  constitute  the  ovum.  Hence,  in  a 
certain  case,  the  plane  of  the  first  cleavage-furrow  may  coincide  with 
the  plane  of  the  median  axis  of  the  embryo,  and  the  sundering  of 
the  protoplasmic  material  may  take  place  into  right  and  left,  accord- 
ing to  the  preexisting  organization  of  the  ^gg  at  the  time  of  cleav- 
age ;  and  in  another  case  the  first  cleavage  may  roughly  correspond 
to  the  differentiation  of  the  ectoderm  and  the  entoderm,  also  accord- 
ing to  the  preorganized  constitution  of  the  protoplasmic  materials  of 
the  ovum. 

"  It  does  not  appear  strange,  therefore,  that  we  may  detect  a  cer- 
tain structural  differentiation  in  the  unsegmented  ovum,  with  all  the 
axes  foreshadowed  in  it,  and  the  axial  symmetry  of  the  embryonic 
organism  identical  with  that  of  the  adult."  ^ 

This  passage  contains,  I  believe,  the  gist  of  the  whole  matter,  as 
far  as  the  promorphological  relations  of  the  ovum  and  of  cleavage- 

1  '81,  p.  512.  2  'gi^  p_  2S0. 


PROMORPIIOLOGICAL  RELATIONS   OF  CLEAVAGE  387 

forms  are  concerned,  though  Watase  does  not  enter  into  the  question 
as  to  how  the  arrangement  of  protoplasmic  materials  is  effected.  In 
considering  this  question,  we  must  hold  fast  to  the  fundamental  fact 
that  the  ^%g  is  a  cell,  like  other  cells,  and  that  from  an  a  priori  point 
of  view  there  is  every  reason  to  believe  that  the  cytoplasmic  differ- 
entiations that  it  undergoes  must  arise  in  essentially  the  same  way  as 
in  other  cells.  We  know  that  such  differentiations,  whether  in  form 
or  in  internal  structure,  show  a  definite  relation  to  the  environment 
of  the  cell  —  to  its  fellows,  to  the  source  of  food,  and  the  like.  We 
know  further,  as  Korschelt  especially  has  pointed  out,  that  the  egg- 
axis,  as  expressed  by  the  eecentricity  of  the  gennitial  vesicle,  often 
sJiozvs  a  definite  relation  to  the  ovanaji  tissues,  the  germinal  vesicle 
lying  near  the  point  of  attachment  or  of  food-supply.  Mark  made 
the  pregnant  suggestion,  in  1881,  that  the  primary  polarity  of  the  egg 
might  be  determined  by  ''the  topographical  relation  of  the  egg  {\\\\k:\\ 
still  in  an  indifferent  state)  to  the  remaining  cells  of  the  maternal  tis- 
sue f'om  ivhich  it  is  differentiated,''  2i\iA  added  that  this  relation  might 
operate  through  the  nutrition  of  the  ovum.  "  It  would  certainly  be 
interesting  to  know  if  that  phase  of  polar  differentiation  which  is 
manifest  in  the  position  of  the  nutritive  substance  and  of  the  germi- 
nal vesicle  bears  a  constant  relation  to  the  free  surface  of  the  epithe- 
lium from  which  the  egg  takes  its  origin.  If,  in  cases  where  the  egg 
is  directly  developed  from  epithelial  cells,  this  relationship  were 
demonstrable,  it  would  be  fair  to  infer  the  existence  of  correspond- 
ing, though  obscured,  relations  in  those  cases  where  (as,  for  example, 
in  mammals)  the  origin  of  the  ovum  is  less  directly  traceable  to  an 
epithelial  surface."  ^  The  polarity  of  the  egg  would  therefore  be 
comparable  to  the  polarity  of  epithelial  or  gland-cells,  where,  as 
pointed  out  at  page  57,  the  nucleus  usually  lies  toward  the  base  of  the 
cell,  near  the  source  of  food,  while  the  centrosomes,  and  often  also 
characteristic  cytoplasmic  products,  such  as  zymogen  granules  and 
other  secretions,  appear  in  the  outer  portion. ^  The  exact  conditions 
under  which  the  ovarian  egg  develops  are  still  too  little  known  to 
allow  of  a  positive  conclusion  regarding  Mark's  suggestion.  More- 
over, the  force  of  Korschelt's  observation  is  weakened  by  the  fact  that 
in  many  eggs  of  the  extreme  telolecithal  type,  where  the  j^olarity  is 
very  marked,  the  germinal  vesicle  occupies  a  central  or  sub-central 
position  during  the  period  of  yolk-formation  and  only  moves  toward 
the  periphery  near  the  time  of  maturation. 

Indeed,  in  mollusks,  annelids,  and  many  other  cases,  the  germinal 
vesicle  remains  in  a  central  position,  surrounded  by  yolk  on  all  sides, 
until  the  spermatozoon  enters.     Only  then  does  the  egg-nucleus  move 

i'8i,p.  515. 

2  Hatschek  has  suggested  the  same  CDmparisun  (^Zoologie,  p.  1 12). 


388  CELL-DIVISION  AND  DEVELOPMENT 

to  the  periphery,  the  deutoplasm  become  massed  at  one  pole,  and 
the  polarity  of  the  ^g^  come  into  view  (7V^;r/j,  Figs.  60  and  97).^  In 
such  cases  the  axis  of  the  ^gg  may  perhaps  be  predetermined  by 
the  position  of  the  centrosome,  and  we  have  still  to  seek  the  causes 
by  which  the  position  is  established  in  the  ovarian  history  of  the  ^gg. 
These  considerations  show  that  this  problem  is  a  complex  one,  involv- 
ing, as  it  does,  the  whole  question  of  cell-polarity ;  and  I  know  of 
no  more  promising  field  of  investigation  than  the  ovarian  history  of 
the  ovum  with  reference  to  this  question.  That  Mark's  view  is  cor- 
rect in  principle  is  indicated  by  a  great  array  of  general  evidence 
considered  in  the  following  chapter,  where  its  bearing  on  the  general 
theory  of  development  is  more  fully  dealt  with. 

C.     Cell-division  and  Growth 

The  general  relations  between  cell-division  and  growth,  which  have 
already  been  briefly  considered  at  page  58  and  in  the  course  of  this 
chapter,  may  now  be  more  critically  examined,  together  with  some 
account  of  the  causes  that  incite  or  inhibit  division.  It  has  been 
shown  above  that  every  precise  inquiry  into  the  rate  form,  or  direc- 
tion of  cell-division,  inevitably  merges  into  the  larger  problem  of  the 
general  determination  of  growth.  We  may  conveniently  approach 
this  subject  by  considering  hrst  the  energy  of  division  and  the  limita- 
tion of  growth. 

All  animals  and  plants  have  a  limit  of  growth,  which  is,  how- 
ever, much  more  definite  in  some  forms  than  in  others,  and  differs 
in  different  tissues.  During  the  individual  development  the  energy 
of  cell-division  is  most  intense  in  the  early  stages  (cleavage)  and 
diminishes  more  and  more  as  the  Umit  of  growth  is  approached. 
When  the  Hmit  is  attained  a  more  or  less  definite  equilibrium  is  estab- 
lished, some  of  the  cells  ceasing  to  divide  and  perhaps  losing  this 
power  altogether  (nerve-cells),  others  dividing  only  under  special  con- 
ditions (connective  tissue-cells,  gland-cells,  muscle-cells),  while  others 
continue  to  divide  throughout  life,  and  thus  replace  the  worn-out  cells 
of  the  same  tissue  (Malpighian  layer  of  the  epidermis,  etc.).  The 
limit  of  size  at  which  this  state  of  equilibrium  is  attained  is  an  heredi- 
tary character,  which  in  many  cases  shows  an  obvious  relation  to  the 
environment,  and  has  therefore  probably  been  determined  and  is 
maintained  by  natural  selection.  From  the  cytological  point  of  view 
the  limit  of  body-size  appears  to  be  correlated  with  the  total  number 
of  cells  formed  rather  than  with  their  individual  size.  This  relation 
has  been  carefully  studied  by  Conklin  ('96)  in  the  case  of  the  gastero- 

1  The  immature  egg  of  Nereis  shows,  however,  a  distinct  polarity  in  the  arrangement  of 
the  fat-drops,  which  form  a  ring  in  the  equatorial  regions. 


CELL-DIVISION  AND    GROW  Til  389 

pod  Crepidula,  an  animal  which  varies  greatly  in  size  in  the  mature 
condition,  the  dwarfs  having  in  some  cases  not  more  than  .,\-  the  vol- 
ume of  the  giants.  The  eggs  are,  however,  of  the  same  size  in  all, 
and  their  number  is  proportional  to  the  size  of  the  adult.  The  same 
is  true  of  the  tissue-cells.  Measurements  of  cells  from  the  epidermis, 
the  kidney,  the  liver,  the  alimentary  epithelium,  and  other  tissues 
show  that  they  are  on  the  whole  as  large  in  the  dwarfs  as  in  the 
giants.  The  body-size  therefore  depends  on  the  total  number  of  cells 
rather  than  on  their  size  individually  considered,  and  the  same  appears 
to  be  the  case  in  plants.^ 

A  result  which,  broadly  speaking,  agrees  with  the  foregoing,  is 
given  through  the  interesting  experimental  studies  of  Morgan  ('95,  i, 
'96),  supplemented  by  those  of  Driesch  ('98),  in  which  the  number  of 
cells  in  normal  larvae  of  echinoderms,  ascidians,  and  Aniphioxus  is 
compared  with  those  in  dwarf  larvae  of  the  same  species  developed 
from  egg-fragments  (Morgan)  and  isolated  blastomeres  (Driesch). 
Unless  otherwise  specified,  the  follow^ing  data  are  cited  from  Driesch. 

The  normal  blastula  of  SpJiccrccJiijius  possesses  about  500  cells 
(Morgan),  of  which  from  75  to  90  invaginate  to  form  the  archenteron 
(Driesch).  In  half-gastrulas  the  number  varies  from  35  to  45,  occa- 
sionally reaching  50.  In  the  same  species,  the  normal  number  of 
mesenchyme-cells  is  54  to  60,  in  the  half-larvae  25  to  30.  In  Echinus 
the  corresponding  numbers  are  30 ±  and  13  to  15.  In  the  ascidian 
larvae  —  a  particularly  favourable  object  —  there  are  29  to  35  (excep- 
tionally as  high  as  40)  chorda-cells  ;  in  the  half-larvae,  1 3  to  17.  While 
these  comparisons  are  not  mathematically  precise,  owing  to  the  diffi- 
culty of  selecting  exactly  equivalent  stages,  they  nevertheless  show 
that,  on  the  whole,  the  size  of  the  organ,  as  of  the  entire  organism,  is 
directly  proportional  to  the  number  and  not  to  the  size  of  the  cells, 
just  as  in  the  mature  individuals  of  Crepidula.  The  available  data 
are,  however,  too  scanty  to  justify  any  very  positive  conclusions,  and 
it  is  probable  that  further  experiment  will  disclose  factors  at  present 
unknown.  It  would  be  highly  interesting  to  determine  whether  such 
dwarf  embryos  could  in  the  end  restore  the  normal  number  of  cells, 
and,  hence,  the  normal  size  of  the  body.  In  all  the  cases  thus  far 
determined  the  dwarf  gastrulas  give  rise  to  larvae  {Plutci,  etc.)  corre- 
spondingly dwarfed ;  but  their  later  history  has  not  yet  been  suffi- 
ciently followed  out. 

The  gradual  diminution  of  the  energy  of  division  during  develop- 
ment by  no  means  proceeds  at  a  uniform  pace  in  all  of  the  cells,  and, 
during  the  cleavage,  the  individual  blastomeres  are  often  found  to 
exhibit  entirely  different  rhythms  of  division,  periods  of  active  division 
being  succeeded  by  long  pauses,  and  sometimes  by  an  entire  cessa- 

1  See  Amelung  ('93)  and  Strasburger  ('93). 


390  CELL-DIVISION  AND  DEVELOPMENT 

tion  of  division  even  at  a  very  early  period.     In  the  echinoderms,  for 
example,  it  is  well  established  that  division  suddenly  pauses,  or  changes 
its  rhythm,  just  before  the  gastrulation  (in  Synapta  at  the   512-cell 
stage,  according  to  Selenka),  and  the  same  is  said  to  be  the  case  in 
Aniphioxus  (Hatschek,  Lwoff).     In  Nereis,  one  of  the  blastomeres  on 
each  side  of  the  body  in  the  forty-two-cell  stage  suddenly  ceases  to 
divide,  migrates  into  the  interior  of  the  body,  and  is  converted  into  a 
unicellular  glandular  organ. ^     In  the  same  animal,  the  four  lower  cells 
(macromeres)  of  the  eight-cell  stage  divide  in  nearly  regular  succes- 
sion up  to  the  thirty-eight-cell  stage,  when  a  long  pause  takes  place, 
and  when  the  divisions  are  resumed  they  are  of  a  character  totally 
different  from  those  of  the  earlier  period.      The  cells  of  the  ciliated 
belt  or  prototroch  in  this  and  other  annelids  likewise  cease  to  divide 
at  a  certain  period,  their  number  remaining  fixed  thereafter.^     Again, 
the  number  of  cells  produced  for  the  foundation  of  particular  struc- 
tures is  often  definitely  fixed,  even  when  their  number  is  afterward 
increased  by  division.     In  annelids  and  gasteropods,  for  example,  the 
entire  ectoblast  arises  from  twelve  micromeres  segmented  off  in  three 
successive  quartets  of  micromeres  from  the  blastomeres  of  the  four- 
cell  stage.      Perhaps  the  most  interesting  numerical  relations  of  this 
kind  are  those  recently  discovered  in  the  division  of  teloblasts,  where 
the  number  of  divisions  is  directly  correlated  with  the  number  of  seg- 
ments or  somites.     It  is  well  known  that  this  is  the  case  in  certain  plants 
{C/iaracece),  where  the  alternating  nodes  and  intern  odes  of  the  stem 
are  derived  from  corresponding  single  cells  successively  segmented 
off  from  the  apical  cell.     Vejdovsky  s   observations  on  the  annelid 
Dendrobcena  give  strong  ground  to  believe  that  the  number  of  meta- 
merically  repeated  parts  of  this  animal,  and  probably  of  other  anne- 
hds,   corresponds  in  Uke  manner  with  that  of  the  number  of   cells 
segmented  off  from  the  teloblasts.     The  most  remarkable  and  accu- 
rately determined  case  of  this  kind  is  that  of  the  isopod  Crustacea, 
where  the  number  of  somites  is  limited  and  perfectly  constant.     In 
the  embryos  of  these  animals  there  are  two  groups  of  teloblasts  near 
the  hinder  end  of  the  embryo,  viz.  an  inner  group  of  mesoblasts,  from 
which  arise  the  mesoblast-bands,  and  an  outer  group  of  ectoblasts, 
from  which  arise  the  neural  plates  and  the  ventral  ectoblast.      McMur- 
rich  ('95)  has  recently  demonstrated  that  the  mesoblasts  always  divide 
exactly  sixteen  times,  the  ectoblasts  thirty-two  (or  thirty-three)  times, 
before  relinquishing  their  teleoblastic  mode  of  division  and  breaking 
up  into  smaller  cells.     Now  the  sixteen  groups  of  cells  thus  formed 
give  rise  to  the  sixteen  respective  somites  of  the  post-naupliar  region 
of  the  embryo  {i.e.  from  the  second  maxilla  backward).      In   other 

1  This  organ,  doubtfully  identified  by  me  as  the  head-kidney,  is  probably  a  mucus-gland 
(Mead).  2  cf.  Fig.   171. 


CELL-DIVISION  AND    GROWTH  301 

words,  each  single  division  of  the  mesoblasts  and  each  double  division 
of  the  ectoblasts  splits  off  the  material  for   a   single  somite  '     The 
number  of  these  divisions,  and  hence  of  the  corresponding  somites 
is  a  fixed  inheritance  of  the  species. 

The  causes  that  determine  the  rhythm  of  division,  and  thus  finally 
estabhsh  the  adult  equilibrium,  are  but  vaguely  comprehended      The 
ultimate  causes  must  of  course  lie  in  the  inherited  constitution  of  the 
organism,  and  are  referable  in  the  last  analysis  to  the  structure  of 
the  germ-cells.     Every  division  must,  however,  be  the  response  of  the 
cell  to  a  particular  set  of  conditions  or  stimuh ;  and  it  is  through  the 
investigation  of  these  stimuh  that  we  may  hope  to  penetrate  f'arther 
into  the  nature  of  development.     The  immediate,  specific  causes  of 
cell-division  are  still  imperfectly  known.     In  the  adult,  cells  may  be 
stimulated  to  divide  by  the  utmost  variety  of  agencies  — by  chemical 
stimulus,  as  in  the  formation  of  galls,  or  in  hyperplasia  induced  by 
the   injection  of  foreign   substances  into  the   blood;  by  mechanical 
pressure,  as  in  the  formation  of  calluses;  by  injury,  as  in  the  heahng 
of  wounds  and  in  the  regeneration  of  lost  parts ;  and  by  a  multitude 
of  more  complex  physiological  and  pathological  conditions, —  by  any 
agency,  in  short,  that  disturbs  the  normal  equilibrium  of  the  body. 
In  all  these  cases,  however,  it  is  difficult  to  determine  the  immediate 
stimulus  to   division  ;  for   a   long   chain   of   causes  and   effects   may 
intervene  between  the  primary  disturbance  and  the  ultimate  reaction 
of  the  dividing  cells.     Thus  there  is  reason  to  believe  that  the  for- 
mation of  a  callus  is  not  directly  caused  by  pressure  or  friction,  but 
through  the  determination  of  an  increased  blood-supply  to  the  part 
affected  and  a  heightened  nutrition  of  the  cells.     Cell-division  is  here 
probably  incited  by  local  chemical  changes  ;  and  the  opinion  is  gain- 
ing ground   that  the   immediate   causes   of  division,   whatever  their 
antecedents,  are  to  be  sought  in  this  direction.      That  such   is   the 
case  is  indicated  by  nothing  more  clearly  than  the  recent  experiments 
on  the  Qgg  by  R.  Hertwig,  Mead,  Morgan,  and  Loeb  already  referred 
to  in  part  at  pages  1 1 1  and  215.     The  egg-cell  is,  in  most  cases,  stimu- 
lated to  divide  by  the  entrance  of  the  spermatozoon,  but  in  partheno- 
genesis exactly  the  same  result  is  produced  by  an  apparently  quite 
different  cause.     The  experiments  in  question  give,  however,  ground 
for  the  conclusion  that  the  common  element  in  the  two  cases  is  a 
chemical  stimulus.      In  the  eggs  of  ChcEtopterus  under  normal  condi- 
tions the  first  polar  mitosis  pauses  at  the  anaphase  until  the  entrance 
of  the  spermatozoon,  when  the  mitotic  activity  is  resumed  and  both 
polar  bodies  are  formed.     Mead  ('98)  shows,  however,  that  the  same 
effect  may  be  produced  without  fertilization  by  placing  the  eggs  for 
a  few   minutes   in   a  weak  solution   of   potassium  chloride.      In  like 
manner  R.   Hertwig  ('96)  and  Morgan  ('99)  show   that   unfertilized 


392 


CELL-DIVISION  AND  DEVELOPMENT 


echinoderm-eggs  may  be  stimulated  to  division  by  treatment  with 
weak  solution  of  strychnine,  sodium-chloride,  and  other  reagents,  the 
result  being  here  more  striking  than  in  the  case  of  CJicEtopterns,  since 
the  entire  mitotic  system  is  formed  anew  under  the  chemical  stimulus. 
The  cUmax  of  these  experiments  is  reached  in  Loeb's  artificial  pro- 
duction of  parthenogenesis  in  sea-urchin  eggs  by  treatment  with  dilute 
magnesium  chloride.  Beside  these  interesting  results  may  be  placed 
the  remarkable  facts  of  gall-formation  in  plants,  which  seem  to  leave 
no  doubt  that  extremely  complex  and  characteristic  abnormal  growths 
may  result  from  specific  chemical  stimuli,  and  many  pathologists 
have  held  that  tumours  and  other  pathological  growths  in  the  animal 
body  may  be  incited  through  disturbances  of  circulation  or  other 
causes  resulting  in  abnormal  local  chemical  conditions.^ 

But  while  we  have  gained  some  light  on  the  immediate  causes  of 
division,  we  have  still  to  inquire  how  those  causes  are  set  in  opera- 
tion and  are  coordinated  toward  a  typical  end;  and  w^e  are  thus 
brought  again  to  the  general  problem  of  growth.  A  very  interesting 
suggestion  is  the  resistance-theory  of  Thiersch  and  Boll,  according 
to  which  each  tissue  continues  to  grow  up  to  the  limit  afforded  by 
the  resistance  of  neighbouring  tissues  or  organs.  The  removal  or 
lessening  of  this  resistance  through  injury  or  disease  causes  a  resump- 
tion of  growth  and  division,  leading  either  to  the  regeneration  of  the 
lost  parts  or  to  the  formation  of  abnormal  growths.  Thus  the 
removal  of  a  salamander's  limb  would  seem  to  remove  a  barrier  to 
the  proliferation  and  growth  of  the  remaining  cells.  These  processes 
are  therefore  resumed,  and  continue  until  the  normal  barrier  is  re- 
established by  the  regeneration.  To  speak  of  such  a  "barrier"  or 
**  resistance  "  is,  however,  to  use  a  highly  figurative  phrase  which  is 
not  to  be  construed  in  a  rude  mechanical  sense.  There  is  no  doubt 
that  hypertrophy,  atrophy,  or  displacement  of  particular  parts  often 
leads  to  compensatory  changes  in  the  neighbouring  parts ;  but  it  is 
equally  certain  that  such  changes  are  not  a  direct  mechanical  effect 
of  the  disturbance,  but  a  highly  complex  physiological  response  to  it. 
How  complex  the  problem  is,  is  shown  by  the  fact  that  even  closely 
related  animals  may  differ  widely  in  this  respect.  Thus  Fraisse  has 
shown  that  the  salamander  may  completely  regenerate  an  amputated 
Hmb,  while  the  frog  only  heals  the  wound  without  further  regenera- 
tion.2  Again,  in  the  case  of  coelenterates,  Loeb  and  Bickford  have 
shown  that  the  tubularian  hydroids  are  able  to  regenerate  the  ten- 
tacles at  both  ends  of  a  segment  of  the  stem,  while  the  polyp  Cerian- 
thiis  can  regenerate  them  only  at  the  distal  end  of  a  section  (Fig.  194;. 

1  QC  p.  97.     For  a  good  discussion  of  this  subject,  see  E.  Ziegler,  '89. 

2  In  salamanders  regeneration  only  takes  place  when  the  bone  is  cut  across,  and  does  not 
occur  if  the  limb  be  exarticulated  and  removed  at  the  joint. 


CELL-DIVISION  AND    GROWTH 


393 


In  the  latter  case,  therefore,  the  body  possesses  an  inherent  polarity 
which  cannot  be  overturned  by  external  conditions.  A  very  curious 
case  is  that  of  the  earthworm,  which  has  long  been  known  to  possess 
a  high  regenerative  capacity.  If  the  posterior  region  of  the  worm 
be  cut  off,  a  new  tail  is  usually  regenerated.  If  the  same  operation 
be  performed  far  forward  in  the  anterior  region,  a  new  head  is  often 
formed  at  the  front  end  of  the  posterior  piece.  If,  however,  the  sec- 
tion be  in  the  middle  region  the  posterior  piece  sometimes  regenerates 
a  head,  but  more  usually  a  tail,  as  was  long  since  shown  by  Spallanzani 
and  recently  by  Morgan  ('99).  Why  such  a  blunder  should  be  com- 
mitted remains  for  the  present  quite  unexplained. 

It  remains  to  inquire  more  critically  into  the  nature  of  the  correla- 
tion between  growth  and  cell-division.  In  the  growing  tissues  the 
direction  of  the  division-planes  in  the  individual  cells  evidently  stands 
in  a  definite  relation  with  the  axes  of  growth  in  the  body,  as  is  espe- 
cially clear  in  the  case  of  rapidly  elongating  structures  (apical  buds, 
teloblasts,  and  the  like),  where  the  division-planes  are  predominantly 
transverse  to  the  axis  of  elongation.  Which  of  these  is  the  primary 
factor,  the  direction  of  general  growth  or  the  direction  of  the  division- 
planes  }  This  question  is  a  difficult  one  to  answer,  for  the  two  phe- 
nomena are  often  too  closely  related  to  be  disentangled.  As  far  as 
the  plants  are  concerned,  however,  it  has  been  conclusively  shown  by 
Hofmeister,  De  Bary,  and  Sachs  that  tJie  groivth  of  tJic  mass  is  tlic 
primary  factor ;  for  the  characteristic  mode  of  growth  is  often  shown 
by  the  growing  mass  before  it  splits  up  into  cells,  and  the  form  of 
cell-division  adapts  itself  to  that  of  the  mass  :  *'  Die  Pflanze  bildet 
Zellen,  nicht  die  Zelle  bildet  Pflanzen  "  (De  Bary). 

Much  of  the  recent  work  in  normal  and  experimental  embryology, 
as  well  as  that  on  regeneration,  indicates  that  the  same  is  true  in  prin- 
ciple of  animal  growth.  Among  recent  writers  who  have  urged  this 
view  should  be  mentioned  Rauber,  Hertwig,  Adam  Sedgwick,  and 
especially  Whitman,  whose  fine  essay  on  the  Inadequacy  of  ilic  Cell- 
theory  of  Development  ('93)  marks  a  distinct  advance  in  our  point  of 
view.  Still  more  recently  this  view  has  been  almost  demonstrated 
through  some  remarkable  experiments  on  regeneration,  which  show 
that  definitely  formed  material,  in  some  cases  even  the  adult  tissues, 
may  be  directly  moulded  i?ito  new  structures.  Driesch  has  shown 
('95,  2,  '99)  that  if  gastrulas  of  SpJicerechinus  be  bisected  through  the 
equator  so  that  each  half  contains  both  ectoderm  and  entoderm,  the 
wounds  heal,  each  half  forming  a  typical  gastrula,  in  which  the  ente- 
ron  differentiates  itself  into  the  three  typical  regions  (fore,  middle, 
and  hind  gut)  correctly  proportioned,  though  the  whole  structure  is 
but  half  the  normal  size.  Here,  therefore,  the  formative  process  is  in 
the  main  independent  of  cell-division  or  increase  in  size.    Miss  Bickford 


394  CELL-DIVISION  AND  DEVELOPMENT 

('94)  found  that  in  the  regeneration  of  decapitated  hydranths  of  tubu- 
larians  the  new  hydranth  is  primarily  formed,  not  by  new  cell-formation 
and  growth  from  the  cut  end,  but  by  direct  transformation  of  the  distal 
portion  of  the  stem.^  Morgan's  remarkable  observations  on  Planarin, 
finally,  show  that  here  also,  when  the  animal  is  cut  into  pieces,  com- 
plete animals  are  produced  from  these  pieces,  but  only  in  small  degree 
through  the  formation  of  new  tissue,  and  mainly  by  direct  remould- 
ing of  the  old  material  into  a  new  body  having  the  correct  propor- 
tions of  the  species.  As  Driesch  has  well  said,  it  is  as  if  a  plan  or 
mould  of  the  new  little  worm  were  first  prepared  and  then  the  old 
material  were  poured  into  it.^ 

Facts  of  this  kind,  of  which  a  considerable  store  has  been  accumu- 
lated, give  strong  ground  for  the  view  that  cell-formation  is  subordi- 
nate to  growth,  or  rather  to  the  general  formative  process  of  which 
growth  is  an  expression  ;  and  they  furnish  a  powerful  argument  against 
Schwann's  conception  of  the  organism  as  a  cell-composite  (p.  58). 
That  conception  is,  however,  not  to  be  rejected  /;/  toto,  but  contains  a 
large  element  of  truth;  for  there  are  many  cases  in  which  cells  pos- 
sess so  high  a  degree  of  independence  that  profound  modifications 
may  occur  in  special  regions  through  injury  or  disease,  without  affect- 
ing the  general  equilibrium  of  the  body.  The  most  striking  proof  of 
this  lies  in  the  fact  that  grafts  or  transplanted  structures  may  perfectly 
retain  their  specific  character,  though  transferred  to  a  different  region 
of  the  body,  or  even  to  another  species.  Nevertheless  the  facts  of 
regeneration  prove  that  even  in  the  adult  the  formative  processes  in 
special  parts  are  in  many  cases  definitely  correlated  with  the  organi- 
zation of  the  entire  mass  ;  and  there  is  reason  to  conclude  that  such 
a  correlation  is  a  survival,  in  the  adult,  of  a  condition  characteristic 
of  the  embryonic  stages,  and  that  the  independence  of  special  parts 
in  the  adult  is  a  secondary  result  of  development.  The  study  of  cell- 
division  thus  brings  us  finally  to  a  general  consideration  of  develop- 
ment which  forms  the  subject  of  the  following  chapter. 

LITERATURE.     VIII 

Berthold,  G.  —  Studien  liber  Protoplasma-mechanik.     Leipzig,  1886. 

Boll,  Fr.  —  Das  Princip  des  Wachsthums.     Berlin.  1876. 

Bourne,  G.  C.  —  A  Criticism  of  the  Cell-theory;    being  an  answer  to  Mr.  Sedgwick's 

article  on  the  Inadequacy  of  the  Cellular  Theory  of  Development :  Quart.  Jcmni. 

Mic.Sci.,XXXN\l\.\.     1895. 

1  Driesch  suggests  for  such  a  process  the  term  reparation  in  contradistinction  to  true 
regeneration. 

2  '99,  p.  55.  It  is  mainly  on  these  considerations  that  Driesch  ('99)  has  built  his  recent 
theory  of  vitalism  (^cf.  p.  417),  the  nature  of  the  formative  power  being  regarded  as  a 
problem  sui generis,  and  one  which  the  "machine-theory  of  life  "  is  powerless  to  solve.  Cf. 
also  the  views  of  Whitman,  p.  416. 


LITER  A  TURE  395 

Castle,  W.  E.— The  early  Embryology  of  Ciona.      Bull.  Mus.  Conip.  ZooL,  XXVII. 

1896. 
Conklin,  E.  G.  —  The  Embryology  of  Crepidula  :  Journ.  Morpli.,  XIII.     1 897. 
Driesch,  H. —  (See  Literature.  IX.) 
Errera,  L.  —  Zellformen  unci  Seifenblaseii :    Tivj^cbl.  dcr  60  Vcrsatiuiilniij^  licutsilwr 

Nafnrforsc/ia'-  imd  Aerzte  zii  Wiesbaden.     1887. 
Hertwig,  0.  —  Das  Problem  der  Befruchtung  und  der  Isotropic  des  Eies,  eine  Theo- 

rie  der  Vererbung.    Jena^  1884. 
Hofmeister.  — ^Die  Lehre  von  der  Pflanzenzelle.     Leipzig.,  1867. 
Jennings,  H.  S.  —  The  Early  Development  of  Asplanchna  :  BulL  Miis.  Coup.  Zool., 

XXX.  I.     Ca;/ibridge,  1896. 
Kofoid,  C.  A.  —  On    the    Early   Development   of   Limax  :    Bidl.  Miis.   Conip.  Zool., 

XXVII.     1895. 
Lillie,  F.  R.  — The  Embryology  of  the  Unionidae  :  Journ.  Morph..,  X.     1895. 
Id. — Adaptation  in  Cleavage  :    Wood'' s  Hall  Biol.  Lectures.     1899. 
McMurrich,  J.  P.  —  Embryology  of  the  Isopod  Crustacea:  Journ.  Morpli.,  XI.   i. 

1895. 
Mark,  E.  L.  —Limax.     (See  list  IV.) 
Morgan,  T.  H.  —  (See  Literature,  IX.) 
Rauber,  A.  —  Neue  Grundlegungen  zur  Kenntniss  der  Zelle  :  Morph.  Ja/irb.,  \'I1I. 

1883. 
Rhumbler,  L.  —  Allgemeine  Zellmechanik  :    Merkel  u.  Bonnet.,  Ergeb.,  WW.     1898. 
Sachs,  J.  —  Pflanzenphysiologie.     (See  list  VII.) 
Sedgwick,  H.  —  On  the  Inadequacy  of  the  Cellular  Theory  of  Development,  etc.: 

Quart.  Journ .  Mic .  Sci. ,  XXXVII.  i.     1894. 
Strasburger,  E. — Uber  die  Wirkungssphare  der  Kerne  und  die  Zellgrcisse  :  Histo- 

logische  Beitrdge,  V.     1893. 
Zur  Strassen,  0. —  Embryonalentwickelung  der  Ascaris  :  Arch.  Ento/n..  III.     1896. 
Watase,  S.  —  Studies  on  Cephalopods  ;   I.,  Cleavage  of  the  Ovum  :  Journ.  Morph., 

IV.  3.     1891. 
Whitman,  C.  0. — The  Inadequacy  of  the  Cell-theory  of  Development :  Wood's  Moll 

Biol.  Lectures.     1893. 
Wilson,  Edm.  B.  —  The  Cell-lineage  oi Nereis :  Journ.  Morph.,  VI.  3.     1892. 
Id.  — Amphioxus  and  the  Mosaic  Theory  of  Development :  Journ.  Morph.,  \'I1I.  3. 

1893- 
Id.  — Considerations    on    Cell-lineage   and  Ancestral    Reminiscence:    Ann.  X.   ). 

Acad.,  XI.  1898;  also  Wood's  H oil  Biol.  Lectures,  1899. 


CHAPTER   IX 

THEORIES   OF   INHERITANXE   AND    DEVELOPMENT 

"  It  is  certain  that  the  germ  is  not  merely  a  body  in  which  life  is  dormant  or  potential, 
but  that  it  is  itself  simply  a  detached  portion  of  the  substance  of  a  preexisting  living  body." 

HUXLEY.I 

"  Inheritance  must  be  looked  at  as  merely  a  form  of  growth."  Darwin.2 

"  Ich  mochte  daher  wohl  den  Versuch  wagen,  durch  eine  Darstellung  des  Beobachteten 
Sie  zu  einer  tiefern  Einsicht  m  die  Zeugungs-  und  Entwickelungsgeschichte  der  organischen 
Korper  zu  fiihren  und  zu  zeigen,  wie  dieselben  weder  vorgebildet  sind,  noch  auch,  wie  man 
sich  crewohnlich  denkt,  aus  ungeformter  Masse  in  einem  bestimmten  Momente  plotzlich 
ausschiessen."  Von  Baer.3 

Every  discussion  of  inheritance  and  development  must  take  as  its 
point  of  departure  the  fact  that  the  germ  is  a  single  cell  similar  in  its 
essential  nature  to  any  one  of  the  tissue-cells  of  which  the  body  is 
composed.  That  a  cell  can  carry  with  it  the  sum  total  of  the  heritage 
of  the  species,  that  it  can  in  the  course  of  a  few  days  or  weeks  give 
rise  to  a  mollusk  or  a  man,  is  the  greatest  marvel  of  biological  science. 
In  attempting  to  analyze  the  problems  that  it  involves,  we  must  from 
the  outset  hold  fast  to  the  fact,  on  which  Huxley  insisted,  that  the 
wonderful  formative  energy  of  the  germ  is  not  impressed  upon  it 
from  without,  but  is  inherent  in  the  &gg  as  a  heritage  from  the  paren- 
tal life  of  which  it  was  originally  a  part.  The  development  of  the 
embryo  is  nothing  new.  It  involves  no  breach  of  continuity,  and  is 
but  a  continuation  of  the  vital  processes  going  on  in  the  parental 
.  body.  What  gives  development  its  marvellous  character  is  the  rapid- 
ity with  which  it  proceeds  and  the  diversity  of  the  results  attained  in 
a  span  so  brief. 

But  when  we  have  grasped  this  cardinal  fact,  we  have  but  focussed 
our  instruments  for  a  study  of  the  real  problem.  How  do  the  adult 
characteristics  lie  latent  in  the  germ-cell ;  and  how  do  they  become 
patent  as  development  proceeds  ?  This  is  the  final  question  that  looms 
in  the  background  of  every  investigation  of  the  cell.  In  approaching 
it  we  may  well  make  a  frank  confession  of  ignorance ;  for  in  spite  of 
all  that  the  microscope  has  revealed,  we  have  not  yet  penetrated  the 
mystery,  and  inheritance  and  development  still  remain  in  their  fun- 

1  Evolution,  Science  and  Culture,  p.  291. 

2  Variation  of  Anitnals  and  Plants,  II.,  p.  398. 

3  Entwick.  der  Thieve,  II.,  1837,  p.  8. 

396 


THE    THEORY   OF  GERMINAL   LOCALIZATION  397 

damental  aspects  as  great  a  riddle  as  they  were  to  the  Greeks.  What 
we  have  gained  is  a  tolerably  precise  acquaintance  with  the  external 
aspects  of  development.  The  gross  errors  of  the  early  preformation- 
ists  have  been  dispelled. ^  We  know  that  the  germ-cell  contains  no 
predelineated  embryo  ;  that  development  is  manifested,  on  the  one 
hand,  by  the  cleavage  of  the  ^gg,  on  the  other  hand,  by  a  process  of 
differentiation,  through  which  the  products  of  cleavage  gradually 
assume  diverse  forms  and  functions,  and  so  accomplish  a  physiological 
division  of  labour.  We  can  clearly  recognize  the  fact  that  these  pro- 
cesses fall  in  the  same  category  as  those  that  take  place  in  the  tissue- 
cells  ;  for  the  cleavage  of  the  ovum  is  a  form  of  mitotic  cell-division, 
while,  as  many  eminent  naturalists  have  perceived,  differentiation  is 
nearly  related  to  growth  and  has  its  root  in  the  phenomena  of  nutri- 
tion and  metabolism.  The  real  problem  of  development  is  the  orderly 
sequence  and  congelation  of  these  pJic7iomena  toward  a  typical  result. 
We  cannot  escape  the  conclusion  that  this  is  the  outcome  of  the 
organization  of  the  germ-cells  ;  but  the  nature  of  that  which,  for  lack 
of  a  better  term,  we  call  ''organization,"  is  and  doubtless  long  will 
remain  almost  wholly  in  the  dark. 

In  the  following  discussion,  which  is  necessarily  compressed  within 
narrow  limits,  we  shall  disregard  the  earlier  baseless  speculations, 
such  as  those  of  the  seventeenth  and  eighteenth  centuries,  which 
attempted  a  merely  formal  solution  of  the  problem,  confining  our- 
selves to  more  recent  discussions  that  have  grown  directlv  out  of 
modern  research.  An  introduction  to  the  general  subject  may  be 
given  by  a  preliminary  examination  of  two  central  hypotheses  about 
which  most  recent  discussions  have  revolved.  These  are,  first,  the 
theory  of  Germinal  Localization'^  of  Wilhelm  His  ('74),  and,  second, 
the  Idioplasm  Hypothesis  of  Nageli  ('84).  The  relation  between  these 
two  conceptions,  close  as  it  is,  is  not  at  first  sight  very  apparent ; 
and  for  the  purpose  of  a  preliminary  sketch  they  may  best  be  con- 
sidered separately. 

A.     The  Theory  of  Germinal  Localization 

Although  the  naive  early  theory  of  preformation  and  evolution  was 
long  since  abandoned,  yet  we  find  an  after-image  of  it  in  the  theory 
of  germinal  localization  which  in  one  form  or  another  has  been  advo- 
cated by  some  of  the  foremost  students  of  development.  It  is  main- 
tained that,  although  the  embryo  is  not  \)X^formed  in  the  germ,  it  must 
nevertheless   be   Y>^Qdetermined  in   the    sense  that  the  <tgg  contains 

^  Cf.  Introduction,  p.  8. 

2  I  venture  to  suggest  this  term  as  an  English  equivalent  for  the  awkward  expression 
"  Organbildende  Keimbezirke  "  of  His. 


398  INHERITANCE  AND  DEVELOPMENT 

definite  areas  or  definite  substances  predestined  for  the  formation  of 
corresponding  parts  of  the  embryonic  body.  The  first  clear  state- 
ment of  this  conception  is  found  in  the  interesting  and  suggestive 
work  of  Wilhehii  His  ('74)  entitled  Unsere  Kdrperform.  Considering 
the  development  of  the  chick,  he  says  :  '*  It  is  clear,  on  the  one  hand, 
that  every  point  in  the  embryonic  region  of  the  blastoderm  must  rep- 
resent a  later  organ  or  part  of  an  organ,  and,  on  the  other  hand,  that 
every  organ  developed  from  the  blastoderm  has  its  preformed  germ 
(vorgebildete  Anlage)  in  a  definitely  located  region  of  the  flat  germ- 
^  disc.  .  .  .  The  material  of  the  germ  is  already  present  in  the  flat 
germ-disc,  but  is  not  yet  morphologically  marked  off  and  hence 
not  directly  recognizable.  But  by  following  the  development  back- 
wards we  may  determine  the  location  of  every  such  germ,  even  at  a 
period  when  the  morphological  differentiation  is  incomplete  or  before 
it  occurs ;  logically,  indeed,  we  must  extend  this  process  back  to  the 
fertihzed  or  even  the  unfertilized  Q,gg.  According  to  this  principle, 
the  germ-disc  contains  the  organ-germs  spread  out  in  a  flat  plate,  and, 
conversely,  every  point  of  the  germ-disc  reappears  in  a  later  organ ; 
I  call  this  the  principle  of  organ- forming  germ-regions T  ^  His  thus 
conceived  the  embryo,  not  as  ^x^formed,  but  as  having  all  of  its  parts 
//  ^x^localized'm  the  egg-protoplasm  (cytoplasm). 

A  great  impulse  to  this  conception  was  given  during  the  follow- 
ing decade  by  discoveries  relating,  on  the  one  hand,  to  protoplasmic 
structure,  on  the  other  hand,  to  the  promorphological  relations  of  the 
ovum.  Ray  Lankester  writes,  in  1877:  ''Though  the  substance  of  a 
cell 2  may  appear  homogeneous  under  the  most  powerful  microscope, 
it  is  quite  possible,  indeed  certain,  that  it  may  contain,  already  formed 
and  individualized,  various  kinds  of  physiological  molecules.  The 
visible  process  of  segregation  is  only  the  sequel  of  a  differentiation 
already  estabUshed,  and  not  visible."^  The  egg-cytoplasm  has  a  defi- 
nite molecular  organization  directly  handed  down  from  the  parent; 
cleavage  sunders  the  various  "  physiological  molecules "  and  iso- 
lates them  in  particular  cells.  Whitman  expresses  a  similar  thought 
in  the  following  year :  "  While  we  cannot  say  that  the  embryo  is  pre- 
delineated,  we  can  say  that  it  is  predetermined.  The  '  histogenetic 
sundering  '  of  embryonic  elenients  begins  with  the  cleavage,  and  every 
step  in  the  process  bears  a  definite  and  invariable  relation  to  antece- 
dent and  subsequent  steps.  ...  It  is,  therefore,  not  surprising  to 
find  certain  important  histological  differentiations  and  fundamental 
structural  relations  anticipated  in  the  early  phases  of  cleavage,  and 
foreshadowed  even  before  cleavage  begins."  ^     It  was,  however,  Flem- 

1  /.  c,  p.  19. 

2  It  is  clear  from  the  context  that  by  "  substance  "  Lankester  had  m  nimd  the:_Qto;^iasiii, 
though  this  is  not  specifically  stated.  ^  '77,  p.  14.  ,  *  '785  P-  49- 


THE    THEORY  OF  GERMINAL   LOCALIZATION  399 

ming  who  gave  the  first  specific  statement  of  the  matter  from  the  cyto- 
logical  point  of  view  :   '^  But  if  the  substance  of  the  egg-cell  has  a 
definite  structure  (Bau),  and  if  this  structure  and  the  nTture  of  the 
network  varies  in  different  regions  of  the  cell-body,  we  may  seek  in 
it  a  basis  for  the  predetermination  of  development  wherein  one  ec'-o- 
differs  from  another,  and  it  will  be  possible  to  look  for  it  luitk  7he 
microscope.     How  far  this  search  can  be  carried  no  one  can  say,  but 
its  ultimate  aim  is  nothing  less  than  a  true  morphology  of  inhcritancer  1 
In  the  following  year  Van  Beneden  pointed  out  how  nearly  this  con- 
ception approaches  to  a  theory  of  preformation:  "If  this  were  the 
case  {i.e.  if  the  egg-axis  coincided  with  the  principal  axis  of  the  adult 
body),  the  old  theory  of  evolution  would  not  be  as  baseless  as  we 
think  to-day.     The  fact  that  in  the  ascidians,  and  probably  in  other 
bilateral  animals,  the  median   plane  of  the   body  of  the  future  ani- 
mal is  marked  out  from  the  beginning  of  cleavage,  fully  justifies  the 
hypothesis  that  the  materials  destined  to  form  the  right  side  of  the 
body  are  situated  in  one  of  the  lateral  hemispheres  of  the  ^gg,  while 
the  left  hemisphere  gives  rise  to  all  of  the  organs  of  the  left  half."  2 
The  hypothesis  thus  suggested  seemed,  for  a  time,  to  be  placed  on 
a  secure  basis  of  fact  through  a  remarkable  experiment  subsequently 
performed  by  Roux  i^'^^)  on  the  frog's  ^gg.     On  killing  one  of  the 
blastomeres  of  the  two-cell  stage  by  means  of  a  heated  needle  the  un- 
injured half  developed  in  some  cases  into  a  well-formed    half-larva 
(Fig.    182),  representing  approximately  the  right  or  left  half  of  the 
body,  containing  one  medullary  fold,  one  auditory  pit,  etc.^     Analo- 
gous, though  less  complete,  results  were  obtained  by  operating  with 
the  four-cell  stage.     Roux  was  thus  led  to  the  declaration  (made  with 
certain  subsequent  reservations)  that  "  the  development  of  the  fro^-- 
gastrula  and  of  the  embryo  formed  from  it  is  from  the  second  cleavage 
onward  a  mosaic-work,  consisting  of  at  least  four  vertical  indepen- 
dently developing  pieces."*     This  conclusion  seemed  to  form  a  very 
strong  support  to  His's  theory  of  germinal  localization,  though,  as 
will  appear  beyond,  Roux  transferred  this  theory  to  the  nucleus,  and 
thus  developed  it  in  a  very  different    direction  from    Lankester  or 
Van  Beneden.     His's  theory  also  received  very  strong  apparent  sup- 
port through  investigations  on  cell-lineage  by  Whitman.  Rabl,   and 

^  Zellsubstanz,  '82,  p.  70 :  the  italics  are  in  the  original. 

-'83,P-57i. 

^  The  accuracy  of  this  result  was  disputed  by  Oscar  Hertwig  ('93,  i),  who  found  that  the 
uninjured  blastomere  gave  rise  to  a  defective  larva,  in  which  certain  parts  were  missing,  but 
not  to  a  true  half-body.  Later  observers,  especially  Schultze,  Endres,  and  Morgan,  have, 
however,  shown  that  both  Hertwig  and  Roux  were  right,  proving  that  the  uninjured  blasto- 
mere may  give  rise  to  a  true  half-larva,  to  a  larva  with  irregular  defects,  or  to  a  whole 
larva  of  half-size,  according  to  circumstances  (p.  422). 
*  l-C;  P-  30- 


400 


INHERITANCE   AND  DEVEIOPMENT 


many  later  observers,  which  have  shown  that  in  the  cleavage  of  anne- 
lids, moUusks,  platodes,  tunicates,  and  many  other  animals,  every  cell 
has  a  definite  origin  and  fate,  and  plays  a  definite  part  in  the  building 
of  the  body.^ 


;?2    s 


Fig.  182.  —  Half-embryos  of  the  frog  (in  transverse  section)  arising  from  a  blastomere  of  the 
two-cell  stage  after  killing  the  other  blastomere,     [Roux.] 

A.  Half-blastula  (dead  blastomere  on  the  left).  B.  Later  stage.  C.  Half-tadpole  with  one 
medullary  fold  and  one  mesoblast  plate  ;  regeneration  of  the  missing  (right)  half  in  process. 

ar.  archenteric  cavity ;  c.c.  cleavage-cavity ;  ch.  notochord  ;  ?«./.  medullary  fold ;  m.s.  meso- 
blast-plate. 


In  an  able  series  of  later  works  Whitman  has  followed  out  the  sug- 
gestion made  in  his  paper  of  1878,  cited  above,  pointing  out  how 
essential  a  part  is  played  in  development  by  the  cytoplasm  and  insist- 
ing that  cytoplasmic  preorganization  must  be  regarded  as  a  leading 
factor  in  the  ontogeny.  Whitman's  interesting  and  suggestive  views 
are  expressed  with  great  caution  and  with  a  full  recognition  of  the 

1  Cf.  p.  378. 


THE  IDIOPLASM   THEORY 


401 


difficulty  and  complexity  of  the  problem.  From  his  latest  essay,  in- 
deed ('94),  it  is  not  easy  to  gather  his  precise  position  regarding  the 
theory  of  cytoplasmic  localization.  Through  all  his  writings,  never- 
theless, runs  the  leading  idea  that  the  germ  is  definitely  organized 
before  development  begins,  and  that  cleavage  only  reveals  an  organi- 
zation that  exists  from  the  beginning.  ''  That  organization  precedes 
cell-formation  and  regulates  it,  rather  than  the  reverse,  is  a  conclu- 
sion that  forces  itself  upon  us  from  many  sides."  ^  "  The  organism 
exists  before  cleavage  sets  in,  and  persists  throughout  every  stage  of 
cell-multiplication."  ^ 

All  of  these  views,  excepting  those  of  Roux,  lean  more  or  less 
distinctly  toward  the  conclusion  that  the  cytoplasm  of  the  egg-cell 
is  from  the  first  mapped  out,  as  it  were,  into  regions  which  corre- 
spond with  the  parts  of  the  future  embryonic  body.  The  cleavage 
of  the  ovum  does  not  create  these  regions,  but  only  reveals  them  to 
view  by  marking  off  their  boundaries.  Their  topographical  arrange- 
ment in  the  ^gg  does  not  necessarily  coincide  with  that  of  the  adult 
parts,  but  only  involves  the  latter  as  a  necessary  consequence  —  some- 
what as  a  picture  in  the  kaleidoscope  gives  rise  to  a  succeeding  pic- 
ture composed  of  the  same  parts  in  a  different  arrangement.  The 
germinal  localization  may,  however,  in  a  greater  or  less  degree,  fore- 
shadow the  arrangement  of  adult  parts  —  for  instance,  in  the  ^gg  of 
the  tunicate  or  cephalopod,  where  the  bilateral  symmetry  and  antero- 
posterior differentiation  of  the  adult  is  foreshadowed  not  only  in  the 
cleavage  stages,  but  even  in  the  un segmented  ^gg. 

By  another  set  of  wTiters,  such  as  Roux,  De  Vries,  Hertwig,  and 
Weismann,  germinal  localization  is  primarily  sought  not  in  the  cyto- 
plasm, but  in  the  nucleus;  but  these  views  can  be  best  considered 
after  a  review  of  the  idioplasm  hypothesis,  to  which  we  now  proceed. 


B.     The  Idioplasm  Theory 

We  owe  to  Nageli  the  first  systematic  attempt  to  discuss  heredity 
regarded  as  inherent  in  a  definite  physical  basis ;  ^  but  it  is  hardly 
necessary  to  point  out  his  great  debt  to  earlier  writers,  foremost 
among  them  Darwin,  Herbert  Spencer,  and  Hackel.  The  essence  of 
Nageli's  hypothesis  was  the  assumption  that  inheritance  is  effected 
by  the  transmission  not  of  a  cell,  considered  as  a  whole,  but  of  a  par- 
ticukr  sub'stance,  the  idioplasm,  contained  within  a  cell,  and  forming 
"the  physical  basis  of  heredity.  The  idioplasm  is  to  be  sharply  dis- 
tinguished from  the  other  constituents  of  the  cell,  which  play  no 
direct  part  in  inheritance  and  form  a  "nutritive  plasma"  or  tropJio- 

1  '93,  p.  11^.  '^  l.c.i  p.  112.  ^  Theorie  der  Abstammungslehre,  18S4. 

2  D 


L^ 


402  INHERITANCE  AND  DEVELOPMENT 

plasm.  ■■  Hereditary  traits  are  the  outcome  of  a  definite  molecular 
organization  of  the  idioplasm.  The  hen's  ^gg  differs  from  the  frog's 
because  it  contains  a  different  idioplasm.  The  species  is  as  com- 
pletely contained  in  the  one  as  in  the  other,  and  the  hen's  ^gg  differs 
from  a  frog's  ^gg  as  widely  as  a  hen  from  a  frog. 

The  idioplasm  was  conceived  as  an  extremely  complex  substance, 
consisting  of  elementary  complexes  of  molecules  known  as  micellce. 
These  are  variously  grouped  to  form  units  of  higher  orders,  which, 
as  development  proceeds,  determine  the  development  of  the  adult 
cells,  tissues,  and  organs.  The  specific  peculiarities  of  the  idioplasm 
are  therefore  due  to  the  arrangement  of  the  micellae  ;  and  this,  in  its 
turn,  is  owing  to  dynamic  properties  of  the  micellae  themselves. 
During  development  the  idioplasm  undergoes  a  progressive  trans- 
formation of  its  substance,  not  through  any  material  change,  but 
through  dynamic  alterations  of  the  conditions  of  tension  and  move- 
ment of  the  micella.  These  changes  in  the  idioplasm  cause  reactions 
on  the  part  of  surrounding  structures  leading  to  definite  chemical  and 
plastic  changes,  i.e.  to  differentiation  and  development. 

Nageli  made  no  attempt  to  locate  the  idioplasm  precisely  or  to 
identify  it  with  any  of  the  known  morphological  constituents  of  the 
cell,     it  was  somewhat  vaguely  conceived  as  a  network  extending 
through  both  nucleus  and  cytoplasm,  and  from  cell  to  cell  through- 
out the  entire  organism.     Almost  immediately  after  the  publication 
of  his  theory,  however,  several  of  the  foremost  leaders  of  biological 
investigation  were  led   to  locate   the  idioplasm  in  the  nucleus,  and 
concluded  that  it  is  to  be  identified  with  chromatin.     The  grounds 
for  this  conclusion,  which  have  already  been  stated  in  Chapter  VII., 
may  be   here   again    briefly   reviewed.      The    beautiful    experiments 
of  Nussbaum,  Gruber,  and   Verworn    proved    that   the  regeneration 
of   differentiated    cytoplasmic   structures    in    the    Protozoa   can   only 
take  place  when  nuclear  matter  is  present  {cf.  p.  342).     The  study  of 
fertilization  by  Hertwig,  Strasburger,  and  Van  Beneden  proved  that 
in  the  sexual  reproduction  of  both  plants  and  animals  the  nucleus  of 
the  germ  is  equally  derived  from  both  sexes,  while  the  cytoplasm  is 
derived  almost  entirely  from  the  female.     The  two  germ-nuclei,  which 
by  their  union  give  rise  to. that  of   the  germ,  were  shown   by  Van 
Beneden  to  be  of  exactly  the  same  morphological  nature,  since  each 
gives  rise  to  chromosomes  of  the  same  number,  form,  and  size.     Van 
Beneden  and  Boveri  proved  (p.  182)  that  the  paternal  and  maternal 
nuclear  substances  are  equally  distributed  to  each  of   the  first  two 
cells,  and   the    more  recent   work  of    Hacker,   Rlickert,   Herla,  and 
Zoja  estabhshes  a  strong  probability  that  this  equal  distribution  con- 
tinues in  the  later  divisions.      Roux  pointed  out  the  telling  fact  that 
the  entire  complicated  mechanism  of  mitosis  seems  designed  to  affect 


UNION   OF   THE    TWO    THEORIES 


403 


the  most  accurate  division  of  the  entire  nuclear  substance  in  all  of 
its  parts,  while  fission  of  the  cytoplasmic  cell-body  is  in  the  main 
a  mass-division,  and  not  a  meristic  division  of  the  individual  parts. 
Again,  the  complicated  processes  of  maturation  show  the  significant 
fact  that  while  the  greatest  pains  is  taken  to  prepare  the  germ-nuclei 
for  their  coming  union,  by  rendering  them  exactly  equivalent,  the 
cytoplasm  becomes  widely  different  in  the  two  germ-cells  and  is 
devoted  to  entirely  different  functions. 

It  was  in  the  main  these  considerations  that  led  Hertwig,  Stras- 
burger,  Kolliker,  and  Weismann  independently  and  almost  simultane- 
ously to  the  conclusion  that  the  nucleus  contains  the  physical  basis  of 
inJietitance,  and  that  chromatin,  its  essential  constituent,  is  the  idio- 
plasm postulated  in  NdgelVs  theory.  This  conclusion  is  now  widely 
accepted  and  rests  upon  a  basis  so  firm  that  it  must  be  regarded  as  a 
working  hypothesis  of  high  value.  To  accept  it  is,  however,  to  reject 
the  theory  of  germinal  localization  in  so  far  as  it  assumes  a  prelocali- 
zation  of  the  egg-cytoplasm  as  a  fundamental  character  of  the  tgg. 
For  if  the  specific  character  of  the  organism  be  determined  by  an 
idioplasm  contained  in  the  chromatin,  then  every  characteristic  of  the 
cytoplasm  must  in  the  long  run  be  determined  from  the  same  source. 
A  striking  illustration  of  this  point  is  given  by  the  phenomena  of 
colour-inheritance  in  plant-hybrids,  as  De  Vries  has  pointed  out. 
Pigment  is  developed  in  the  embryonic  cytoplasm,  which  is  derived 
from  the  mother-cell ;  yet  in  hybrids  it  may  be  inherited  from  the 
male  through  the  nucleus  of  the  germ-cell.  The  specific  form  of 
cytoplasmic  metabolism  by  which  the  pigment  is  formed  must  there- 
fore be  determined  by  the  paternal  chromatin  in  the  germ-nucleus, 
and  not  by  a  predetermination  of  the  egg-cytoplasm. 

C.     Union  of  the  Two  Theories 

We  have  now  to  consider  the  attempts  that  have  been  made  to 
transfer  the  localization-theory  from  the  cytoplasm  to  the  nucleus, 
and  thus  to  bring  it  into  harmony  with  the  theory  of  nuclear  idio- 
plasm. These  attempts  are  especially  associated  with  the  names  of 
Roux,  De  Vries,  Weismann,  and  Hertwig  ;  but  all  of  them  may  be 
traced  back  to  Darwin's  celebrated  hypothesis  of  pangenesis  as  a 
prototype.  This  hypothesis  is  so  well  known  as  to  require  but  a 
brief  review.  Its  fundamental  postulate  assumes  that  the  germ-cells 
contain  innumerable  ultra-microscopic  organized  bodies  or  geinjnules, 
each  of  which  is  the  germ  of  a  cell  and  determines  the  development 
of  a  similar  cell  during  the  ontogeny.  The  germ-cell  is,  therefore, 
in  Darwin's  words,  a  microcosm  formed  of  a  host  of  inconceivably 
minute  self-propagating  organisms,  every  one  of  which  predetermines 


404  INHERITANCE  AND  DEVEIOPMENT 

the  formation  of  one  of  the  adult  cells.  De  Vries  ('89)  brought  this 
conception  into  relation  with  the  theory  of  nuclear  idioplasm  by 
assuming  that  the  gemmules  of  Darwin,  which  he  z?^^^  pangcns,  are 
contained  in  the  nucleus,  migrating  thence  into  the  cytoplasm  step 
by  step  during  ontogeny,  and  thus  determining  the  successive  stages 
of  development.  The  hypothesis  is  further  modified  by  the  assump- 
tion that  the  pangens  are  not  cell-germs,  as  Darwin  assumed,  but 
ultimate  protoplasmic  units  of  which  cells  are  built,  and  which  are 
the  bearers  of  particular  hereditary  qualities.  The  same  view  was 
afterward  accepted  by  Hertwig  and  Weismann.^ 

The  theory  of  germinal  localization  is  thus  transferred  from  the 
cytoplasm  to  the  nucleus.  It  is  not  denied  that  the  egg-cytoplasm 
may  be  more  or  less  distinctly  differentiated  into  regions  that  have  a 
constant  relation  to  the  parts  of  the  embryo.  This  differentiation  is, 
however,  conceived,  not  as  a  primordial  characteristic  of  the  ^gg,  but 
as  one  secondarily  determined  through  the  influence  of  the  nucleus. 
Both  De  Vries  and  Weismann  assume,  in  fact,  that  the  entire  cyto- 
plasm is  a  product  of  the  nucleus,  being  composed  of  pangens  that 
migrate  out  from  the  latter,  and  by  their  active  growth  and  multipli- 
cation build  up  the  cytoplasmic  substance.^ 


D.     The  Roux-Weismann  Theory  of  Development 

We  now  proceed  to  an  examination  of  two  sharply  opposing  hy- 
potheses of  development  based  on  the  theory  of  nuclear  idioplasm. 
One  of  these  originated  with  Roux  i^^^  and  has  been  elaborated 
especially  by  Weismann.  The  other  was  clearly  outlined  by  De  Varies 
('89),  and  has  been  developed  in  various  directions  by  Oscar  Hertwig, 
Driesch,  and  other  writers.  In  discussing  them,  it  should  be  borne 
in  mind  that,  although  both  have  been  especially  developed  by  the 
advocates  of  the  pangen-hypothesis,  neither  necessarily  involves  that 
hypothesis  in  its  strict  form,  i.e.  the  postulate  of  discrete  self-propa- 
gating units  in  the  idioplasm.     This  hypothesis  may  therefore  be  laid 

1  QC  p.  290. 

2  The  neo-pangenesis  of  De  Vries  differs  from  Darwin's  hypothesis  in  one  very  important 
respect.  Darwin  assumed  that  the  gemmules  arose  in  the  body,  being  thrown  off  as  germs 
by  the  individual  tissue-cells,  transported  to  the  germ-cells,  and  there  accumulated  as  in  a 
reservoir;  and  he  thus  endeavoured  to  explain  the  transmission  of  acquired  characters.  De 
Vries,  on  the  other  hand,  denies  such  a  transportal  from  cell  to  cell,  maintaining  that  the 
pangens  arise  or  preexist  in  the  germ-cell,  and  those  of  the  tissue-cells  are  derived  from  this 
source  by  cell-division. 

^  This  conception  obviously  harmonizes  with  the  role  of  the  nucleus  in  the  synthetic 
process.  In  accepting  the  view  that  the  nuclear  control  of  the  cell  is  effected  by  an  emana- 
tion of  specific  substances  from  the  nucleus,  we  need  not,  however,  necessarily  adopt  the 
pangen-hypothesis. 


THE  ROUX-WEISMANN   THEORY   OF  DEVELOPMENT  405 

aside  as  an  open  question,^  and  will  be  considered  only  in   so  far  as  it 
is  necessary  to  a  presentation  of  the  views  of  individual  writers. 

The  Roux-Weismann  hypothesis  has  already  been  touched  on  at 
page  245.  Roux  conceived  the  idioplasm  {i.e.  the  chromatin)  not  as  a 
single  chemical  compound  or  a  homogeneous  mass  of  molecules,  but 
as  a  highly  complex  mixture  of  different  substances,  representing 
different  qualities,  and  having  their  seat  in  the  individual  chromatin- 
granules.  In  mitosis  these  become  arranged  in  a  linear  series  to 
form  the  spireme-thread,  and  hence  may  be  precisely  divided  by  the 
splitting  of  the  thread.  Roux  assumes,  as  a  fundamental  postulate, 
that  division  of  the  granules  may  be  either  quantitative  or  qualitative. 
In  the  first  mode  the  group  of  qualities  represented  in  the  mother- 
granule  is  first  doubled  and  then  split  into  equivalent  daughter-groups, 
the  daughter-cells  therefore  receiving  the  same  qualities  and  remain- 
ing of  the  same  nature.  In  ''qualitative  division,"  on  the  other  hand, 
the  mother-group  of  qualities  is  split  into  dissimilar  groups,  which, 
passing  into  the  respective  daughter-nuclei,  lead  to  a  eorrespondin^^ 
diff'erentiation  in  the  daughter-cells.  By  qualitative  di\isions,  occur- 
ring in  a  fixed  and  predetermined  order,  the  idioplasm  is  thus  split 
up  during  ontogeny  into  its  constituent  qualities,  which  are,  as  it  were, 
sifted  apart  and  distributed  to  the  various  nuclei  of  the  embrvo. 
Every  cell-nucleus,  therefore,  receives  a  specific  form  of  chromatin  which 
determines  the  nature  of  the  cell  at  a  given  period  and  its  later  his- 
tory. Every  cell  is  thus  endowed  with  a  power  of  self  determination, 
which  lies  in  the  specific  structure  of  its  nucleus,  and  its  course  of 
development  is  only  in  a  minor  degree  capable  of  modification  through 
the  relation  of  the  cell  to  its  fellows  ("correlative  differentiation  "). 

Roux's  hypothesis,  be  it  observed,  does  not  commit  him  to  the 
theory  of  pangenesis.  It  was  reserved  for  Weismann  to  develop  the 
hypothesis  of  qualitative  division  in  terms  of  the  pangen-hypothesis, 
and  to  elaborate  it  as  a  complete  theory  of  development.  In  his 
Hrst  essay  ('85),  published  before  De  Vries's  paper,  he  went  no  far- 
ther than  Roux.  "  I  believe  that  we  must  accept  the  hypothesis  that 
in  indirect  nuclear  division,  the  formation  of  non-equivalent  halves 
may  take  place  quite  as  readily  as  the  formation  of  equivalent  halves, 
and  that  the  equivalence  or  non-equivalence  of  the  subsecjuently  i)ro- 
duced  daughter-cells  must  depend  upon  that  of  the  nuclei.  Thus, 
during  ontogeny  a  gradual  transformation  of  the  nuclear  substance 
takes  place,  necessarily  imposed  upon  it,  according  to  certain  laws, 
by  its  own  nature,  and  such  transformation  is  accompanied  by  a 
gradual  change  in  the  character  of  the  cell-bodies."  -  In  later  writ- 
ings Weismann  advanced  far  beyond  this,  building  up  an  elaborate 
artificial  system,  which  appears  in  its  final  form  in  the  remarkable 

1  Cf.  Chapter  VI.  2  Kssay  IV.,  p.  19.5,  1SS5. 


406  INHERITANCE  AND  DEVEIOPMENT 

book  on  the  germ-plasm  ('92).  Accepting  De  Vries's  conception  of 
the  pangens,  he  assumes  a  definite  grouping  of  these  bodies  in  the 
germ-plasm  or  idioplasm  (chromatin),  somewhat  as  in  Nageli's  concep- 
tion. The  pangens  or  biopJiores  are  conceived  to  be  successively  ag- 
gregated in  larger  and  larger  groups;  namely,  (i )  determinants,  which 
are  still  beyond  the  limits  of  microscopical  vision ;  (2)  ids,  which  are 
identified  with  the  visible  chromatin-granules ;  and  (3)  idants,  or 
chromosomes.  The  chromatin  has,  therefore,  a  highly  complex  fixed 
architecture,  which  is  transmitted  from  generation  to  generation,  and 
determines  the  development  of  the  embryo  in  a  definite  and  specific 
manner.  Mitotic  division  is  conceived  as  an  apparatus  which  may 
distribute  the  elements  of  the  chromatin  to  the  daughter-nuclei  either 
equally  or  unequally.  In  the  former  case  {''  Jiomoeokinesis,''  integral 
or  quantitative  division),  the  resulting  nuclei  remain  precisely  equiva- 
lent. In  the  second  c^lSQ  {'' /ietej'oki7iesis,''  qualitative  ox  dijferential 
division),  the  daughter-cells  receive  different  groups  of  chromatin- 
elements,  and  hence  become  differently  modified.  During  ontogeny, 
through  successive  qualitative  divisions,  the  elements  of  the  idioplasm 
or  germ-plasm  (chromatin)  are  gradually  sifted  apart,  and  distributed 
in  a  definite  and  predetermined  manner  to  the  various  parts  of  the 
body.  "  Ontogeny  depends  on  a  gradual  process  of  disintegration  of 
the  id  of  germ-plasm,  which  splits  into  smaller  and  smaller  groups  of 
determinants  in  the  development  of  each  individual.  .  .  .  Finally, 
if  we  neglect  possible  complications,  only  one  kind  of  determinant  re- 
mains in  each  cell,  viz.  that  which  has  to  control  that  particular  cell  or 
group  of  cells.  ...  In  this  cell  it  breaks  up  into  its  constituent  bio- 
phores,  and  gives  the  cell  its  inherited  specific  character."^  Devel- 
opment is,  therefore,  essentially  evolutionary  and  not  epigenetic ;  ^  its 
point  of  departure  is  a  substance  in  which  all  of  the  adult  characters 
are  represented  by  preformed,  prearranged  germs  ;  its  course  is  the 
result  of  a  predetermined  har^iony  in  the  succession  of  the  qualitative 
divisions  by  which  the  hereditary  substance  is  progressively  disinte- 
grated. In  order  to  account  for  heredity  through  successive  genera- 
tions, Weismann  is  obliged  to  assume  that,  by  means  of  quantitative 
or  integral  division,  a  certain  part  of  the  original  germ-plasm  is  car- 
ried on  unchanged,  and  is  finally  delivered,  with  its  original  architecture 
unaltered,  to  the  germ-nuclei.  The  power  of  regeneration  is  explained, 
in  like  manner,  as  the  result  of  a  transmission  of  unmodified  or  sHghtly 
modified  germ-plasm  to  those  parts  capable  of  regeneration. 

1  Germ-plasm,  pp.  76,  77.  ^  I.e.,  p.  15. 


CRITIQUE   OF  THE   ROUX-WEISMANN   THEORY 


407 


E.     Critique  of  the  Roux-Weismann  Thkorv 

It  is  impossible  not  to  admire  the  thorou<(hness,  candour,  and  lo^^ical 
skill  with  which  Weismann  has  developed  his  theory,  or  to  deny  That, 
in  its  final  form,  it  does  afford  up  to  a  certain  point  a  forma/  solution 
of  the  problems  with  which  it  deals.  Its  fundamental  weakness  is  its 
^//^?j-/-metaphysical  character,  which,  indeed,  almost  places  it  outside 


A 


D 


C  D 

Fig.  183.  —  Half  and  whole  cleavage  in  the  eggs  of  sea-urchins. 

A.  Normal  sixteen-cell  stage,  showing  the  four  micromeres  above  (from  Driesch,  after  Selenka). 
B.  Half  sixteen-cell  stage  developed  from  one  blastomere  of  the  two-cell  stage  after  killing  the  other 
by  shaking  (Driesch).  C.  Half  blastula  resulting,  the  dead  blastomere  at  the  right  (Driesch), 
D.  Half-sized  sixteen-cell  stage  of  Toxopneustes,  viewed  from  the  micromere-pole  (the  «yght  lower 
not  shown).  This  embryo,  developed  from  an  isolated  blastomere  of  the  two-cell  stage,  segmented 
like  an  entire  normal  ovum. 


the  sphere  of  legitimate  scientific  hypothesis.  Save  in  the  maturation 
of  the  germ-cells  (''reducing  divisions"),  none  of  the  visible  phenom- 
ena of  cell-division  give  even  a  remote  suggestion  of  qualitative  divi- 
sion. All  the  facts  of  ordinary  mitosis,  on  the  contrary,  indicate  that 
the  division  of  the  chromatin  is  carried  out  with  the  most  e.xact  equality. 


4o8 


INHERITANCE  AND  DEVELOPMENT 


The  hypothesis  mainly  rests  upon  a  quite  different  order  of  phenom- 
ena, namely,  on  facts  indicating  that  isolated  blastomeres,  or  other 
cells,  have  a  certain  power  of  self-determination,  or  **  self-differentia- 
tion" (Roux),  pecuKar  to  themselves,  and  which  is  assumed  to  be  pri- 
marily due  to  the  specific  quality  of  the  nuclei.  This  assumption, 
which  may  or  may  not  be  true,^  is  itself  based  upon  the  further  assump- 
tion of  quahtative  nuclear  division  of  which  we  actually  know^  nothing 
whatever.  The  fundamental  hypothesis  is  thus  of  purely  a  priori 
character;    and  every  fact  opposed  to  it   has   been  met  by  subsidi- 


A  B 

Fig.  184.  —  Normal  and  dwarf  gastrulas  oi  Amphioxus. 

A.  Normal  gastrula.     B.  Half-sized  dwarf,  from  an  isolated  blastomere  of  ihe  two-cell  stage. 
C.  Quarter-sized  dwarf,  from  an  isolated  blastomere  of  the  four-cell  stage. 


ary  hypotheses,  which,  like  their  principal,  relate  to  matters  beyond 
the  reach  of  observation. 

Such  an  hypothesis  cannot  be  actually  overturned  by  a  direct 
appeal  to  fact.  We  can,  however,  make  an  indirect  appeal,  the 
results  of  which  show  that  the  hypothesis  of  qualitative  division  is 
not  only  so  improbable  as  to  lose  all  semblance  of  reality,  but  is  in 
fact  quite  superfluous.  It  is  rather  remarkable  that  Roux  himself 
led  the  way  in  this  direction.  In  the  course  of  his  observations  on 
the  development  of  a  half-embryo  from  one  of  the  blastomeres  of 
the  two-cell  stage  of  the  frog's  ^gg,  he  determined  the  significant 
fact  that  the  half-embryo  in  the  end  restores  more  or  less  completely 

1  Cf.  p.  426. 


CRITIQUE    OF   THE  ROUX-WEISMANN   THEORY 


409 


the  missing  half  \iy  a  peculiar  process,  related  to  regeneration,  which 
Roux  designated  as  post-generation.  Later  studies  showed  that  an 
isolated  blastomere  is  able  to  give  rise  to  a  complete  embryo  in  manv 
other  animals,  sometimes  developing  in  its  earlier  stages  as  though 


E 

Fig.  185.  —  Dwarf  and  double  embryos  of  Amphioxus. 
A.  Isolated  blastomere  of  the  two-cell  stage  segmenting  like  an  entire  egg  Kef.  Fig.  183.  />>). 
B.  Twin  gastrulas  from  a"  single  egg.     C.  Double  cleavage  resulting  from  the  partial  separation, 
by  shaking,  of  the  blastomeres  of  the  two-cell  stage.     D.E.F.  Double  gastrulas  arising  from  such 
forms  as  the  last. 


still  forming  part  of  a  complete  embryo  (**  partial  development"), 
but  in  other  cases  developing  directly  into  a  complete  dwarf  embryo, 
as  if  it  were  an  ^g%  of  diminished  size.  In  1891  Driesch  w^is  ab^e 
to  follow  out  the  development  of  isolated  blastomeres  of  sea-nrchin 


410 


INHERITANCE  AND  DEVELOPMENT 


eggs  separated  by  shaking  to  pieces  the  two-cell  and  four-cell  stages. 
Blastomeres  thus  isolated  segment  as  if  still  forming  part  of  an  entire 
larva,  and  give  rise  to  a  half-  (or  quarter-)  blastula  (Fig.  183).  The 
opening  soon  closes,  however,  to  form  a  small  complete  blastula,  and 
the  resulting  gastrula  and  Pluteus  larva  is  a  perfectly  formed  dwarf 
of  only  half  (or  quarter)  the  normal  size.  Incompletely  separated 
blastomeres  give  rise  to  double  embryos  Uke  the  Siamese  twins. 
Shortly  afterward  the  writer  obtained  similar  results  in  the  case  of 
AnipJiioxHS^  but  here  tJie  isolated  blastoinere  behaves  from  the  begin- 
7iing  like  a  complete  ovum  of  half  the  usual  sice,  and  gives  rise  to  a 
complete  blastula,  gastrula,  and  larva.  Complete  embryos  have  also 
been  obtained  from  a  single  blastomere  in  the  teleost  Fuiidulus 
(Morgan,  '95,  2),  in  Triton  (Herlitzka,  '95),  and  in  a  number  of 
hydromedusae  (Zoja,  '95,  Bunting,  '99);  and  nearly  complete  em- 
bryos in  the  tunicates  Ascidiella  (Chabry,  "^'j\  PJiallusia  (Driesch, 
'94),  and  Molgula  (Crampton,  '98).^  Perhaps  the  most  striking  of 
these  cases  is  that  of  the  hydroid  Clytia,  in  which  Zoja  was  able  to 
obtain  perfect  embryos,  not  only  from  the  blastomeres  of  the  two- 
cell  and  four-cell  stages,  but  from  eight-cell  and  even  from  sixteen- 
cell  stages,  the  dwarfs  in  the  last  case  being  but  one-sixteenth  the 
normal  size. 

These  experiments  render  highly  improbable  the  hypothesis  of 
qualitative  division  in  its  strict  form,  for  they  demonstrate  that  the 
earlier  cleavages,  at  least,  do  not  in  these  cases  sunder  fundamentally 
different  materials,  either  nuclear  or  cytoplasmic,  but  only  split  the 
^&g  up  into  a  number  of  parts,  each  of  which  is  capable  of  producing 
an  entire  body  of  diminished  size,  and  hence  m.ust  contain  all  of  the 
material  essential  to  complete  development.  Both  Roux  and  Weis- 
mann  endeavour  to  meet  this  adverse  evidence  with  the  assumption 
of  a  "  reserve  idioplasm,"  containing  all  of  the  elements  of  the  germ- 
plasm  which  is  in  these  cases  distributed  equally  to  all  the  cells  in 
addition  to  the  specific  chromatin  conveyed  to  them  by  qualitative 
division.  This  subsidiary  hypothesis  renders  the  principal  one  {i.e. 
that  of  qualitative  division)  superfluous,  and  brings  us  back  to  the 
same  problems  that  arise  when  the  assumption  of  qualitative  division 
is  discarded. 

The  theory  of  qualitative  nuclear  division  has  been  practically  dis- 
proved in  another  way  by  Driesch,  through  the  pressure-experiments 
already  mentioned  at  page  375.  Following  the  earlier  experiments  of 
Pfluger  ('84)  and  Roux  ('85)  on  the  frog's  Q,g^,  Driesch  subjected 
segmenting  eggs  of  the  sea-urchin  to  pressure,  and  thus  obtained  flat 
plates  of  cells  in  which  the  arrangement  of  the  nuclei  differed  totally 

1  The  "partial"  development  in  the  earlier  stages  of  some  of  these  forms  is  considered 
at  page  419. 


CRITIQUE    OF   THE  ROUX-WEISMANN   THEORY  411 

from  the  normal  (Fig.  186);  yet  such  eggs  when  released  from  press- 
ure continue  to  segment,  zvitJwiit  rearrangeme^it  of  the  nuclei,  and 
give  rise  to  perfectly  normal  larvae.  I  have  repeated  these  experi- 
ments not  only  with  sea-urchin  eggs,  but  also  with  those  of  an  annelid 
i^Nereis\  which  yield  a  very  convincing  result,  since  in  this  case  the 
histological  differentiation  of  the  cells  appears  very  early.  In  the 
normal  development  of  this  animal  the  archenteron  arises  from  four 
large  cells  or  macromeres  (entomeres),  which  remain  after  the  suc- 
cessive formation  of  three  quartets  of  micromeres  (ectomeres)  and  the 
parent-cell  of  the  mesoblast.  After  the  primary  differentiation  of 
the  germ-layers  the  four  entomeres  do  not  divide  again  until  a  very 
late  period  (free-swimming  trochophore),  and  their  substance  always 
retains  a  characteristic  appearance,  differing  from  that  of  the  other 


Fig.  186.  —  Modification  of  cleavage  in  sea-urchin  eggs  by  pressure. 

A.   Normal  eight-cell  stage  of  Toxopneustes.     B.   Eight-cell  stage  of  £'c/^///«j  segmentino- under 
pressure.     Both  forms  produce  normal  Plutei. 

blastomeres  in  its  pale  non-granular  character  and  in  the  presence  of 
large  oil-drops.  If  unsegmented  eggs  be  subjected  to  pressure,  as  in 
Driesch's  echinoderm  experiments,  they  segment  in  a  flat  plate,  all 
of  the  cleavages  being  vertical.  In  this  way  are  formed  eight-celled 
plates  in  which  all  of  the  cells  contain  oil-drops  (Fig.  187,  D).  If 
they  are  now  released  from  the  pressure,  each  of  the  cells  divides  in 
a  plane  approximately  horizontal,  a  smaller  granular  micromere  being 
formed  above,  leaving  below  a  larger  clear  macromere  in  which  the 
oil-drops  remain.  The  sixteen-cell  stage,  therefore,  consists  of  eight 
deutoplasm-laden  macromeres  and  eight  protoplasmic  micromeres 
(instead  of  four  macromeres  and  twelve  micromeres,  as  in  the  usual 
development).  These  embryos  developed  into  free-swimming  trocho- 
phores  containing  eight  instead  of  four  macromeres,  which  have  the 
typical  clear  protoplasm  containing  oil-drops.      In  this  case  there  can 


412 


INHERITANCE  AND  DEVEIOPMENT 


be  no  doubt  whatever  that  four  of  the  entoblastic  nuclei  were  nor- 
mally destined  for  the  first  quartet  of  micromeres  (Fig.  187,  B\  from 
which  arise  the  apical  gangha  and  the  prototroch.  Under  the  condi- 
tions of  the  experiment,  however,  they  have  given  rise  to  the  nuclei 
of  cells  which  differ  in  no  wise  from  the  other  entoderm-cells.     Even 


Fig.  187.  —  Modifications  of  cleavage  by  pressure  in  Nereis. 

A.  B.  Normal  four-  and  eight-cell  stages.  C.  Normal  trochophore  larva  resulting,  with  four 
entoderm-cells.  D.  Eight-cell  stage  arising  from  an  egg  flattened  by  pressure  ;  such  eggs  give  rise 
10  trochophores  with  eight  instead  of  four  entoderm-cells.  Numerals  designate  the  successive 
cleavages. 


in  a  highly  differentiated  type  of  cleavage,  therefore,  the  nuclei  of  the 
segmenting  Qgg  are  not  specifically  different,  as  the  Roux-Weismann 
hypothesis  demands,  but  contain  the  same  materials  even  in  the  cells 
that  undergo  the  most  diverse  subsequent  fate.  But  there  is,  further- 
more, very  strong  reason  for  believing  that  this  may  be  true  in  later 


NATURE  AND    CAUSES  OF  DIFFERENTIATION  413 

stages  as  well,  as  Koiliker  insisted  in  opposition  to  Weismann  as 
early  as  1886,  and  as  has  been  urged  by  many  subsequent  writers. 
The  strongest  evidence  in  this  direction  is  afforded  by  the  facts  of 
regeneration;  and  many  cases  are  known  —  for  instance,  among  the 
hydroids  and  the  plants  —  in  which  even  a  small  fragment  of  the 
body  is  able  to  reproduce  the  whole.  It  is  true  that  the  power  of 
regeneration  is  always  limited  to  a  greater  or  less  extent  according 
to  the  species.  But  there  is  no  evidence  whatever  that  such  hmita- 
tion  arises  through  specification  of  the  nuclei  by  quaHtative  division, 
and,  as  will  appear  beyond,  its  explanation  is  probably  to  be  sought 
in  a  very  different  direction. 

F.     On  the  Nature  and  Causes  of  Differentiation 

We  have  now  cleared  the  ground  for  a  restatement  of  the  problem 
of  development  and  an  examination  of  the  views  opposed  to  the 
Roux-Weismann  theory.  After  discarding  the  hypothesis  of  quaH- 
tative division  the  problem  confronts  us  in  the  following  form.  If 
chromatin  be  the  idioplasm  in  which  inheres  the  sum  total  of  heredi- 
tary forces,  and  if  it  be  equally  distributed  at  every  cell-division,  how 
can  its  mode  of  action  so  vary  in  different  cells  as  to  cause  diversity 
of  structure,  i.e.  dijferentiatio7i  ?  It  is  perfectly  certain  that  differen- 
tiation is  an  actual  progressive  transformation  of  the  egg-substance 
involving  both  physical  and  chemical  changes,  occurring  in  a  definite 
order,  and  showing  a  definite  distribution  in  the  regions  of  the  Qgg. 
These  changes  are  sooner  or  later  accompanied  by  the  cleavage 
of  the  Qgg  into  cells  whose  boundaries  may  sharply  mark  the 
areas  of  differentiation.  What  gives  these  cells  their  specific  char- 
acter.? Why,  in  the  four-cell  stage  of  an  annelid  Qgg,  should  the 
four  cells  contribute  equally  to  the  formation  of  the  ahmentary  canal 
and  the  cephaUc  nervous  system,  while  only  one  of  them  (the  left- 
hand  posterior)  gives  rise  to'  the  nervous  system  of  the  trunk-region 
and  to  the  muscles,  connective  tissues,  and  the  germ-cells.'  (Figs. 
171,  188,  B.)  There  cannot  be  a  fixed  relation  between  the  various 
regions  of  the  &gg  which  these  blastomeres  represent  and  the  adult 
pa'I-ts  arising  from  them;  for  in  some  eggs  these  relations  maybe 
artificially  changed.  A  portion  of  the  Qgg  which  under  normal  con- 
ditions would  give  rise  to  only  a  fragment  of  the  body  will,  if  split  off 
from  the  rest,  give  rise  to  an  entire  body  of  diminished  size.  W  hat 
then  determines  the  history  of  such  a  portion.?  What  influence 
moulds  it  now  into  an  entire  body,  now  into  a  part  of  a  body  .? 

De  Vries,  in  his  remarkable  essay  on  Intracellular  l\iuo;cmsis 
('89),  endeavoured  to  cut  this  Gordian  knot  by  assuming  that  the 
character  of  each  cell  is  determined  by  pangens  that  migrate  from 


414 


INHERITANCE  AND   DEVEIOPMENT 


the  nucleus  into  the  cytoplasm,  and,  there  becoming  active,  set  up 
specific  changes  and  determine  the  character  of  the  cell,  this  way 
or  that,  according  to  their  nature.  But  what  influence  guides  the 
migrations  of  the  pangens,  and  so  correlates  the  operations  of  devel- 
opment?     Both    Driesch    and    Oscar    Hertwig    have    attempted    to 


f — 


Fig.  i88.  — Diagrams  illustrating  the  value  of  the  quartets  in  a  polyclade  {Leptoplana),  a  lamel- 
libranch  {Ufiio),  and  a  gasteropod  {Crepiduld).  A.  Leptoplana,  showing  mesoblast-formation 
in  the  second  quartet.  B.  Crepidula,  showing  source  of  ectomesoblast  (from  a~,  b-,  c'^)  and  en- 
tomesoblast  (from  quadrant  Z)).     C.  6'>/2c, 'ectomesoblast  formed  only  from  u^. 

In  all  the  figures  the  successive  quartets  are  numbered  with  Arabic  figures  ;  ectoblast  unshaded, 
mesoblest  dotted,  entoblast  vertically  lined. 

answer  this  question,  though  the  first-named  author  does  not  commit 
himself  to  the  pangen-hypothesis.  These  writers  have  maintained 
that  the  particular  mode  of  development  in  a  given  region  or  blasto- 
mere  of  the  tgg  is  a  result  of  its  relation  to  the  remainder  of  tJie  7nass^ 
i.e.  a  product  of    what  may  be  called  the  intra-embryonic  environ- 


NATURE   AND    CAUSES   OF  DIFEERENTIATION  415 

ment.     Hertwig  insisted   that  the  organism   develops  as  a  whole  as 
the  result  of    a  physiological  interaction  of    equivalent  blastomeres 
the  transformation   of    each   being   due   not    to   an    inherent   specific 
power  of    self-differentiation,  as   Roux's  mosaic-theory  assumed    but 
to   the   action    upon    it  of    the   whole   system   of  which    it   is  a   part 
''According  to  my  conception,"  said   Hertwig,  -  each  of  the  first  two 
blastomeres  contains    the    formative    and    differentiating    forces   not 
simply  for  the  production  of  a  half-body,  but  for  the  entire  organism  ; 
the  left  blastomere  develops  into  the  left  half  of  the  bodv  only  because 
it  is   placed  in  relation  to  a  right   blastomere."  1     Again,  in   a   later 
paper  :   "  The  ^gg  is  a  specifically  organized  elementary  organism  that 
develops  epigenetically  by  breaking  up  into  cells  and  their  subsequent 
differentiation.     Since  every  elementary  part  {i.e.  cell)  arises  through 
the  division  of  the  germ,  or  fertilized  ^gg,  it  contains  also  the  ger'm 
of  the  w^hole,  but  during  the  process  of  development  it  becomes  ever 
more   precisely   differentiated   and   determined   by  the   formation   of 
cytoplasmic  products  according  to  its  position  with  reference  to  the 
entire  organism  (blastula,  gastrula,  etc.)."  2 

An  essentially  similar  view  was  advocated  by  the  writer  ('93,  '94) 
nearly  at  the  same  time,  and  the  same  general  conception  was  ex- 
pressed with  great  clearness  and  precision  by  Driesch  shortlv  after 
Hertwig:  "The  fragments  {i.e.  cells)  produced  by  cleavage  are  com- 
pletely equivalent  or  indifferent."  "The  blastomeres  of  the  sea- 
urchin  are  to  be  regarded  as  forming  a  uniform  material,  and  they 
may  be  thrown  about,  like  balls  in  a  pile,  without  in  the  least  degree 
impairing  thereby  the  normal  power  of  development." '"^  '' The  rehi- 
tive positio7i  of  a  blastomere  iji  tJie  zi'/io/e  determines  in  general  what 
develops  from  it ;  if  its  position  be  e hanged,  it  gives  rise  to  something 
different;  in  other  words,  its  prospective  valne  is  a  function  of  its 
position.  "^ 

Jn/this  last  aphorism  the  whole  problem  of  development  is  brought 
to  a  focus.  It  is  clearly  not  a  solution  of  the  j^roblem,  but  (Mily  a 
highly  suggestive  restatement  of  it;  for  everything  turns  upon  how 
the  relation  of  the  part  to  the  whole  is  conceived.  W^-y  little  con- 
sideration is  required  to  show  that  this  relation  cannot  be  a  merelv 
geometrical   or  rudely  mechanical  one,  for  in  the   eggs  of  different 

1  '92,  I,  p.  481. 

^  '93»  P-  793-  It  should  be  pointed  out  that  Roux  himself  in  several  papers  expressl\ 
recognizes  the  fact  that  development  cannot  be  regarded  as  a  ]iure  mosaic-work,  and  that 
besides  the  power  of  self-differentiation  postulated  by  his  hypothesis  we  must  assume  a 
"correlative  differentiation"  or  differentiating  interaction  of  parts  in  the  embryo.      Cf.  Koux, 

'92,  '93.  I- 

3  Studien  IV.,  p.  25. 

*  Studien  IV.,  p.  39.  Cf.  His,  "  Es  muss  die  Wachsthumserregbarkeit  dcs  pjcs  cine 
Function  des  Raumes  sein."     ('74,  p.  153.) 


41 6  INHERITANCE  AND  DEVELOPMENT 

animals    blastomeres  may  almost    exactly  correspond    in   origin   and 
relative  position,  yet  differ  widely  in  their  relation  to  the  resulting 
embryo.     Thus  we  find  that  the  cleavage  of  polyclades,  annelids,  and 
gasteropods  (Fig.  i88)  shows  a  really  wonderful  agreement  in  form, 
yet  the  individual  cells  differ  markedly  in  prospective  value.     In  all 
of  these  forms  three  quartets  of  micromeres  are  successively  formed 
according  to  exactly  the  same  remarkable  law  of  the  alternation  of  the 
spirals ;  ^  and,  in  all,  the  posterior  cell  of  a  fourth  quartet  lies  at  the 
hinder  end  of  the  embryo  in  precisely  the  same  geometrical  relation 
to  the  remainder  of  the  embryo  ;  yet  in  the  gasteropods  and  annelids 
this  cell  gives  rise  to  the  mesoblast-bands  and  their  products,  in  the 
polyclade  to  a  part  of    the  archenteron,  while  important  differences 
also  exist  in  the  value  of   the  other  quartets.     The  relation  of   the 
part  to  the  whole  is  therefore  of  a  highly  subtle  character,  the  pro- 
spective value  of  a  blastomere  depending  not  merely  upon  its  geomet- 
rical position,  but  upon  its  relation  to  the  whole  complex  inherited 
organization  of  which  it  forms  a  part.     The  apparently  simple  con- 
clusion stated  in  Driesch's  clever  aphorism  thus  leads  to  further  prob- 
lems of  the  highest  complexity.     It  should  be  here  pointed  out  that 
Driesch  does  not  accept  Hertwig's  theory  of  the  interaction  of  blasto- 
meres as  such,  but,  hke  Whitman,  Morgan,  and  others,  has  brought 
forward  effective  arguments  against  that  too  simple  and  mechanical 
conception.     That  theory  is,  in  fact,  merely  Schwann's  cell-composite 
theory  of  the  organism  applied  to  the  developing  embryo,  and  the 
general  arguments  against  that  theory  find  some  of    their  strongest 
\     support  in  the  facts  of  growth  and  development.^     Th]s  has  been 
'     forcibly  urged  by  Whitman  ('93),  who  almost  simultaneously  with  the 
statements  of  Driesch  and  Hertwig,  cited  above,  expressed  the  con- 
viction that  the  morphogenic  process  cannot  be  conceived  as  merely 
the  sum  total  or  resultant  of  the  individual  cell-activities,  but  operates 
as  a  unit  without  respect  to  cell-boundaries,  precisely  as  De  Bary  con- 
cludes in  the  case  of  growing  plant-tissues  (p.  393),  and  the  nature 
of  that  process  is  due  to  the  organization  of  the  ^gg  as  a  whole.  ^ 

While  recognizing  fully  the  great  value  of  the  results  attained 
during  the  past  few  years  in  the  field  of  experimental  and  specula- 
tive embryology,  we  are  constrained  to  admit  that  as  far  as  the 
essence  of  the  problem  is  concerned  we  have  not  gone  very  far 
beyond  the  conclusions  stated  above  ;  for  beyond  the  fact  that  the 
inherited  organization  is  involved  in  that  of  the  germ-cells  we  remain 
quite  ignorant  of  its  essential  nature.  This  has  been  recognized  by 
no  one  more  clearly  than  by  Driesch  himself,  to  whose  critical 
researches  we  owe  so  much  in  this  field.  At  the  cUmax  of  a  recent 
elaborate  analysis,  the  high  interest  of  which  is  somewhat  obscured  by 
1  Cf.  p.  368.  ^  Cf.  pp.  388-394. 


NATURE  AND    CAUSES   OE  DIFEERENTIATION  417 

its  too  abstruse  form,  Driesch  can  only  reiterate  his  former  aphorism,^ 
finally  taking  refuge  in  an  avowed  theory  of  vitalism  which  assumes 
the  localization  of  morphogenic  phenomena  to  be  determined  bv  "a 
wholly  unknown  principle  of  correlation,"^  and  forms  a  problem  sni 
generis^^  This  conclusion  recognizes  the  fact  that  the  fundamental 
problem  of  development  remains  wholly  unsolved,  thus  confirming 
from  a  new  point  of  view  a  conclusion  which  it  is  only  fair  to  point 
out  has  been  reached  by  many  others. 

-  But  while  the  fundamental  nature  of  the  morphogenic  process  thus 
remains  unknown,  we  have  learned  some  very  interesting  facts  regard- 
ing the  conditions  under  which  it  ta'kes  place,  and  which  show  that 
Driesch's  aphorism  loses  its  meaning  unless  carefully  qualified.  The 
experiments  referred  to  at  pages  353,  410,  show  that  up  to  a  certain 
stage  of  development  the  blastomeres  of  the  early  echinoderm,  AnipJii- 
oxiis  or  medusa-embryo,  are  "totipotent"  (Roux),  or  "equipotential  " 
(Driesch),  i.e.  capable  of  producing  any  or  all  parts  of  the  body. 
Even  in  these  cases,  however,  we  cannot  accept  the  early  conclusion 
of  Pfliiger  i^'^Z),  applied  by  him  to  the  frog's  ^gg,  and  afterward 
accepted  by  Hertwig,  that  the  material  of  the  ^gg,,  or  of  the  blasto- 
meres into  which  it  splits  up,  is  absolutely  "  isotropic,"  i.e.  consists  of 
quite  uniform  indifferent  material,  devoid  of  preestablished  axes. 
Whitman  and  Morgan,  and  Driesch  himself,  showed  that  this  cannot 
be  the  case  in  the  echinoderm  Qgg\  for  the  ovum  possesses  a  polarity 
predetermined  before  cleavage  begins,  as  proved  by  the  fact  that  at 
the  fourth  cleavage  a  group  of  small  cells  or  micromeres  always  arises 
at  a  certain  point,  which  may  be  precisely  located  before  cleavage  by 
reference  to  the  eccentricity  of  the  first  cleavage-nucleus,'*  and  which, 
as  Morgan  showed,^  is  indicated  before  the  third,  and  sometimes 
before  the  second  cleavage,  by  a  migration  of  pigment  away  from  the 
micromere-pole.  These  observers  are  thus  led  to  the  assumption  of 
a  primary  polarity  of  the  egg-protoplasm,  to  which  Driesch,  in  the 
course  of  further  analysis  of  the  phenomena,  is  compelled  to  add  the 
assumption  of  a  secondary  polarity  at  right  angles  to  the  first.'^  The.se 
polarities,  inherent  not  only  in  the  entire  ^gg,  but  also  in  each  of  the 
blastomeres  into  which  it  divides,  form  the  primary  conditions  under 
which  the  bilaterally  symmetrical  organism  develops  by  epigenesis. 
To  this  extent,  therefore,  the  material  of  the  blastomeres,  though 
''totipotent,"  shows  a  certain  predetermination  with  respect  to  the 
adult  body. 

1  '99,  pp.  86-87. 

2  This  phrase  is  cited  by  Driesch  from  an  earher  work  (^'92,  p.  59O)  as  ^ivm-  a  cncct 
though  "  unanalytical  "  statement  of  his  view.  It  may  be  questioned  whether  many  readers 
will  regard  as  an  improvement  the  ''  analytical  "  form  it  assumes  in  his  last  work. 

3  I.e.,  p.  90.  *  'Cf.  Fig.  103.  ^  '94,  I'.  142. 
6  See  Driesch,  '93,  pp.  229,  241 ;   '96,  and  '99,  p.  44- 

2  E 


4i8 


ISIIERITAXCE   AXD   DEVELOPMEXT 


~\Ve  now  proceed  to  the  consideration  of  experiments  which  show- 
that  in  some  animal  egK'^  ^^»ch  ]-)redetermination  may  go  much  farther, 
so  that  the  development  does,  in  fact,  show  many  of  the  features  of 
a  mosaic-work,  as  maintained  ]:>y  Roux.  The  best-determined  of  these 
cases  is  that  of  the  ctenoph()re-e«;K.  ^^^  shown  by  the  work  of  Chun, 


Fig.  189.  —  Partial  larvcne  of  the  ctenophore  Beroc.     [Dkiesch  and  Morgan.] 

A.  Half  sixteen-cell  stage,  from  an  isolated  blastomere.  li.  Resulting  larva,  with  four  rows  of 
swimming-plates  and  three  gastric  pouches.  C.  One-fourth  sixtecn-cell  stage,  from  an  isolated 
blastomere.  D.  Resulting  larva,  with  two  rows  of  plates  and  two  gastric  pouches.  E.  Defective 
larva,  with  six  rows  of  plates  and  three  gastric  pouches,  from  a  nucleated  fragment  of  an  unseg- 
mented  egg.     F.  Similar  larva  with  five  rows  of  plates,  from  above. 

Driesch,  and  Morgan  ('95),  and  Fischel  ('98).  These  observers  have 
demonstrated  that  isolated  blastomeres  of  the  two-,  four-,  or  eight-cell 
stage  undergo  a  cleavage  which,  through  the  earliest  stages,  is  exactly 
like  that  which  it  would  have  undergone  if  forming  part  of  a  com- 


NATURE  AND    CAUSES   OF  DIFEERENTIATION  419 

plete  embryo,  and  gives  rise  to  a  defective  larva,  having  only  four 
two,  or  one  row  of  swimming-plates  (Fig.  189);  and  Fischel's  obser' 
vations  give  strong  reason  to  believe  that  each  of  the  eight  micromeres 
of  the  sixteen-cell  stage  is  definitely  specified  for  the  formation  of  one 
of  the  rows  of  plates.  In  like  manner  Crampton  (96)  found  that  in 
case  of  the  marine  gasteropod  Ilyanassa  isolated  blastomeres  of  two- 
cell  or  four-cell  stages  segmented  exactly  as  if  forming  part  of  an 
entire  embryo,  and  gave  rise  to  fmgmaits  of  a  larva,  not  to  complete 
dwarfs,  as  in  the  echinoderm  (Fig.  190).  Further,  in  embryos  from 
which  the  ''yolk-lobe"  (a  region  of  that  macromere  from  which  the 
primary  mesoblast  normally  arises)  had  been  removed,  no  mesoblast- 
bands  were  formed.  Most  interesting  of  all,  Driesch  and  Morgan 
discovered  that  if  a  part  of  the  cytoplasm  of  an  imscgjnaitcd  cte^'no- 
phore-egg  were  removed,  the  remainder  gave  rise  to  an  incomplete 
larva,  showing  definite  defects  (Fig.  189,  E,  F). 

In  none  of  these  cases  is  the  embryo  able  to  complete  itself,  though 
it  should  be  remarked  that  neither  in  the  ctenophore  nor  in  the  snail 
is  the  partial  embryo  identical  with  a  fragment  of  a  whole  embryo, 
since  the  micromeres  finally  enclose  the  macromeres,  leaving  no  sur- 
face of  fracture.     This  extreme  is,  however,  connected  by  a  series  of 
forms  with  such  cases  as  those  of  Amphioxns  or  the  medusa,  where 
the  fragment  develops  nearly  or  quite  as  if  it  were  a  whole.      In  the 
tunicates  the  researches  of  Chabry  i^^y),  Driesch  ('94),  and  Crampton 
('97)  show  that  an  isolated  blastomere  of  the  two-cell  stage  undergoes 
a  typical  half-cleavage  (Crampton),  but  finally  gives  rise  to  a  nearly 
perfect  tadpole  larva   lacking  only  one  of  the  asymmetrically  placed 
sense-organs  (Driesch).     Next  in  the  series  may  be  placed  the  frog, 
where,  as  Roux,  Endres,  and  Walter  have  shown,  a  blastomere  of  the 
two-cell  stage  may  give  rise  to  a  typical  half-morula,  half-gastrula, 
and  half-embryo  1  (Fig.  182),  yet  finally  produces  a  perfect  larva.     A 
further   stage   is  given    by  the    echinoderm-egg,  which,  as    Driesch 
showed,  undergoes  a  half-cleavage  and  produces  a  half-blastula,  which, 
however,  closes  to  form  a  whole  before  the  gastrula-stage  (Fig.  183). 
Perfectly  formed  though  dwarf  larvae  result.    Finally,  we  reach  Auiphi- 
oxus  and  the  hydromasae  in  which  a  perfect  **  whole  develoj)mcnt  " 
usually  takes  place  from  the  beginning,  though   it  is  a  verv  interest- 
ing fact  that  the  isolated  blastomeres  of  Aiiiphioxus  sometimes  show, 
in  the  early  stages  of  cleavage,  peculiarities  of  development  that  recall 
their  behaviour  when  forming  part  of  an  entire  embryo.- 

We  see  throughout  this  series  an  effort,  as  it  were,  on  the  part  of 
the  isolated  blastomere  to  assume  the  mode  of  development  character- 
istic of  a  complete  ^^^,  but  one  that  is  striving  against  conditions  that 

^  This  is  not  invariably  the  case,  as  described  beyond. 
2  Cf.  Wilson,  '93,  pp.  590,  608. 


420 


INHERITANCE  AND  DEVEIOPMENT 


tend  to  confine  its  operations  to  the  role  it  would  have  played  if  still 
forming  part  of  an  entire  developing  ^g^.  In  Amphioxits  or  Clytia 
this  tendency  is  successful  almost  from  the  beginning.  In  other 
forms  the  Umiting  conditions  are  only  overcome  at  a  later  period, 
while  in  the  ctenophore  or  snail  they  seem  to  afford  an  insurmount- 


Fig.  190.  —  Partial  development  of  isolated  blastomeres  of  the  gasteropod  ^gg,  Ilyanassa. 
[Crampton.] 

A.  Normal  eight-cell  stage.  B.  Normal  sixteen-cell  stage.  C.  Half  eight-cell  stage,  from 
isolated  blastomere  of  the  two-cell  stage.  D.  Half  twelve-cell  stage  succeeding.  E.  Two  stages 
in  the  cleavage  of  an  isolated  blastomere  of  the  four-cell  stage  ;  above  a  one-fourth  eight-cell  stage, 
below  a  one-fourth  sixteen-cell  stage. 

able  barrier  to  complete  development.  What  determines  the  limita- 
tions of  development  in  these  various  cases  ?  They  cannot  be  due  to 
nuclear  specification  ;  for  in  the  ctenophore  the  fragment  of  an  iniseg- 
mented  ^%^,  containing  the  normal  egg-nucleus,  gives  rise  to  a  defec- 
tive larva ;  and  my  experiments  on  Nereis  show  that  even  in  a  highly 


NATURE  AND    CAUSES   OF  DIFFERENTIATION 


421 


determinate  cleavage,  essentially  like  that  of  the  snail,  the  nuclei  may 
be  shifted  about  by  pressure  without  altering  the  end-result.  Neither 
can  they  lie  in  the  form  of  the  dividing  mass  as  some  authors  have 
assumed ;  for  in  Crampton's  experiments  the  half  or  quarter  blasto- 
mere  does  not  retain  the  form  of  a  half  or  quarter  sphere,  but  rounds 


A 


B 


Fig.  191.  —  Double  embryos  of  frog  developed  from  eggs  inverted  when  in  the  two-cell  stage. 
[O.  SCHULTZE.] 

A.  Twins  with  heads  turned  in  opposite  directions.  B.  Twins  united  back  to  back.  C.  Twins 
united  by  their  ventral  sides.     D.  Double-headed  tadpole. 


off  to  a  spheroid  like  the  ^gg.  But  if  the  limiting  conditions  lie 
neither  in  the  nucleus  nor  in  the  form  of  the  mass,  we  must  seek  them 
in  the  cytoplasm  ;  and  if  we  find  here  factors  by  which  the  tendency 
of  the  part  to  develop  into  a  whole  may  be,  as  it  were,  hemmed  in,  we 
shall  reach  a  proximate  explanation  of  the  mosaic-Hke  character  of 
cleavage  shown   in  the  forms   under  consideration,  and   the   mosaic 


422  INHERITANCE  AND  DEVEIOPMENT 

theory  of  cytoplasmic  localization  will  find  a  substantial  if  somewhat 
restricted  basis. 

That  we  are  here  approaching  the  true  explanation  is  indicated  by 
certain  very  remarkable  and  interesting  experiments  on  the  frog's  ^gg, 
which  prove  that  each  of  the  first  two  blastomeres  may  give  rise  either 
to  a  half-embryo  or  to  a  whole  embryo  of  half  size,  according  to  cir- 
cumstances, and  which  indicate,  furthermore,  that  these  circumstances 
lie  in  a  measure  in  the  arrangement  of  the  cytoplasmic  materials. 
This  most  important  result,  which  we  owe  especially  to  Morgan, ^  was 
reached  in  the  following  manner.  Born  had  shown,  in  1885,  that  if 
frogs'  eggs  be  fastened  in  an  abnormal  position,  —  e.g.  upside  down,  or 
on  the  side,  —  a  rearrangement  of  the  egg-material  takes  place,  the 
heavier  deutoplasm  sinking  toward  the  lower  side,  while  the  nucleus 
and  protoplasm  rise.  A  new  axis  is  tints  established  in  the  egg,  which 
has  the  same  relation  to  the  body-axes  as  in  the  ordinary  develop- 
ment (though  the  pigment  retains  its  original  arrangement).  This 
proves  that  in  eggs  of  this  character  (telolecithal)  the  distribution  of 
deutoplasm,  or  conversely  of  protoplasm,  is  one  of  the  primary  forma- 
tive conditions  of  the  cytoplasm  ;  and  the  significant  fact  is  that  by 
artificially  changing  this  distribution  the  axis  of  the  ejnbryo  is  shifted. 
Oscar  Schultze( '94)  discovered  that  if  the  Qgg  be  turned  upside  down 
when  in  the  two-cell  stage,  a  whole  embryo  (or  half  of  a  double 
embryo)  may  arise  from  each  blastomere  instead  of  a  half-embryo 
as  in  the  normal  development,  and  that  the  axes  of  these  embryos 
show  no  constant  relation  to  one  another  (Fig.  191).  Morgan  (95,  3) 
added  the  important  discovery  that  either  a  half-embryo  or  a  whole 
half-sized  dwarf  might  be  formed,  according  to  the  position  of  the  blas- 
tomere. If,  after  destruction  of  one  blastomere,  the  other  be  allowed 
to  remain  in  its  normal  position,  a  half-embryo  always  results,^  pre- 
cisely as  described  by  Roux.  If,  on  the  other  hand,  the  blastomere 
be  inverted,  it  may  give  rise  either  to  a  half-embryo  ^  or  to  a  whole 
dwarf.*  Morgan  therefore  concluded  that  the  production  of  whole 
embryos  by  the  inverted  blastomeres  was,  in  part  at  least,  due  to  a 
rearrangement  or  rotation  of  the  egg-materials  under  the  influence  of 
gravity,  the  blastomere  thus  returning,  as  it  were,  to  a  state  of  equilib- 
rium like  that  of  an  entire  ovum. 

This  beautiful  experiment  gives  most  conclusive  evidence  that  each 
of  the  two  blastomeres  contains  all  the  materials,  nuclear  and  cyto- 
plasmic, necessary  for  the  formation  of  a  whole  body ;  and  that  these 
materials  may  be  used  to  build  a  whole  body  or  half-body,  according 
to  the  grouping  that  they  assume.     After  the  first  cleavage  takes 


1  Ajiat.  Anz.,  X.  19,  1895.  ^  Three  cases. 

*  Eleven  cases  observed.  *  Nine  cases  observed. 


NATURE  AND    CAUSES   OF  DIFFERENTIATION  423 

place,  each  blastomere  is  set,  as  it  were,  for  a  half-development,  but 
not  so  firmly  that  a  rearrangement  is  excluded. 

I  have  reached  a  nearly  related  result  in  the  case  of  both  Aniphi- 
oxus  and  the  echinoderms.  In  AmpJiioxus  the  isolated  blastomere 
usually  segments  like  an  entire  ovum  of  diminished  size.  This  is, 
however,  not  invariable,  for  a  certain  number  of  such  bla.stomeres 
show  a  more  or  less  marked  tendency  to  divide  as  if  still  forming  part 
of  an  entire  embryo.  The  sea-urchin  Toxopnenstcs  reverses  this  rule, 
for  the  isolated  blastomere  of  the  two-cell  stage  usually  shows  a  per- 
fectly typical  half-cleavage,  as  described  by  Driesch,  but  in  rare  cases 
it  may  segment  like  an  enth'e  ovum  of  half-size  (Fig.  1.83,  />>)and  give 
rise  to  an  entire  blastula.  We  may  interpret  this  to  mean  that  in 
Aniphioxus  the  differentiation  of  the  cytoplasmic  substance  is  at  first 
very  shght,  or  readily  alterable,  so  that  the  isolated  blastomere,  as  a 
rule,  reverts  at  once  to  the  condition  of  the  entire  ovum.  In  the  sea- 
urchin,  the  initial  differentiations  are  more  extensive  or  more  firmlv 
established,  so  that  only  exceptionally  can  they  be  altered.  In  the 
snail  and  ctenophore  we  have  the  opposite  extreme  to  Avipliioxiis,  the 
cytoplasmic  conditions  having  been  so  firmly  established  that  they  can- 
not be  readjusted,  and  the  development  must,  from  the  outset,  proceed 
within  the  limits  thus  set  up. 

Through  this  conclusion  we  reconcile,  as  I  believe,  the  theories  of 
cytoplasmic  localization  and  mosaic  development  with  the  hypothesis 
of  cytoplasmic  totipotence.  Primarily  the  egg-cytoplasm  is  totipotent 
in  the  sense  that  its  various  regions  stand  in  no  fixed  relation  with  the 
parts  to  which  they  respectively  give  rise,  and  the  substance  of  each 
of  the  blastomeres  into  which  it  splits  up  contains  all  of  the  materials 
necessary  to  the  formation  of  a  complete  body.  Secondaril)-,  how- 
ever, development  may  assume  more  or  less  of  a  mosaic-like  character 
through  differentiations  of  the  cytoplasmic  substance  involving  local 
chemical  and  physical  changes,  deposits  of  metaplasmic  material, 
and  doubtless  many  other  unknown  subtler  processes.  Both  the  ex- 
tent and  the  rate  of  such  differentiations  seem  to  vary  in  different 
cases ;  and  here  probably  lies  the  explanation  of  the  fact  that  the 
isolated  blastomeres  of  different  eggs  vary  so  widely  in  their  mode 
of  development.  When  the  initial  differentiation  is  of  small  extent 
or  is  of  such  a  kind  as  to  be  readily  modified,  cleavage  is  imictcrmi- 
nate  in  character  and  may  easily  be  remodelled  (as  in  AmpJiioxus). 
When  they  are  more  extensive  or  more  rigid,  cleavage  assumes  a 
mosaic-like  or  determinate  character,^  and  qualitative  division,  in  a 
certain  sense,  becomes  a  fact.  Conklin's  ('99)  interesting  observa- 
tions on  the  highly  determinate  cleavage  of  gasteropods  {Crepitiit/a) 

1  The  convenient  terms  iuJeterminatc  and  determinate  cleavage  were  suggested  by 
Conklin  ('98). 


424 


INHERITANCE  AND  DEVELOPMENT 


show  that  here  the  substance  of  the  attraction-spheres  is  unequally 
distributed,  in  a  quite  definite  way,  among  the  cleavage-cells,  each 
sphere  of  a  daughter-cell  being  carried  over  bodily  into  one  of  the 
granddaughter-cells  (Fig.  192).  We  have  here  a  substantial  basis  for 
the  conclusion  that  in  cleavage  of  this  type  qualitative  division  of  the 
cytoplasm  may  occur. 

It  is  important  not  to  lose  sight  of  the  fact  that  development  and 
differentiation  do  not  in  any  proper  sense  first  begin  with  the  cleavage 
of  the  ovum,  but  long  before  this,  during  its  ovarian  history. ^  The 
primary  differentiations  thus  established  in  the  cytoplasm  form  the 
immediate  conditions  to  which  the  later  development  must  conform ; 
and  the   difference  between  Aiuphioxiis  on   the   one   hand,  and  the 


Fig.  192.  —  Two  successive  stages  in  the  third  cleavage  of  the  egg  of  Crepidula,  seen  from  the 
upper  pole.     [CONKLIN.] 

In  both  figures  the  old  spheres  (dotted)  lie  at  the  upper  pole  of  the  embryo,  and  at  the  third 
cleavage  they  pass  into  the  four  respective  cells  of  the  first  quartet  of  micromeres.  The  centro- 
somes  are  seen  in  the  new^  spheres. 


snail  or  ctenophore  on  the  other,  simply  means,  I  think,  that  the 
initial  differentiation  is  less  extensive  or  less  firmly  established  in 
the  one  than  in  the  other. 

The  origin  of  the  cytoplasmic  differentiations  existing  at  the  be- 
ginning of  cleavage  has  already  been  considered  (p.  386).  If  the 
conclusions  there  reached  be  placed  beside  the  above,  we  reach  the 
following  conception.  The  primary  determining  cause  of  develop- 
ment lies  in  the  nucleus,  which  operates  by  setting  up  a  continuous 
series  of  specific  metabolic  changes  in  the  cytoplasm.  This  process 
begins  during  ovarian  growth,  establishing  the  external  form  of  the 
^^g,  its  primary  polarity,  and  the  distribution  of  substances  within  it. 
The  cytoplasmic  differentiations  thus  set  up  form  as  it  were  a  frame- 

1  See  Wilson  ('96),  Driesch  ('98,  i). 


THE   NUCLEUS  IN  LATER  DEVELOPMENT  425 

work  within  which  the  subsequent  operations  take  place  in  a  course 
which  is  more  or  less  firmly  fixed  in  different  cases.  If  the  cyto- 
plasmic conditions  be  artificially  altered  by  isolation  or  other  dis- 
turbance of  the  blastomeres,  a  readjustment  may  take  place  and 
development  may  be  correspondingly  altered.  Whether  such  a  read- 
justment is  possible  depends  on  secondary  factors  —  the  extent  of 
the  primary  differentiations,  the  physical  consistency  of  the  e^^g- 
substance,  the  susceptibility  of  the  protoplasm  to  injury,  and  doubtless 
a  multitude  of  others.  The  same  doubtless  applies  to  the  later  stages 
of  development ;  and  we  must  here  seek  for  some  of  the  factors  by 
which  the  power  of  regeneration  in  the  adult  is  determined  and  lim- 
ited. It  is,  however,  not  improbable,  as  pointed  out  below,  that  in  the 
later  stages  differentiation  may  occur  in  the  nuclear  as  well  as  in  the 
cytoplasmic  substance. 

G.     The  Nucleus  in  Later  Development 

The  foregoing  conception,  as  far  as  it  goes,  gives  at  least  an  in- 
telligible view  of  the  more  general  features  of  early  development  and 
in  a  measure  harmonizes  the  apparently  conflicting  results  of  experi- 
ment on  various  forms.  But  there  are  a  very  large  number  of  facts 
relating  especially  to  the  later  stages  of  differentiation,  which  it 
seems  to  leave  unexplained,  and  which  indicate  that  the  nucleus  as 
well  as  the  cytoplasm  may  undergo  progressive  changes  of  its  sub- 
stance. It  has  been  assumed  by  most  critics  of  the  Roux-Weismann 
theory  that  all  of  the  nuclei  of  the  body  contain  the  same  idioplasm, 
and  that  each  therefore,  in  Hertwig's  words,  contains  the  germ  of  the 
whole.  It  is,  however,  doubtful  whether  this  assumption  is  well 
founded.  The  power  of  a  single  cell  to  produce  the  entire  body  is  in 
general  limited  to  the  earliest  stages  of  cleavage,  rapidly  diminishes, 
and  as  a  rule  soon  disappears  entirely.  When  once  the  germ-layers 
have  been  definitely  separated,  they  lose  entirely  the  power  to  regener- 
ate one  another  save  in  a  few  exceptional  cases.  In  asexual  repro- 
duction, in  the  regeneration  of  lost  parts,  in  the  formation  of  morbid 
growths,  each  tissue  is  in  general  able  to  reproduce  only  a  tissue  of  its 
own  or  a  nearly  related  kind.  Transplanted  or  transposed  groups  of 
cells  (grafts  and  the  like)  retain  more  or  less  completely  their  autonomy 
and  vary  only  within  certain  well-defined  limits,  despite  their  change 
of  environment.  All  of  these  statements  are,  it  is  true,  subject  to 
exception  ;  yet  the  facts  afford  an  overwhelming  demonstration  that 
differentiated  cells  possess  a  specific  character,  that  their  power  of 
development  and  adaptability  to  changed  conditions  becomes  in  a 
greater  or  less  degree  limited  with  the  progress  of  development. 
As  indicated  above,  this  progressive  specification  of  the  tissue-cells 


426  INHERITANCE  AND   DEVELOPMENT 

is  no  doubt  due  in  part  to  differentiation  of  the  cytoplasm.  There  is, 
however,  reason  to  suspect  that,  beyond  this,  differentiation  may  sootier 
or  later  involve  a  specification  of  the  nuclear  substance.  When  we 
reflect  on  the  general  role  of  the  nucleus  in  metabolism  and  its  signifi- 
cance as  the  especial  seat  of  the  formative  power,  we  may  well  hesi- 
tate to  deny  that  this  part  of  Roux's  conception  may  be  better  founded 
than  his  critics  have  admitted.  Nageli  insisted  that  the  idioplasm 
must  undergo  a  progressive  transformation  during  development,  and 
many  subsequent  writers,  including  such  acute  thinkers  as  Boveri  and 
Nussbaum,  and  many  pathologists,  have  recognized  the  necessity  for 
such  an  assumption.  Boveri's  remarkable  observations  on  the  nuclei 
of  the  primordial  germ-cells  in  Ascaris  demonstrate  the  truth  of  this 
view  in  a  particular  case ;  for  here  all  of  the  somatic  nuclei  lose  a  portion 
of  their  chromatin,  and  only  the  progenitors  of  the  germ-neclei  retain  the 
entire  ancestral  heritage.  Boveri  himself  has  in  a  measure  pointed  out 
the  significance  of  his  discovery,  insisting  that  the  specific  develop- 
ment of  the  tissue-cells  is  conditioned  by  specific  changes  in  the 
chromatin  that  they  receive,^  though  he  is  careful  not  to  commit  him- 
self to  any  definite  theory.  It  hardly  seems  possible  to  doubt  that  in 
Ascaris  the  limitation  of  the  somatic  cells  in  respect  to  the  power  of 
development  arises  through  a  loss  of  particular  portions  of  the 
chromatin.  One  cannot  avoid  the  thought  that  further  and  more 
specific  limitations  in  the  various  forms  of  somatic  cells  may  arise 
through  an  analogous  process,  and  that  we  have  here  a  key  to  the 
origin  of  nuclear  specification  zvithout  recourse  to  the  theory  of  qualita- 
tive division.  We  do  not  need  to  assume  that  the  unused  chromatin 
is  cast  out  bodily  ;  for  it  m.ay  degenerate  and  dissolve,  or  may  be 
transformed  into  linin-substance  or  into  nucleoli. 

This  suggestion  is  made  only  as  a  tentative  hypothesis,  but  the 
phenomena  of  mitosis  seem  well  worthy  of  consideration  from  this 
point  of  view.  Its  appUcation  to  the  facts  of  development  becomes 
clearer  when  we  consider  the  nature  of  the  nuclear  ''control"  of  the 
cell,  i.e.  the  action  of  the  nucleus  upon  the  cytoplasm.  Strasburger, 
following  in  a  measure  the  lines  laid  down  by  Nageli,  regards  the 
action  as  essentially  dynamic,  i.e.  as  a  propagation  of  molecular 
movements  from  nucleus  to  cytoplasm  in  a  manner  which  might  be 
compared  to  the  transmission  of  a  nervous  impulse.  When,  however, 
we  consider  the  role  of  the  nucleus  in  synthetic  metaboHsm,  and  the 
relation  between  this  process  and  that  of  morphological  synthesis, 
we  must  regard  the  question  in  another  light ;  and  opinion  has  of 
late  strongly  tended  to  the  conclusion  that  nuclear  "control"  can 
only  be  explained  as  the  result  of  active  exchanges  of  material 
between  nucleus   and   cytoplasm.     De   Vries,  followed   by  Hertwig, 


1 ' 


9^,  P-  433- 


THE  NUCLEUS  IN  LATER  DEVELOPMENT 


427 


assumes  a  migration  of  pangens  from  nucleus  to  cytoplasm,  the 
character  of  the  cell  being  determined  by  the  nature  of  the  mi^a'at- 
ing  pangens,  and  these  being,  as  it  were,  selected  by  circumstances 
(position  of  the  cell,  etc.).  But,  as  already  pointed  out,  the  pangen- 
hypothesis  should  be  held  quite  distinct  from  the  purely  physiologi- 
cal aspect  of  the  question,  and  may  be  temporarily  set  aside  ;  for 
specific  nuclear  substances  may  pass  from  the  nucleus  into  the 
cytoplasm  in  an  unorganized  form.  Sachs,  followed  by  Loeb,  has 
advanced  the  hypothesis  that  the  development  of  particular  organs 
is  determined  by  specific  "  formative  substances "  which  incite  cor- 
responding forms  of  metabolic  activity,  growth,  and  differentiation. 
It  is  but  a  step  from  this  to  the  very  interesting  suggestion  of 
Driesch  that  the  nucleus  is  a  storehouse  of  ferments  which  pass 
out  into  the  cytoplasm  and  there  set  up  specific  activities.  Under 
the  influence  of  these  ferments  the  cytoplasmic  organization  is  deter- 
mined at  every  step  of  the  development,  and  new  conditions  are 
established  for  the  ensuing  change.  This  view  is  put  forward  only 
tentatively  as  a  "fiction"  or  working  hypothesis;  but  it  is  certainly 
full  of  suggestion.  Could  we  establish  the  fact  that  the  number  of 
ferments  or  formative  substances  in  the  nucleus  diminishes  with  the 
progress  of  differentiation,  we  should  have  a  comparatively  simple 
and  intelligible  explanation  of  the  specification  of  nuclei  and  the 
limitation  of  development.  The  power  of  regeneration  might  then 
be  conceived,  somewhat  as  in  the  Roux-Weismann  theorv,  as  due  to 
a  retention  of  idioplasm  or  germ-plasm  —  i.e.  chromatin  —  in  a  less 
highly  modified  condition,  and  the  differences  between  the  various 
tissues  in  this  regard,  or  between  related  organisms,  would  find  a 
natural  explanation. 

Development  may  thus  be  conceived  as  a  progressive  transforma- 
tion of  the  egg-substance  primarily  incited  by  the  nucleus,  first  mani- 
festing itself  by  specific  changes  in  the  cytoplasm,  but  sooner  or  later 
involving  in  some  measure  the  nuclear  substance  itself.  This  process, 
which  one  is  tempted  to  compare  to  a  complicated  and  progressive 
form  of  crystallization,  begins  with  the  youngest  ovarian  Q,^g  and  pro- 
ceeds continuously  until  the  cycle  of  individual  life  has  run  its  course. 
Cell-division  is  an  accompaniment  but  not  a  direct  cause  of  differen- 
tiation. The  cell  is  no  more  than  a  particular  area  of  the  germinal 
substance  comprising  a  certain  quantity  of  cytoplasm  and  a  mass  of 
idioplasm  in  its  nucleus.  Its  character  is  primarily  a  manifestation 
of  the  general  formative  energy  acting  at  a  particular  point  under 
given  conditions.  When  once  such  a  circumscribed  area  has  been 
established,  it  may,  however,  emancipate  itself  in  a  greater  or  less 
degree  from  the  remainder  of  the  mass,  and  acquire  a  specific  char- 
acter so  fixed  as  to  be  incapable  of  further  change  save  within  the 
limits  imposed  by  its  acquired  character. 


428 


INHERITANCE  AND   DEVELOPMENT 


H.     The  External  Conditions  of  Development 

We  have  thus  far  considered  only  the  internal  conditions  of  devel- 
opment which  are  progressively  created  by  the  germ-cell  itself.  We 
must  now  briefly  glance  at  the  external  conditions  afforded  by  the 
environment  of  the  embryo.  That  development  is  conditioned  by 
the   external   environment   is   obvious.      But   we   have   only   recently 

come  to  realize  how  intimate  the  rela- 
tion is;  and  it  has  been  especially  the 
service  of  Loeb,  Herbst,  and  Driesch  to 
show  how  essential  a  part  is  played  by 
the  environment  in  the  development  of 
specific  organic  forms.  The  hmits  of 
this  work  will  not  admit  of  any  adequate 
review  of  the  vast  array  of  known  facts 
in  this  field,  for  which  the  reader  is  re- 
ferred to  the  works  especially  of  Herbst. 
I  shall  only  consider  one  or  two  cases 
which  may  serve  to  bring  out  the  general 
principle  that  they  involve.  Every  liv- 
ing organism  at  every  stage  oFlt?  exist^ 
ence  reacts  to  its  environment  by  physio- 
logical and  morphological  changes.  The 
developing  embryo,  like  the  adult,  is  a 
moving  equilibrium  —  a  product  of  the 
response  of  the  inherited  organization  to 
the  external  stimuli  working  upon  it.  If 
these  stimuli  be  altered,  development  is 
aftered..  This  is  beautifully  shown  by  the 
experiments  of  Herbst  and  others  on  the 
larvae  of  sea-urchins.    [Herbst.]  development    of    sea-urchins.      Pouchet 

A.  Normal  Piuteus  {strongyiocen-  ^"^  Chabry  showcd  that  if  the  cmbryos 
trotiis).    B.  Larva  {sphcerechinus)  at  of  thcsc  auimals  bc  made  to,develop  in 

the  same  stage  as  the  foresroinsr,  devel-    ^^„  ,,r^4-^^     ^       <-    •     *  t  li.       ^i 

^^  .  •        ^    ,  .  •  •  r  1.   sea-water   contammg;  no   hme-salts,  the 

oped   m   sea-water   contammg  a  slight  ^  &        ^  ^  oi.i±».o,    i.±iv. 

excess  of  potassium  chloride.  larva  fails  to  dcvclop  not  ouly  its  calca- 

reous  skeleton,  but  also  its  ciHated  arms, 
and  a  larva  thus  results  that  resembles  in  some  particulars  an  entirely 
different  specific  form ;  namely,  the  Toriiai'ia  larva  of  Balanoglossus. 
This  result  is  not  due  simply  to  the  lack  of  necessary  material ;  for 
Herbst  showed  that  the  same  result  is  attained  if  a  shght  excess  of 
potassium  chloride  be  added  to  sea-water  containing  the  normal 
amount  of  Hme  (Fig.  193).  In  the  latter  case  the  specific  metabolism 
of  the  protoplasm  is  altered  by  a  particular  chemical  stimulus,  and  a 
new  form  results. 


Fig.    193.  —  Normal    and    modified 


THE   EXTERNAL    CONDITIONS   OE  DEVELOPMENT  429 

The  changes  thus  caused  by  shght  chemical  alterations  in  the 
water  may  be  still  more  profound.  lierbst  (92)  observed,  for 
example,  that  when  the  water  contains  a  very  small  percentage  of 
lithium  chloride,  the  blastula  of  sea-urchins  fails  to  invaginate  to 
form  a  typical  gastrula,  but  evaginatcs  to  form  an  hour-glass-shai)ed 


Fig.  194.  —  Regeneration  in  ccelenterates  {A,  /?.  from  f.OF.R ;    C,  D,  from  BlCKFORP). 

A.  Polyp  (Or/a»/'//«j),  producing  new  tentacles  from  the  aboral  side  of  a  lateral  wound. 
B.  Hydroid  ( 7>//7«/a^-/'c?),  generating  a  head  at  each  end  of  a  fragment  of  the  stem  susr  •  '  1  in 
water.     C.  D.  Similar  generation  of  heads  at  both  ends, of  short  pieces  of  the  stem,  in  E 

larva,  one  half  of  which  represents  the  archenteron.  the  other  halt 
the  ectoblast.  Moreover,  a  much  larger  number  of  the  blastula-cells 
undergo  the  differentiation  into  entoblast  than  in  the  nc^-mal  de- 
velopment, the  ectoblast  sometimes  becoming  greatl\-  reduced  and 
occasionally  disappearing  altogether,  so   that   the  entire    blastula  is 


430  INHERITANCE  AND  DEVEIOPMENT 

differentiated  into  cells  having  the  histological  character  of  the  nor- 
mal entoblast !  One  of  the  most  fundamental  of  embryonic  differen- 
tiations is  thus  shown  to  be  intimately  conditioned  by  the  chemical 
environment. 

The  observations  of  botanists  on  the  production  of  roots  and  other 
structures  as  the  result  of  local  stimuli  are  famiUar  to  all.  Loeb's 
interesting  experiments  on  hydroids  give  a  similar  result  ('91).  It 
has  long  been  known  that  Tubularia,  like  many  other  hydroids,  has 
the  power  to  regenerate  its  ''  head  "  —  i.e.  hypostome,  mouth,  and  ten- 
tacles—  after  decapitation.  Loeb  proved  that  in  this  case  the  power 
to  form  a  new  head  is  conditioned  by  the  environment.  For  if  a 
Tiibiilaria  stem  be  cut  off  at  both  ends  and  inserted  in  the  sand 
upside  down,  i.e.  with  the  oral  end  buried,  a  new  head  is  regen- 
erated at  the  free  (formerly  aboral)  end.  Moreover,  if  such  a  piece 
be  suspended  in  the  water  by  its  middle  point,  a  new  head  is  produced 
at  each  e7id  (Fig.  194);  while  if  both  ends  be  buried  in  the  sand, 
neither  end  regenerates.  This  proves  in  the  clearest  manner  that 
in  this  case  the  power  to  form  a  definite  compUcated  structure  is 
called  forth  by  the  stimulus  of  the  external  environment. 
"  These  cases  must  suffice  for  our  purpose.  They  prove  incontesta- 
bly  that  normal  development  is  in  a  greater  or  less  degree  the  response 
of  tJie  developing  organism  to  normal  conditions  ;  and  they  show  that 
we  cannot  hope  to  solve  the  problems  of  development  without  reckon- 
ing with  these  conditions.  But  neither  can  we  regard  specific  forms 
of  development  as  directly  caused  by  the  external  conditions  ;  for  the 
^gg  of  a  fish  and  that  of  a  polyp  develop,  side  by  side,  in  the  same 
drop  of  water,  under  identical  conditions,  each  into  its  predestined 
form.  Every  step  of  development  is  a  physiological  reaction,  involv- 
ing a  long  and  complex  chain  of  cause  and  effect  between  the  stimu- 
lus and  the  response.  The  character  of  the  response  is  determined, 
not  by  the  stimulus,  but  by  the  inJierited  orga7iization.  While,  there- 
fore, the  study  of  the  external  conditions  is  essential  to  the  analysis 
of  embryological  phenomena,  it  serves  only  to  reveal  the  mode  of 
action  of  the  germ  and  gives  but  a  dim  insight  into  its  ultimate 
nature. 

L     Development,  Inheritance,  and  Metabolism 

In  bringing  the  foregoing  discussion  into  more  direct  relation  with 
the  general  theory  of  cell-action,  we  may  recall  that  the  cell-nucleus 
appears  to  us  in  two  apparently  different  roles.  On  the  one  hand,  it 
is  a  primary  factor  in  morphological  synthesis  and  hence  in  inheri- 
tance, on  the  other  hand  an  organ  of  metabolism  especially  concerned 
with   the   constructive  process.     These  two  functions  we  may   with 


DEVELOPMENT,   INHERITANCE,   AND  METABOLISM  431 

Claude  Bernard  regard  as  but  different  phases  of  one  process.  The 
building  of  a  definite  cell-product,  such  as  a  muscle-fibre,  a  nerve- 
process,  a  cilium,  a  pigment-granule,  a  zymogen-granulc,  is  in  the  last 
analysis  the  result  of  a  specific  form  of  metabolic  activity,  as  we  may 
conclude  from  the  fact  that  such  products  have  not  only  a  definite 
physical  and  morphological  character,  but  also  a  definite  chemical 
character.  In  its  physiological  aspect,  therefore,  inheritance  is  the 
recurrence,  in  successive  generations,  of  like  forms  of  metabolism  ; 
and  this  is  effected  through  the  transmission  from  generation  to  gen- 
eration of  a  specific  substance  or  idioplasm  which  we  have  seen 
reason  to  identify  with  chromatin.  The  validity  of  this  conception 
is  not  affected  by  the  form  in  which  we  conceive  the  morphological 
nature  of  the  idioplasm  —  whether  as  simply  a  mixture  of  chemical 
substances,  as  a  microcosm  of  invisible  germs  or  pangens,  as  assumed 
by  De  Vries,  Weismann,  and  Hertwig,  as  a  storehouse  of  specific  fer- 
ments as  Driesch  suggests,  or  as  a  complex  molecular  substance  grouped 
in  micellae  as  in  NageU's  hypothesis.  It  is  true,  as  Verworn  insists, 
that  the  cytoplasm  is  essential  to  inheritance ;  for  without  a  specifi- 
cally organized  cytoplasm  the  nucleus  is  unable  to  set  up  specific 
forms  of  synthesis.  This  objection,  which  has  already  been  con- 
sidered from  different  points  of  view,  by  both  De  Vries  and  Driesch, 
disappears  as  soon  as  we  regard  the  egg-cytoplasm  as  itself  a  product 
of  the  7mclcar  activity  ;  and  it  is  just  here  that  the  general  role  of  the 
nucleus  in  metabolism  is  of  such  vital  importance  to  the  theory  of 
inheritance.  If  the  nucleus  be  the  formative  centre  of  the  cell,  if 
nutritive  substances  be  elaborated  by  or  under  the  influence  of  the 
nucleus  while  they  are  built  into  the  living  fabric,  then  the  specific 
character  of  the  cytoplasm  is  determined  by  that  of  the  nucleus, 
and  the  contradiction  vanishes.  In  accepting  this  view  we  admit 
that  the  cytoplasm  of  the  egg  is,  in  a  measure,  the  substratum  of 
inheritance,  but  it  is  so  only  by  virtue  of  its  relation  to  the  nucleus, 
which  is,  so  to  speak,  the  ultimate  court  of  appeal.  The  nucleus 
cannot  operate  without  a  cytoplasmic  field  in  which  its  peculiar 
powers  may  come  into  play;  but  this  field  is  created  and  moulded 
by  itself. 

J.     Preformation   and    Epigenesis.      The   Unknown    Factor   in 

Development 

We  have  now  arrived  at  the  farthest  outposts  of  cell-research,  and 
here  we  find  ourselves  confronted  with  the  same  unsolved  problems 
before  which  the  investigators  of  evolution  have  made  a  halt.  For 
we  must  now  inquire  what  is  the  guiding  principle  of  embryological 
development  that  correlates  its  complex  phenomena  and  directs  them 


432  INHERITANCE  AND  DEVEIOPMENT 

to  a  definite  end.  However  we  conceive  the  special  mechanism  of 
development,  we  cannot  escape  the  conclusion  that  the  power  behind 
it  is  involved  in  the  structure  of  the  germ-plasm  inherited  from  fore- 
going generations.  What  is  the  nature  of  this  structure  and  how 
has  it  been  acquired .''  To  the  first  of  these  questions  we  have  as 
yet  no  certain  answer.  The  second  question  is  merely  the  general 
problem  of  evolution  stated  from  the  standpoint  of  the  cell-theory. 
The  first  question  raises  once  more  the  old  puzzle  of  preformation 
or  epigenesis.  The  pangen-hypothesis  of  De  Vries  and  Weismann 
recognizes  the  fact  that  development  is  epigenetic  in  its  external 
features  ;  but  like  Darwin's  hypothesis  of  pangenesis,  it  is  at  bottom 
a  theory  of  preformation,  and  Weismann  expresses  the  conviction 
that  an  epigenetic  development  is  an  impossibility.^  He  thus  ex- 
plicitly adopts  the  view,  long  since  suggested  by  Huxley,  that  "the 
process  which  in  its  superficial  aspect  is  epigenesis  appears  in  es- 
sence to  be  evolution  in  the  modified  sense  adopted  in  Bonnet's  later 
writings ;  and  development  is  merely  the  expansion  of  a  potential 
organism  or  'original  preformation'  according  to  fixed  laws."^  Hert- 
wig  ('92,  2),  while  accepting  the  pangen-hypothesis,  endeavours  to 
take  a  middle  ground  between  preformation  and  epigenesis,  by 
assuming  that  the  pangens  (idioblasts)  represent  only  ccll-cJiaracters, 
the  traits  of  the  multicellular  body  arising  epigenetically  by  permu- 
tations and  combinations  of  these  characters.  This  conception  cer- 
tainly tends  to  simpHfy  our  ideas  of  development  in  its  outward 
features,  but  it  does  not  explain  why  cells  of  different  characters 
should  be  combined  in  a  definite  manner,  and  hence  does  not  reach 
the  ultimate  problem  of   inheritance. 

What  hes  beyond  our  reach  at  present,  as  Driesch  has  very  ably 
urged,  is  to  explain  the  orderly  rhythm  of  development  —  the  co- 
ordinating power  that  guides  development  to  its  predestined  end. 
We  are  logically  compelled  to  refer  this  power  to  the  inherent 
organization  of  the  germ,  but  we  neither  know  nor  can  we  even 
conceive  what  that  organization  is.  The  theory  of  Roux  and  Weis- 
mann demands  for  the  orderly  distribution  of  the  elements  of  the 
germ-plasm  a  prearranged  system  of  forces  of  absolutely  incon- 
ceivable complexity.  Hertwig's  and  De  Yries's  theory,  though  ap- 
parently simpler,  makes  no '  less  a  demand;  for  how  are  we  to 
conceive  the  power  which  guides  the  countless  hosts  of  migrating 
pangens  throughout  all  the  long  and  complex  events  of  development.^ 
The  same  difficulty  confronts  us  under  anv  theory  we  can  frame.  If 
with  Herbert  Spencer  we  assume  the  germ-plasm  to  be  an  aggrega- 
tion of  like  units,  molecular  or  supra-molecular,  endowed  with  prede- 
termined  polarities  which  lead   to  their  grouping   in   specific  forms, 

^  Germ-plasm,  p.  14.  2  Evolution,  Science,  and  Culture,  p.  296. 


PREFORMATION  AND   EPIGENESIS  433 

we  but  throw  the  problem  one  stage  farther  back,  and,  as  Weismann 
himself  has  pomted  out,i  substitute  for  one  difficulty  another  of 
exactly  the  same  kind. 

The  truth  is  that  an  explanation  of  development  is  at  present 
beyond  our  reach.  The  controversy  between  preformation  and 
epigenesis  has  now  arrived  at  a  stage  where  it  has  little  meanin£( 
apart  from  the  general  problem  of  physical  causality.  What  we 
know  is  that  a  specific  kind  of  living  substance,  derived  from  the 
parent,  tends  to  run  through  a  specific  cycle  of  changes  during  which 
It  transforms  itself  into  a  body  like  that  of  which  it  formed  \  part  • 
and  we  are  able  to  study  with  greater  or  less  precision  the  mechanism' 
by  which  that  transformation  is  effected  and  the  conditions  under 
which  it  takes  place.  But  despite  all  our  theories  we  no  more  know 
how  the  organization  of  the  germ-cell  involves  the  properties  of  the 
adult  body  than  we  know  how  the  properties  of  hydrogen  and  oxygen 
involve  those  of  water.  So  long  as  the  chemist  and  physicist  ""are 
unable  to  solve  so  simple  a  problem  of  physical  causality  as  this, 
the  embryologist  may  well  be  content  to  reserve  his  judgment  on  a 
problem  a  hundred-fold  more  complex. 

The  second  question,  regarding  the  historical  origin  of  the  idio- 
plasm, brings  us  to  the  side  of  the  evolutionists.  The  idioplasm  of 
every  species  has  been  derived,  as  we  must  believe,  by  the  modifica- 
tion of  a  preexisting  idioplasm  through  variation,  and  the  survival 
of  the  fittest.  Whether  these  variations  first  arise  in  the  idioplasm 
of  the  germ-cells,  as  Weismann  maintains,  or  whether  they  may  arise 
in  the  body-cells  and  then  be  reflected  back  upon  the  idioplasm,  is 
a  question  to  which  the  study  of  the  cell  has  thus  far  given 
no  certain  answer.  Whatever  position  we  take  on  this  question,  the 
same  difficulty  is  encountered;  namely,  the  origin  of  that  coordi- 
nated fitness,  that  power  of  active  adjustment  between  internal  and 
external  relations,  which,  as  so  many  eminent  biological  thinkers 
have  insisted,  overshadows  every  manifestation  of  life.  The  nature 
and  origin  of  this  power  is  the  fundamental  problem  of  biology. 
When,  after  removing  the  lens  of  the  eye  in  the  larval  salamander, 
we  see  it  restored  in  perfect  and  typical  form  by  regeneration  from 
the  posterior  layer  of  the  iris,^  we  behold  an  adaptive  response  to 
changed  conditions  of  which  the  organism  can  have  had  no  antece- 
dent experience  either  ontogenetic  or  phylogenetic,  and  one  of  so 
marvellous  a  character  that  we  are  made  to  realize,  as  by  a  flash  of 
light,  how  far  we  still  are  from  a  solution  of  this  problem.  It  may 
be  true,  as  Schwann  himself  urged,  that  the  adaptive  power  of 
living  beings  differs  in  degree  only,  not  in  kind,  from  that  of  unor- 

1  Gerjuinal  Selection,  January,  1896,  p.  284. 

2  See  Wolff,  '95,  and  Muller,''96. 

2F 


434 


INHERITANCE  AND   DEVEIOPMENT 


ganized  bodies.     It  is  true  that  we  may  trace  in  organic  nature  long 
and  finely  graduated  series  leading  upward   from  the   lower  to  the 
higher  forms,  and  we  must  believe  that  the  wonderful  adaptive  mani- 
festations of  the  more  complex  forms  have  been  derived  from  simpler 
conditions  through  the  progressive  operation  of  natural  causes.     But 
when    all   these    admissions    are    made,    and   when    the    conserving 
action  of  natural  selection  is  in  the  fullest  degree  recognized,  we  can- 
not close  our  eyes  to  two  facts  :    first,  that  we  are  utterly  ignorant  of 
the  manner  in  which  the  idioplasm  of  the  germ-cell  can  so  respond 
to    the    influence    of   the    environment    as  to  call  forth  an  adaptive 
variation  ;    and  second,  that  the  study  of  the  cell  has  on  the  whole 
seemed  to  widen  rather  than  to  narrow  the  enormous  gap  that  sepa- 
rates even  the  lowest  forms  of  Hfe  from,  the  inorganic  world. 
r-      \  am  well  aware  that  to  many  such  a  conclusion  may  appear  reac- 
!   tionary  or  even  to  involve  a  renunciation  of  what  has  been  regarded 
as  the  ultimate  aim  of  biology.      In  reply  to  such  a  criticism  I  can 
■    only  express  my  conviction  that  the  magnitude  of  the   problem  of 
development,  whether  ontogenetic  or  phylogenetic,  has  been  under- 
estimated ;    and  that  the  progress  of  science  is  retarded  rather  than 
advanced  by  a  premature  attack  upon  its  ultimate  problems.      Yet 
the  splendid  achievements  of  cell-research  in  the  past  twenty  years 
stand  as  the  promise  of  its  possibilities  for  the  future,  and  we  need 
set  no  Umit  to  its  advance.     To  Schleiden  and  Schwann  the  present 
standpoint  of  the  cell-theory  might  well  have  seemed  unattainable. 
We  cannot  foretell  its  future  triumphs,  nor  can  we  doubt  that   the 
way  has  already  been  opened  to  better  understanding  of  inheritance 
\  and  development. 


LITERATURE.     IX 

Barfurth,  D.  —  Regeneration  und  Involution:    Merkel  u.  Bonnet^  Ergeb.,  I.-VIIL 

1891-99. 
Boveri,   Th.  — Ein  geschlechtlich  erzeugter  Organismus   ohne   miitterliche  Eigen- 

schaften:  Sitz.-Ber.  d.  Ges.f.  Morph.  und  Phys.  in  Milnchen,  V.     1889.     See 

also  Arch.  /.  Entw.     1 895 . 
Brooks,  W.  K.  —  The  Law  of  Heredity.     Baltimore,  1883. 
Id.  —  The  Foundations  of  Zoology.    N^eiu  York,  1899. 

Davenport,  C.  B.  —  Experimental  Morphology  :   L,  11.     New  York,  1897,  1899. 
Driesch,  H.  —  Analytische  Theorie  der  organischen  Entwicklung.     Leipzig,  1894. 
Id.— Die  Localisation  morphogenetischer  Vorgange  :  Arch.  Entw., WW.  i.     1899. 
Id.  —  Resultate  und  Probleme  der  Entwickelungs-physiologie  der  Tiere  :  Merkel  u. 

Bonnet,  Ergeb.,  VIII.,  1898.     (Full  literature.) 
Herbst,  C— iJber  die   Bedeutung  der  Reizphysiologie  fiir  die  kausale  Auffassung 

von   Vorgangen   in   der   tierischen    Ontogenese :    Biol.    Centralb.,  XIV.,  XV. 

1894-95. 
Eertwig,  0.  — Altere  und  neuere  Entwicklungs-theorien.     Berlin.  1892. 


LITERATURE 


435 


Hertwig,  0.  — Urmund  unci  Spina  Bifida:  Arch.  niik.  Anat.,  XXXIX.      1892. 
Id.  — tJber  den  Werth  der  Ersten  Furchungszellen  fiir  die  Organbildung  des  Em- 
bryo: Arch.  mik.  Anat.yXhW.     1893. 
Id.  — Zeit  und  Streitfragen  der  Biologic.     I.  Berlin,  1894.     II.  Jena,  1897. 
Id.  —  Die  Zelle  und  die  Gewebe,  II.    Jena,  1898. 

His,  W.  — Unsere  Korperform  und  das  physiologische  Problem  ihrer  Entstehung. 
Leipzig,  1874. 

Loeb,  J.  —  Untersuchungen  zur  physiologischen  Morphologie  :    I.  Heteromorphosis. 

Wurzburg,  1891.     II.  Organbildung  und  Wachsthum.      Wiirsburg.  1892. 
Id.  — Some  Facts  and  Principles  of  Physiological  Morphology:    Wood's  El  oil  BioL 

Lectures.     1893. 
Morgan,  T.  H.  —  Experimental  Studies  of  the  Regeneration  of  Phanaria  iMaculata : 

Arch.  Entiv.,  VII.  2.  3.     1898. 
Id. — The  Development  of  the  Frog's  Egg.     New  York.,  1897. 
Nageli,  C.  —  Mechanisch-ph3-siologische    Theorie   der   Abstammungslehre.     AEiin- 

chen  u.  Leipzig,  1884. 
Roux,  W.  —  tJber  die  Bedeutung  der  Kernteilungsfiguren.     Leipzig,  1883. 
Id.  — Uber  das  klinstliche  Hervorbringen  halber  Embryonen  durch  Zerstorung  einer 

der  beiden  ersten  Furchungskugeln,  etc. :    VirchoTifs  Archiv,  114.     1888. 
Id. — Fiir  unsere  Programme  und  seine  Verwirklichung  :  Arch.  Entiu.,  V.  2.      1897. 

(See  also  Gesammelte  Abhandlungen  liber  Entwicklungsmechanik  der  Organ- 

ismen,  1895.) 
Sachs,  J.  —  Stofif  und  Form  der  Pflanzenorgane  :   Ges.  Abhandlitngen,  II.     1893. 
Weismann,  A.  —  Essays  upon  Heredity.  First  Series.     OxJ'onl,  1891. 
Id.  —  Essays  upon  Heredity.  Second  Series.     Oxford,  1892. 
Id.  —  Aussere  Einfliisse  als  Entwicklungsreize.    Jena,  1894. 
Id.  —  The  Germ-plasm,     A^ew  York.  1893. 

Whitman,  C.  0.  — Evolution  and  Epigenesis  :    Wood's  H oil  Biol.  Lectures.     1894. 
Wilson,  Edm.  B. — On  Cleavage   and   Mosaic-work:    Arch,  fur  Entiuicklungsni., 

III.   I.     1896.     See  also  Literature,  VIII..  p.  394.) 


I 


GLOSSARY 

[Obsolete  terms  are  enclosed  in  brackets.    The  name  and  date  refer  to  the  first  use  of  the  word; 
subsequent  changes  of  meaning  are  indicated  in  the  definition.] 

Achro'matin    (see    Chromatin),  the    non-staining   substance  of  the    nucleus,  as 

opposed  to  chromatin  ;  comprising  the  ground-substance  and  the  linin-network. 

(Flemming,  1879.) 
A'crosome  (  aKpov.  apex,  o-co/xa,  body),  the  apical  body  situated  at  the  anterior  end 

of  head  of  spermatozoon.     (Lenhossek,  1897.) 
[Akaryo'ta]  (see  Karyota),  non-nucleated  cells.     (Flemming,  1882.) 
Ale'cithal  (d-priv.  ;  AcKt^os,  the  yolk  of  an  egg),  having  little  or  no  yolk  (applied 

to  eggs).     (Balfour,  1880.) 
Alloplasma'tic  (aAAos,  ditferent).     Applied  to  active  substances  formed  by  dit^er- 

entiation  from  the  protoplasm  proper,  e.g.  the  substance  of  cilia,  of  nerve-hbrillar, 

and  of  muscle-fibrillas.     Alloplasmatic  organs  are  opposed  to  •*  protoplasmatic."' 

which  arise  only  by  division  of  preexisting  bodies  of  the  same  kind.      (A.  Meyer. 

1896.) 
Aniito'sis  (see   Mitosis),  direct  or  amitotic    nuclear   division ;    mass-division   of 

the  nuclear  substance  without  the  formation  of  chromosomes  and  amphiaster. 

(Flemming,  1882.) 
Am'phiaster  {afxcj^L,  on  both  sides ;  dar-^p,  a  star),  the  achromatic  figure  formed 

in  mitotic  cell-division,  consisting  of  two  asters  connected  by  a  spindle.     ( FoL. 

1877.) 
Amphipy 'renin     (see     Pyrenin),    the     substance    of    the    nuclear     membrane. 

(Schwarz,  1887.) 

Amy'loplasts  (a/xyXov,  starch:  TrAao-ros,  TrXdaaeLv.  form),  the  colourless  starch- 
forming  plastids  of  plant-cells.     (Errera,  1882.) 

An'aphase  (am,  back  or  again),  the  later  period  of  mitosis  during  the  divergence 
of  the  daughter-chromosomes.     (Strasburger.  18S4.) 

Aniso'tropy  (see  Isotropy),  having  a  predetermined  axis  or  axes  (as  applied  to 
the  egg).     (Pfluger,  1883.) 

Antherozo'id.  the  same  as  Sperniatozoid. 

Anti'podal  cone,  the  cone  of  astral  rays  opposite  to  the  spindle-fibres.  (\'an' 
Bexedex.  1883.) 

Archiam'phiaster  (apxi-  =  first,  +  amphiaster),  the  amphiaster  by  which  the  tirst 
or  second  polar  body  is  formed.      (Whit.al-vx,  1878.) 

Ar'choplasma  or  Archoplasm  (dpx(^^V'  a  ruler)  (sometimes  written  <7rc////>/<is///). 
the  substance  from  which  the  attraction-sphere,  the  astral  rays,  and  the  spindle- 
fibres   are  developed,  and  of  which  they  consist.     (Boveri,  1888.) 

Arrhe'noid  (dpprjv,  male).  The  sperm-aster  or  attraction-sphere  formed  during  the 
fertilization  of  the  ovum.     (Hexkix(j.  1890.) 

As'ter  (do-rr/p,  a  star),  i.  The  star-shaped  structure  surrounding  the  centrosome. 
(FoL,  1877.)  [2.  The  star-shaped  group  of  chromosomes  during  mitosis  (see 
Karyaster),     (Flemmlxg,  1892.)] 

[As'trocoele]  (dcTTrjp,  a  star:  koiAo?.  hollow),  a  term  somewhat  vaguely  api)lied  to 
the  space  in  which  the  centrosome  lies.     (Fol,  1891.) 

437 


438 


GLOSSARY 


As'trosphere  (see  Centrosphere).  i.  The  central  mass  of  the  aster,  exckisive 
of  the  rays,  in  which  the  centrosome  Hes.  Equivalent  to  the  '•  attraction-sphere  " 
of  Van  Beneden.  (Fol,  1891  ;  Strasburger,  1892.)  2.  The  entire  aster 
exclusive  of  the  centrosome.  Equivalent  to  the  "astral  sphere"  of  Mark. 
(BovERi,  1895.) 

Attraction-sphere  (see  Centrosphere),  the  central  mass  of  the  aster  from  which 
the  rays  proceed.  Also  the  mass  of  '^  archoplasm,''  derived  from  the  aster,  by 
which  the  centrosome  is  surrounded  in  the  resting  cell.     (Van  Beneden,  1883.) 

[Au'toblast]  (avrds,  self),  applied  by  Altmann  to  bacteria  and  other  minute  organ- 
isms, conceived  as  independent  solitary  "bioblasts."     (1890.) 

Axial  filament,  the  central  filament,  probably  contractile,  of  the  spermatozoon- 
flagellum.     (Elmer,  1874.) 

Basichro'matin  (see  Chromatin),  the  same  as  chromatin  in  the  usual  sense. 
That  portion  of  the  nuclear  network  stained  by  basic  tar-colours.     (Heidexhain, 

1894.) 

Bi'oblast  (/3tos,  life  :  ^Aao-ro'g.  a  germ),  a  term  applied  by  Altmann  to  the  hypo- 
thetical ultimate  vital  unit  (equivalent  to  plasome),  and  identified  by  him  as 
the  "granulum." 

Bi'ogen    {fiio^,  life ;    -yei/?;?,   producing),  equivalent  to  plasome,  etc.     (Verworn, 

1895-) 
Bi'ophores   (/^tos.  life  ;  -<f>6po^,  bearing),  the  ultimate  supra-molecular  vital   units. 

Equivalent  to  the  pangens  of  De  Vries,  the  plasomes  of  Wiesner,  etc.    (Weismann, 

1893.) 
Bi'oplasm  (^to9,  7rXa?/xa).      The  active  "living,  forming   germinal    material,''  as 

opposed  to  ''formed  material."     Nearly  equivalent  to  protoplasm  in  the  wider 

sense.     (Beale,  1870.) 
Bi'oplast.  equivalent  to  cell.     (Beale,  1870.) 
Bi'valent.  applied  to  chromatin-rods  representing  two  chromosomes  joined  end  to 

end.     (Hacker,  1892.) 
Ble'pharoplast   (/:^A£c/)apt9,  eye-lash   or  cilium).     The  centrosome-like    bodies    in 

plant-spermatids   in  connection  with   which  the  ciha  of  the  spermatozoids  are 

formed.     (Webber.  1897.) 
Cell-plate  (see  Mid-body),  the  equatorial   thickening  of  the  spindle-fibres    from 

which  the  partition-wall  arises  during  the  division  of  plant-cells.     (Strasbur- 

ger,  1875.) 
Cell-sap.   the  more  liquid  ground-substance  of   the   nucleus.     [Kolliker,  1865; 

more  precisely  defined  by  R.  Hertwig.  1876.] 
Central  spindle,  the  primary  spindle  by  which  the  centrosomes  are  connected,  as 

opposed  to  the  contractile  mantle-fibres  surrounding  it.     (Hermann,  1891.) 
Cen'triole.  a  term  applied  by  Boveri  to  a  minute  body  or  bodies  ("  Central-korn  ") 

within  the  centrosome.     In  some  cases  not  to  be  distinguished  from  the  centro- 
some.    (Boveri,  1895.) 
Centrodes'mus  (KeVx/oov,  centre;  Ses^o?,  a  band),  the  primary  connection  between 

the  centrosomes,  formed   by  a  substance  from  which  arises  the  central  spindle. 

(Heidenhain,  1894.) 
Centrodeu'toplasm,  the  granular  material  of  the  testis-cells  which  may  contribute 

to  the  formation  of  the  Nebenkern  or  to  that  of  the  idiozome.     (Erlanger, 

1897-) 
Centrole'cithal   (KcVrpov.  centre  :  Acki^os,  yolk),  that  type  of  ovum  m  which  the 

deutoplasm  is  mainly  accumulated  in  the  centre.     (Balfour,  1880.) 
Cen'troplasm  (Kevrpov.  centre;  irXdo-fxa),  the   protoplasm   forming  the  attraction- 
sphere  or  central  region  of  the  aster ;  the  substance  of  the  centrosphere.     (Er- 
langer, 1895.) 


GLOSSARY  AT^Q 

Cen'trosome  (Kevrpov,  centre ;  o-w/xa,  body),  a  body  found  at  the  centre  of  the  aster 
or  attraction-sphere,  regarded  by  some  observers  as   the  active  centre  of  cell- 
division  and  in  this  sense  as  the  dynamic  centre  of  the  cell.     Under  its  influence 
arise    the   asters    and    spindle    (amphiaster)    of    the    mitotic  fi-nire       (Boveim 
1888.) 

Cen'trosphere.  used  in  this  work  as  equivalent  to  the  "  astrosphere "  of  Stras- 
burger;  the  central  mass  of  the  aster  from  which  the  rays  proceed  and  within 
which  Hes  the  centrosome.  The  attraction-sphere.  [Stkasburger,  1892: 
applied  by  him  to  the  "  astrosphere  "  and  centrosome  taken  together.]  ' 

Chloroplas'tids  (xAwpoV  green;  TrAao-ro?,  form),  the  green  plastids  or  chlorophyll- 
bodies  of  plant  and  animal  cells.     (Schimper,  1883.) 

Chromatin  (xpw/xa,  colour),  the  deeply  staining  substance  of  the  nuclear  network 
and  of  the  chromosomes,  consisting  of  nuclein.     (Fle.m.minc;.  1879.) 

Chro'matophore  (xp^/xa,  colour;  -cf>6po^,  bearing),  a  general  term  ai)plied  to  the 
coloured  plastids  of  plant  and  animal  cells,  including  chloroplastids  and  chromo- 

plastids.       (SCHAARSCHMIDT,   1880;    SCHMITZ.   1882.) 

Chro'matoplasm  (xpoifxa.  colour;  irkacrixa,  anything  formed  or  moulded),  the  sub- 
stance of  the  chromoplastids  and  other  plastids.     (Strasrurger,  1882.) 

Chro'miole,  the  smallest  chromatin-granules  which  by  their  aggregation  form  the 
larger  chromomeres  of  which  the  chromosomes  are  composed^     (Eisex,  1899.) 

Chro'momere  (xpoj/xa,  colour;  /xepos,  a  part),  one  of  the  chromatin-granules  of 
which  the  chromosomes  are  made  up.  Identified  by  Weis.manx  as  the  "id." 
See  Chromiole.     (Fol,  1891.) 

Chromoplas'tids  (xpoj/xa,  colour  ;  TrAao-ro's,  form),  the  coloured  plastids  or  pigment- 
bodies  other  than  the  chloroplasts,  in  plant-cells.     (Schimper.  1883.) 

Chro'moplasts,  net-knots  or  chromatin-nucleoli :  also  used  by  some  authors  as 
equivalent  to  Chromoplastid.     (Eisen,  1899.) 

Chro'mosomes  (xpa>/xa,  colour:  o-w/xa.  body),  the  deeply  staining  bodies  into  which 
the  chromatic  nuclear  network  resolves  itself  during  mitotic  cell-division.  (  Wal- 
deyer,  1888.) 

Cleavage-nucleus,  the  nucleus  of  the  fertilized  Qgg,  resulting  from  the  union  of 
egg-nucleus  and  sperm-nucleus.     (O.  Hertwig,  1875.) 

Cortical  zone,  the  outer  zone  of  the  centrosphere.     (Van  Benedex,  1887.) 

Cyano'philous  (kwi/os,  blue;  ^lAetv,  to  love),  having  an  especial  affinity  for  lilue 
or  green  dyes.     (Auerbach.) 

Cy'taster  {KvTo<i,  hollow  (a  cell)  ;  do-XT/p,  star),  the  same  as  Aster,  i.  See  Kary- 
aster.     (Flemming,  1882.) 

[Cy'toblast]  {kvto^,  hollow  (a  cell);  ^Aacrro's,  germ).  i.  The  cell-nucleus. 
(ScHLEiDEN,  1838.)  2.  One  of  the  hypothetical  ultimate  vital  units  (bioblasts  or 
'"  granula '')  of  which  the  cell  is  built  up.  (Altmaxx,  1890.)  3.  A  naked  cell 
or  "protoblast."     (Kolliker.) 

[Cytoblaste'nia]  (see  Cytoblast),  the  formative  material  from  which  cells  were 
supposed  to  arise  by  ••  free  cell-formation."     (Schleidex,  183S.) 

[Cytochyle'ma]  {KVToq,  hollow  (a  cell)  ;  x^'^o?  J*^''^'*^)'  ^'^*^  ground-substance  of  the 
cytoplasm  as  opposed  to  that  of  the  nucleus.     (Strasrurger,  1882.) 

Cy'tode  {KVTo<ij  hollow  (a  cell)  ;  €1809,  form),  a  non-nucleated  cell.    (  Hackee,  1866.) 

Cytodie'resis  {KvTo<i,  hollow  (a  cell)  ;  SLatpeai'i,  division),  the  same  as  Mitosis. 
(Hexxeguy,  188?) 

Cytohy'aloplasma  (kvtos,  hollow  (a  cell)  ;  vaAo9.  glass  ;  irXaafia.  anything  t"ormed), 
the  substance  of  the  cytorcticulum  in  which  are  embedded  the  microsomes: 
opposed  to  nucleohyaloplasma.     (Strasrurger.  1882.) 

Cy'tolymph  {kvto<;.  hollow  (a  cell)  ;  lympha,  clear  water),  the  cytoplasmic  ground- 
substance.     (Hackee,  1891.) 


440 


GLOSSARY 


Cytomi'crosomes  (see  Microsome),  microsomes  of  the  cytoplasm  :  opposed  to 
nucleomicrosomes.     (Strasburger,  1882.) 

Cytomi'tome  {kvto'^,  hollow  (a  cell)  :  ^trto/xa,  from  |UtT09,  thread),  the  cytoplasmic 
as  opposed  to  the  nuclear  thread-work.     (Fle.mmixg,  1882.) 

Cy'toplasm  {KVTo<i,  TrXdafxa).  i.  The  protoplasmic  ground-substance  as  opposed 
to  the  granules.  (Kolliker,  1863.)  2.  Equivalent  to  protoplasm.  (Kolliker, 
1867.)  3.  The  substance  of  the  cell-body  as  opposed  to  that  of  the  nucleus. 
(Strasburger.  1882.) 

Cytoretic'ulum,  the  same  as  Cytomitome.     (Strasburger,  1882.) 

Cy'tosome  (kvVos,  hollow  (a  cell)  :  o-oj/xa.  body),  i.  The  cell-body  or  cytoplasmic 
mass  as  opposed  to  the  nucleus.  (Hackel,  1891.)  2.  A  term  used  as  parallel  to 
chromosome  to  denote  deeply  staining  definitely  organized  cytoplasmic  filaments 
or  other  cytoplasmic  structures  composed  of  "cytochromatin."    (Prenant.  1898.) 

Der'matoplasm  {Sepfxa,  skin),  the  living  protoplasm  asserted  to  form  a  part  of  the 
cell-membrane  in  plants.     (Wiesxer,  1886.) 

Der'matosomes  (8ep/xa,  skin  :  crw/xa,  body),  the  plasomes  which  form  the  cell-mem- 
brane.    (Wiesxer,  1886.) 

Determinant,  a  hypothetical  unit  formed  as  an  aggregation  of  biophores,  determin- 
ing the  development  of  a  single  cell  or  independently  variable  group  of  cells. 
(Weismaxx.  1 89 1.) 

[Deuthy'alosome]  (8evT(epo^),  second;  see  Hyalosome),  the  nucleus  remaining 
in  the  egg  after  formation  of  the  first  polar  body.     (Vax  Bexedex.  1883.) 

Deu'toplasm  (8et'T(e/oos),  second ;  TrAacr/xa,  anything  formed),  yolk,  lifeless  food- 
matters  deposited  in  the  cytoplasm  of  the  egg  :  opposed  to  "protoplasm."  (Vax 
Bexedex.  1870.) 

Diakine'sis  (8ta,  through),  the  segmented-spireme-stage,  following  the  synapsis,  in 
the  primarv  oocyte  or  spermatocyte,  during  which  the  chromosomes  persist  for  a 
considerable  period  in  the  form  of  double  rods.     (Hacker,  1897.) 

Directive  bodies,  the  polar  bodies.     (Fr.  Muller,  1848.) 

Directive  sphere,  the  attraction-sphere.     (Guigxard,  1891.) 

Dispermy,  the  entrance  of  two  spermatozoa  into  the  egg. 

Dispi'reme  (see  Spireme),  that  stage  of  mitosis  in  which  each  daughter-nucleus 
has  given  rise  to  a  spireme.      (Flemmixg,  1882.) 

Dy'aster  (8m5,  two;  see  Aster.  2),  the  double  group  of  chromosomes  during  the 
anaphases  of  cell-division.     (Flemaiixg,  1882.) 

Ectosphere  (eKTo^,  outside),  the  outer  or  cortical  zone  of  the  attraction-sphere. 
(Ziegler,  1899.) 

Egg-nucleus,  the  nucleus  of  the  egg  after  formation  of  the  polar  bodies  and  before 
its  union  with  the  sperm-nucleus.  Equivalent  to  the  '"female  pronucleus"  of  Van 
Bexedex.     (O.  Hertwig,  1875.) 

Enchyle'ma  (ev,  in;  x^^^^'  juice),  i.  The  more  fluid  portion  of  protoplasm, 
consisting  of  "  hyaloplasma."'  (Haxsteix,  1880.)  2.  The  ground-substance 
(cvtolvmph)  of  cytoplasm  as  opposed  to  the  reticulum.     (Carxov,  1883.) 

Endoplast,  the  cell-nucleus.     (Huxley,  1853.) 

Ener'gid,  the  cell-nucleus  together  with  the  cytoplasm  lying  within  its  sphere  of 
influence.     (Sachs.  1892.) 

Entosphere,  (evrog,  inside),  the  inner  or  medullary  zone  of  the  attraction-sphere. 
(Ziegler,  1899.) 

Equatorial  plate,  the  group  of  chromosomes  lying  at  the  equator  of  the  spindle 
during  mitosis.     (Vax  Bexedex,  1875.) 

Ergastic  (epya^o/xat,  to  work).  Applied  to  relatively  passive  substances '•  formed 
anew  through  activity  of  the  protoplasm.''  Equivalent  to  metaplasmic.  C/. 
alloplasmatic.     (A.  Meyer,  1896.) 


GLOSSARY  ^^I 

Ergastoplasm  (epya^o/^ac.  to  work).     Nearly  equivalent    to   the   '*  kinopFasm ''   of 

Strasburger  and  the  ••  ergoplasm  ''  of  Davidoff.     The  more  active  protoplasmic 

substance  from  which  fibrillar  formations  arise.     (Gakniek,  1897.) 
Ergoplasm  (epyov,  work).     The  active  protoplasm  of  the  egg  (in  tunicatcs).  mainly 

derived  from  the  achromatic  part  of  the  germinal  vesicle,  and  giving  rise  in  jxirt 

or   wholly   to    the    polar   spindle.      Analogous   to   archoplasm    and    kmoplasm. 

(Davidoff,  1889.) 
Erythro'philous  (ipvOpoq,  red;  4>Ly€tv,  to  love),  having  an  especial  affinity  for  red 

dyes.     (AuERBACH.)  '  * 

Ga'mete  (ya/xexT/,  wife  ;  ya/xerrys,  husband),  one  of  two  conjugating  cells.     Usually 

applied  to  the  unicellular  forms. 
Gem'mule  (see  Pangen),  one  of  the  ultimate  supra-molecular  germs  of  the  cell 

assumed  by  Darwin.     (Darwix,  1868.) 
[Ge'noblasts]  (yeVos,  sex  ;  fSXaaro^,  germ),  a  term  applied  by  Minot  to  the  mature 

germ-cells.     The  female  genoblast  (egg  or  '•  thelyblast ")  unites  with  the  male 

(spermatozoon  or  "arsenoblast")  to  form  an  hermaphrodite  or  indifferent  cell. 

(Minot,  1877.) 
Germinal  spot,  the  nucleolus  of  the  germinal  vesicle.     (Wagner,  1836.) 
Germinal  vesicle,  the   nucleus  of  the  egg  before  formation  of  the  polar  bodies. 

(PuRKiNjE.  1825.) 
Germ-plasm,  the  same  as  idioplasm.     (Weis.mann.) 
Heterokine'sis  (erepo';,  different),  qualitative  nuclear  division  ;  a  hypothetical  mode 

of    mitosis    assumed    to    separate    chromatins   of   different   quality ;   opposed  to 

homookinesis  or  equation-division.     (Weismann.  1892.) 
Heterole'cithal    (^'Tepo<i,   different :    keKLOoq.    yolk),   having   unequally   distributed 

deutoplasm  (includes  telolecithal  and  centrolecithal).     (Mark,  1892.) 
Heterotyp'ical   mitosis    (erepo?,  different ;    see  Mitosis),  that  mode  of   mitotic 

division  in  which  the  daughter-chromosomes  remain  united  bv  their  ends  to  form 

rings.     (Flemming,  1887.) 
[Holoschi'sis]  (oAo?,  whole ;  crxi^eiv,  to  split),  direct  nuclear  division.     Amitosis. 

(Flemming,  1882.) 
Homole'cithal   (6fx6<;,  the  same,  uniform  :  AeVt^o?,   yolk),  equivalent  to  alecithal. 

Having  little  deutoplasm,  equally  distributed,  or  none.     (Mark,  1892.) 
Homookine'sis  or  Homaeokine'sis  (o/xo's,  the  same),  equation-division,  seijai.iung 

equivalent  chromatins  :  opposed  to  heterokinesis.     (Weismann,  1892.) 
Homoeotyp'ical  mitosis  (o/xoto?,  like:  see  Mitosis),  a   form  of  mitosis  occurring 

in  the  secondary  spermatocytes  of  the  salamander,  differing  from  the  usual  type 

only    in    the    shortness  of  the  chromosomes  and  the  irregular  arrangement    of 

the  daughter-chromosomes.  •  (Flemming,  1887.) 
Hy'aloplasma  (vaXos,  glass  ;  TrAatr/xa,  anything  formed),     i.  The  ground-substance 

of  the  cell  as  distinguished  from  the  granules  or  microsomes.     [Hanstein,  1880.] 

2.    The  achromatic  substance  of  the  nucleus  in  which  the  chromatin-particles  are 

embedded.     (Strasburger,  1882.)     3.    The  ground-substance  as  distinguished 

from  the  reticulum  or '-spongioplasm.^'     (Levdh;,  1885. )     4.    The  exoplasm  or 

peripheral  protoplasmic  zone  in  plant-cells.     (Pfeffer.) 
Hy'alosomes  (vaAos,  glass;  a(ofxa,  body),  nucleolar-like  bodies  but  slightly  stained 

by  either  nuclear  or  plasma  stains.     (  Lukjanow,  1888.) 
[Hy'groplasma]   {vyp(k.  wet:  7rAao-/xa,  something  formed),  the  more   li(juid   part 

of  protoplasm  as  opposed  to  the  firmer  stereoplasm.     (Xageli,  1884.) 
Id,  the   hypothetical   structural  unit   resulting  from   the  successive  aggivgation   of 

biophores  and  determinants.     Identified  by  Weismann  as  the  chromomere,  or 

chromatin-granule.     (Weismann,  1891.) 
Idant,  the  hypothetical  unit  resulting  from  the  successive  aggregation  of  biophore.s. 


442 


GLOSSARY 


determinants,  and  Ids.  Identified  by  Weismann  as  the  chromosome.  (Weis- 
MANN,  1 89 1.) 

Id'ioblasts  (t'Stos,  one's  own;  /^Aacrrd?,  germ),  the  hypothetical  ultimate  units  of 
the  cell;  the  same  as  biophores.     (O.  Hertwig,  1893.) 

Idioplasm  (i,'8ios,  one's  own  ;  TrAacr/Aa,  a  thing  formed),  equivalent  to  the  germ- 
plasm  of  Weismann.  The  substance,  now  generally  identified  with  chromatin, 
which  by  its  inherent  organization  involves  the  characteristics  of  the  species. 
The  physical  basis  of  inheritance.     (Nageli,  1884.) 

Id'iosonie  (I'Sto?.  one's  own;  crco/xa,  body),  the  same  as  idioblast  or  plasome. 
(Whitman,  1893.) 

Idiozome  (tSto?,  specially  formed;  ^oyfxa,  girdle).  The  sphere,  often  called  attrac- 
tion-sphere and  usually  enclosing  the  centrosomes,  found  in  the  spermatids  of 
animals.      (Meves.  1897.) 

Interfilar  substance,  the  ground-substance  of  protoplasm  as  opposed  to  the  thread- 
work.     (Flemming,  1882.) 

Interzonal  fibres  ('•  Filaments  reunissants ''  of  Van  Beneden.  "  Verbindungs- 
fasern "  of  Flemming  and  others).  Those  spindle-fibres  that  stretch  between 
the  two  groups  of  daughter-chromosomes  during  the  anaphase.  Equivalent 
in  some  cases  to  the  central  spindle.     (Mark,  1881.) 

Iso'tropy  (to-os,  equal;  rpoTrrj,  a  turning),  the  absence  of  predetermined  axes  (as 
applied  to  the  egg).     (Pfluger,  1883.) 

[Ka'ryaster]  (Kapvov,  nut,  nucleus  ;  see  Aster,  2),  the  star-shaped  group  of  chromo- 
somes in  mitosis.     Opposed  to  cytaster.     (Flemming,  1882.) 

Karyenchy'ma  (Kapvov,  nut,  nucleus;  iv,  in;  x^H-^'^"  J^ice),  the  "nuclear  sap." 
(Flemming,  1882.) 

Karyokine'sis  (Kapvov,  nut,  nucleus;  KtVr/crts,  change,  movement),  the  same  as 
mitosis.     (Schleicher,  1878.) 

[Karyoly'ma],  the  '' karyolytic '"  (mitotic)  figure.      (Auerbach,  1876.) 

Ka'ryolymph.     The  nuclear  sap.     (Hackel,  1891.) 

[Karyo'lysis]  (Kapvov,  nut,  nucleus  ;  A-ucrts.  dissolution),  the  supposed  dissolution 
of  the  nucleus  during  cell-division.     (Auerbach,  1874.) 

[Karyoly'tic  figure]  (see  Karyolysis),  a  term  applied  by  Auerbach  to  the  mitotic 
figure  in  livino-  cells.  Believed  bv  him  to  result  from  the  dissolution  of  the 
nucleus.     (A^uerbach,  1874.) 

Karyonii'crosonie  (see  Microsome),  the  same  as  nucleo-microsome. 

Karyomi'tome  (Kapvov.  nut,  nucleus ;  /xtVtu/xa,  from  /jllto^,  a  thread),  the  nuclear  as 
opposed  to  the  cytoplasmic  thread-work.     (Flemming,  1882.) 

Karyomito'sis  (Kapvov,  nut,  nucleus ;  see  Mitosis),  mitosis,  (Flemming, 
1882.) 

Ka'ryon  (Kapvov,  nut,  nucleus),  the  cell-nucleus.     (Hackel,  1891.) 

Ka'ryoplasm  (Kapvov,  nut.  nucleus  ;  7rAao-/xa,  a  thing  formed),  nucleoplasm.  The 
nuclear  as  opposed  to  the  cytoplasmic  substance.     (Flemming,  1882.) 

Ka'ryosome  (Kapvov,  nut,  nucleus;  croj/xa,  body),  i.  Nucleoli  of  the  "  net-knot  *' 
type,  staining  with  nuclear  dyes,  -as  opposed  to  plasmosomes  or  true  nucleoli. 
(Ogata,  1883.)  2.  The  same  as  chromosome.  (Platner,  1886.)  3.  Caryo- 
some.     The  cell-nucleus.     (Watase,  1894.) 

[Karyo'ta]   (Kapvov,  nut,  nucleus),  nucleated  cells.     (Flemming,  1882.) 

Karyothe'ca  (Kapi;ov,  nut.  nucleus ;  OrJKr),  case,  box),  the  nuclear  membrane. 
(Hackel,  1891.) 

Ki'noplasm  (Kivelv,  to  move  ■  7rAacr/xa,  a  thing  formed),  nearly  equivalent  to 
archoplasm,  but  used  in  a  broader  sense  to  denote  in  general  the  more 
active  elements  of  protoplasm  from  which  arise  fibril] ae,  the  substance  of  cilia, 
and   (in  plants)    the   peripheral  "  Hautschicht "  from  which    the  membrane   is 


GLOSSARY 


443 


formed;  opposetl  to  the  " troplioplasm '"  or  nutritive  plasm.  (Stkashi'RGER, 
1892.) 

[Lanthanin]  (Aav^ctvetv,  to  conceal),  equivalent  to  oxychromatin.  (  Hi:idknhain, 
1892.) 

Leucoplas'tids  (AevKo?,  white  :  TrAaoro?,  form),  the  colourless  plastids  of  plant- 
cells  from  which  arise  the  starch-formers  (amyloplastids),  chloroplastids,  and 
chromoplastids.     (Schlmper,  1883.) 

Li'nin  (linum,  a  linen  thread),  the  substance  of  the  "achromatic"'  nuclear  reticu- 
lum.    (SCHWARZ,  1887.) 

Lininoplast,  the  true  nucleolus  or  plasmosome.     (Eisen,  1899.) 

Macrocentrosome,  a  term  applied  to  the  "  centrosome  "  in  Boveri's  sense,  i.e.  to 
the  larger  body  in  which  lies  the  central  granule.  (Zikgler,  1898.)  Probably 
svnonymous  with  entosphere. 

Maturation,  the  final  stages  in  the  development  of  the  germ-cells.  .More  spe- 
cifically, the  process  by  which  the  reduction  of  the  number  uf  chromosomes 
is  effected. 

Metakine'sis  (see  Metaphase)  (/xera,  beyond  {i:e.  further)  ;  KLvvrjrrL^;.  movement), 
the  middle  stage  of  mitosis,  when  the  chromosomes  are  grouped  in  the  equatorial 
plate.     (Flemming,  1882.) 

Metanu'cleus,  a  term  applied  to  the  nucleolus  after  its  e.xtrusion  from  the  germi- 
nal vesicle.     (Hacker,  1892.) 

Met'aphase,  the  middle  stage  of  mitosis  during  which  occurs  the  splitting  of  the 
chromosomes  in  the  equatorial  plate.     (Strasburger,  1884.) 

Met'aplasm  (/xera,  after,  beyond;  TrAacr/xa.  a  thing  formed),  a  term  collectively 
applied  to  the  lifeless  inclusions  (deutoplasm.  starch,  etc.)  in  protoplasm  as  op- 
posed to  the  living  substance.     (Hansteix,  1868.) 

Micel'la,  one  of  the  ultimate  supra-molecular  units  of  the  cell.     (N.ageli.  1884.) 

Microcentrosome.  equivalent  to  the  central  granule  or  centriole  of  Bovcri. 
(Ziegler,  1898.) 

Microcen'trum,  the  centrosome  or  group  of  centrosomes  united  by  a  '•  primary 
centrodesmus,"  forming  the  centre  of  the  astral  system.     (Heidenhain,  1894.) 

Mi'cropyle  (fjuKpo^,  small:  ttvXt],  orifice),  the  aperture  in  the  egg-membrane 
through  which  the  spermatozoon  enters.  [First  applied  by  Ti'Ri'i.v,  in  1806, 
to  the  opening  through  which   the  pollen-tube  enters   the  ovule.     /.    Rohert 

Brown.] 
Mi'crosome  (fxiKpoq,  small :  croj/xa,  body),  the  granules  as  opposed  to  the  ground- 
substance  of  protoplasm.     (Hansteix,  1880.) 
Microsphere,  the  central  region  of  the  aster  (centrosphere)  at  the  centre  of  which 

lie  the  centrosomes.     (Kostanecki  and  Siedlecki,  1896.) 
Middle-piece,  that  portion  of  the  spermatozoon  lying  behind  the  nucleus  at  the 

base  of  the  flagellum.     (Schweigger-Seidel.  1865.) 
Mid-body  ("Zwischenkorper ''),  a  body  or  group  of  granules,  i)robal)ly  comparable 

with  the  cell-plate  in  plants,  formed  in  the  equatorial  region  of  the  spindle  during 

the  anaphases  of  mitosis.     (Flemming,  1890.) 
Mi'tome  (/xtrco/xa,  from  ^tVos,  a  thread),  the  reticulum  or  thread-work  as  opposed  to 

the  ground-substance  of  protoplasm.     (Flemming,  1882.) 
[Mitoschi'sis  (/xiro9,  thread ;  ax^C^i-v,  to  split),  indirect  nuclear  division;  mitosis. 

(Flemming,  1882.) 
Mito'sis  (iJLLTo<;,  a    thread),  indirect    nuclear   division    typically   involving:    u,  the 

formation    of  an   amphiaster:    /\  conversion    of   the    chromatin    into   a    thread 

(spireme)  :  r,  segmentation  of  the  thread  into  chromosomes  :  (/.  splitting  of  the 

chromosomes.     (Flemming,  1882.) 
Mi'tosome  (^tros,  a  thread;  aCjfxa,  body),  a  body  derived  from  the  spindle-fibres 


444 


GLOSSARY 


of  the  secondary  spermatocytes,  giving  rise,  according  to  Platxer,  to  the  mid- 
dle-piece and  the  tail-envelope  of  the  spermatozoon.  Equivalent  to  the  Neben- 
kern  of  La  Valette  St.  George.     (Platxer,  1889.) 

Nebenkern  (Paranucleus),  a  name  originally  applied  by  Butschli  (1871)  to  an 
extranuclear  body  in  the  spermatid  :  afterwards  shown  by  La  Valette  St.  George 
and  Platner  to  arise  from  the  spindle-fibres  of  the  secondary  spermatocyte. 
Since  applied  to  many  forms  of  cytoplasmic  bodies  (yolk-nucleus,  etc)  of  the 
most  diverse  nature. 

Nuclear  plate,  i.  The  equatorial  plate.  (Strasburger,  1875.)  2.  The  parti- 
tion-wall which  sometimes  divides  the  nucleus  in  amitosis. 

Nuclein,  the  chemical  basis  of  chromatin  ;  a  compound  of  nucleinic  acid  and  albumin 
or  albumin  radicles.     (Miescher,  1871.) 

Nucleinic  or  nucleic  acid,  a  complex  organic  acid,  rich  in  phosphorus,  and  an 
essential  constituent  of  chromatin. 

Nucleo-albumin,  a  nuclein  having  a  relatively  high  percentage  of  albumin.  Dis- 
tinguished from  nucleo-proteids  by  containing  paranucleinic  acid  which  yields  no 
xanthin-bodies. 

[Nucleochyle'ma]  (x^^Ad?,  juice),  the  ground-substance  of  the  nucleus  as  opposed 
to  that  of  the  cytoplasm.     (Strasburger,  1882.) 

Nucleohy'aloplasma  (see  Hyaloplasm),  the  achromatic  substance  (Hnin)  in  which 
the  chromatin-granules  are  suspended.     (Strasburger,  1882.) 

Nucleomi'crosomes  (see  Microsome),  the  nuclear  (chromatin)  granules  as 
opposed  to  those  of  the  cytoplasm.     (Strasburger,  1882.) 

Nu'cleoplasm.  i.  The  reticular  substance  of  the  (egg-)  nucleus.  (Van  Bene- 
dex,  1875.)  2.  The  substance  of  the  nucleus  as  opposed  to  that  of  the  cell- 
body  or  cytoplasm.     (Strasburger,  1882.) 

Nucleo-pro'teid.  a  nuclein  having  a  relatively  high  percentage  of  albumin.  May 
be  split  into  albumin  and  true  nucleinic  acid,  the  latter  yielding  xanthin-bodies. 

CEde'matin  (otSy^/xa,  a  swelling),  the  granules  or  microsomes  of  the  nuclear  ground- 
substance.     (Reinke,  1893.) 

O'ocyte  (Ovocyte)  (ww,  egg;  kvto?,  hollow  (a  cell)),  the  ultimate  ovarian  ^gg 
before  formation  of  \he  polar  bodies.  The  primary  oocyte  divides  to  form  the 
first  polar  body  and  the  secondary  oocyte.  The  latter  divides  to  form  the  second 
polar  body  and  the  mature  egg.     (Boveri,  1891.) 

Oogenesis,  Ovogenesis  (todv,  (t%g  ;  yeVccrts,  origin),  the  genesis  of  the  ^gg  after  its 
origin  by  division  from  the  mother-cell.  Often  used  more  specifically  to  denote 
the  process  of  reduction  in  the  female. 

Oogo'nium,  Ovogonium  (cudv,  ^gg  ;  yovr;,  generation),  i .  The  primordial  mother- 
cell  from  which  arises  the  egg  and  its  follicle.  (Pfluger.)  2.  The  descend- 
ants of  the  primordial  germ-cell  which  ultimately  give  rise  to  the  oocytes  or 
ovarian  eggs.     (Boveri,  1891.) 

Ookine'sis  (coov,  egg;  KLvrjai^,  movement),  the  mitotic  phenomena  of  the  egg  dur- 
ing maturation  and  fertilization.     (Whitman,  1887.) 

O'vocentre,  the  egg-centrosome  during  fertilization.     (FoL,  1891.) 
Oxychro'matin  (o#i;s.  acid  ;  see  Chromatin),  that  portion  of  the  nuclear  substance 
stained    by   acid   tar-colours.      Equivalent    to   '"linin"     in    the    usual    sense. 
(Heidenhain,  1894.) 
Pangen'esis  (ttS?  (Trav-),  all;  yeVecrt?,  production),  the  theory  of  gemmules,  accord- 
ing to  which  hereditary  traits  are  carried  by  invisible  germs  thrown  off  by  the 
individual  cehs  of  the  body.     (Darwin.  1868.) 
Pangens  (ttS?  (vrav-),  all ;  -yeVvys,  producing),  the  hypothetical uhi mate  supra-molec- 
ular units  of  the  idioplasm,  and  of  the  cell  generally.     Equivalent  to  gemmules, 
micellae,  idioblasts,  biophores,  etc.     (De  Vries,  1889.) 


GLOSSARY 


445 


Parachro 'matin  (see  Chromatin),  the  achromatic  nuclear  substance  (linin  of 
Schwarz)  from  which  the  spindle-tibres  arise.     (Pfitznkk,  1883.) 

Parali'nin  (see  Linin),  the  nuclear  ground-substance  or  nuclear  sap.     (Schwarz 
1887.) 

Parami'tome  (see  Mitome),  the  ground-sub.stance  or  interfilar  sub.stance  of  proto- 
plasm, opposed  to  mitome.     (Flem.ming,  1892.) 

Paranu'clein  (see  Nuclein),  the  substance  of  true  nucleoli  or  plasmosomes. 
Pyrenin  of  Schwarz.  (O.  Hertwig,  1878.)  Applied  by  Kossel  to  "nucleins" 
derived  from  the  cytoplasm.  These  are  compounds  of  aibumin  and  paranucleic 
acid  which  yields  no  xanthin-bodies. 

Paranucleus  (see  Nebenkern). 

Par'aplasm  {irapL  beside;  irXdajxa,  something  formed),  the  less  active  portion,  of 
the  cell-substance.  Originally  applied  by  Kupffer  to  the  cortical  region  of  the 
cell  (exoplasm).  but  now  often  applied  to  the  ground-substance.  (Kl'I'FFFR, 
1875.) 

Periplast  (irepL,  around;  TrXao-TOs,  form),  i.  The  peripheral  j)art  of  the  cell, 
including  those  parts  outside  the  nucleus  or  ^'endoplast.""  (Hl-xlev.  1853.) 
2.  A  term  somewhat  vaguely  applied  to  the  attraction-sphere.  The  term 
daughter-periplast  is  applied  to  the  centrosome.     (Vejoovsky,  188S.) 

Perisphere  (Trepc,  around),  a  term  applied  to  the  outer  region  of  the  attraction- 
sphere  in  nerve-cells,  and  opposed  to  an  inner '•  centrosphere."'  (Lenhossek, 
1895.) 

Plasmocytes  (7rAacr/xa,  K\no%),  colourless  blood-corpuscles  supposed  to  be  free 
attraction-spheres.     (Eisen,  1897.) 

Plasmosphere,  the  same  as  Perisphere. 

Plas'mosome  (7rAao-/xa,  something  formed  {i.e.  protoplasmic)  ;  adfxa,  body),  the 
true  nucleus,  distinguished  by  its  affinity  for  acid  tar-colours  and  other  -plasma- 
stains.''     (Ogata,  1883.) 

Pla'some  (irXdafxa,  a  thing  formed;  o-to/xa,  body),  the  ultimate  supra-molecular 
vital  unit.     See  Biophore,  Pangen.     (Wiesxer.  1890.) 

Plas'tid  (TrXasrds,  form),  i.  A  cell,  whether  nucleated  or  non-nucleated.  (Hackel, 
1866.)  2.  A  general  term  applied  to  permanent  cell-organs  (chloroplasts.  etc.) 
other  than  the  nucleus  and  centrosome.     (Schi.mper,  1883.) 

Plas'tidule,  the  ultimate  supra-molecular  vital  unit.  (Elssbekg,  1874;  H.ackel, 
1876.) 

Plas'tin,  a  term  of  vague  meaning  applied  to  a  substance  related  to  the  nucleo- 
proteids  and  nucleo-albumins  constituting  the  linin-network  (Zacharias)  and  the 
cytoreticulum  (Carnoy).     (Reixke  and  Rodewald.  1881.) 

Pluri'valent  {plus,  more;  valcre,  to  be  worth),  applied  to  chromatin-rods  that 
have  the  value  of  more  than  one  chromosome  soisn  strict  u.     (  H.uker.  1892.) 

Polar  bodies  (Polar  globules),  two  minute  cells  segmented  ort'  from  the  ovum 
before  union  of  the  germ-nuclei.  (Disc,  by  Carus,  1824;  so  named  bv  Robin. 
1862.) 

Polar  corpuscle,  the  centrosome.     (Van  Bexeden,  1S76.) 

Polar  rays  (Polradien),  a  term  sometimes  applied  to  all  of  the  astral  rays  as 
opposed  to  the  spindle-fibres,  sometimes  to  the  group  of  astral  rays  opposite  to 
the  spindle-fibres. 

Pole-plates  (End-plates),  the  achromatic  spheres  or  masses  at  the  poles  of  the 
spindle  in  the  mitosis  of  Protozoa,  probably  representing  the  attraction-spheres. 
(R.  Hertwig,  1877.) 

Polyspermy,  the  entrance  into  the  ovum  of  more  than  one  spermatozoon. 

[Prochro'matin]  (see  Chromatin),  the  substance  of  true  nucleoli,  or  plasmosomes. 
Equivalent  to  paranuclein  of  O.  Hertwig.     (Pfitzxer,  1883.) 


446  GLOSSARY 

Pronuclei,  the  germ-nuclei  during  fertilization  :  i.e.  the  egg-nucleus  (female  pro« 
nucleus)  after  formation  of  the  polar  bodies,  and  the  sperm-nucleus  (male  pro- 
nucleus) after  entrance  of  the  spermatozoon  into  the  egg.     (Van  Beneden, 

1875.) 

[Prothy'alosome]  (see  Hyalosonie).  an  area  in  the  germinal  vesicle  {oi  Ascaris^ 
by  which  the  germinal  spot  is  surrounded,  and  which  is  concerned  in  formation 
of  the  first  polar  body.     (Van  Beneden,  1883.) 

Pro'toblast  (7rfjioro<;.  first;  ySAao-ro?.  a  germ),  i.  A  naked  cell,  devoid  of  a  mem- 
brane. (KoLLiKER.)  2.  A  blastomere  of  the  segmenting  egg  which  is  the 
parent-cell  of  a  definite  part  or  organ.     (Wilson,  1892.) 

Pro'toplasm  (Trpcorog.  first;  TrAacr/xa.  a  thing  formed  or  moulded).  The  active 
or  '•  living  "■  cell-substance.  By  all  earlier  and  some  present  writers  applied  only 
to  the  substance  of  the  cell-body  (equivalent  to  Strasburger's  cytoplasm).  By 
many  later  writers  applied  to  the  entire  active  substance  of  the  cell  (karyoplasm 
plus  cytoplasm).     (Purkinje,  1840;   H.  von  Mohl,  1846.) 

Pro'toplast  (TrpoiTos.  first:  TrAacrro?,  formed),  i.  The  protoplasmic  body  of  the 
cell,  including  nucleus  and  cytoplasm,  regarded  as  a  unit.  Nearly  equivalent  to 
the  energid  of  Sachs.  (Hanstein,  1880.)  2.  Used  by  some  authors  synony- 
mously with  plastid. 

[Psetidochro'matin]  (see  Chromatin),  the  same  as  prochromatin.  (Pfitzner, 
1886.) 

Pseudonu'clein  (see  Nuclein),  the  same  as  the  paranuclein  of  Kossel.  (Ham- 
marsten,  1894.) 

Pseudo-reduction,  the  preliminary  halving  of  the  number  of  chromatin-rods  as  a 
prelude  to  the  formation  of  the  tetrads  and  to  the  actual  reduction  in  the  number 
of  chromosomes  in  maturation.     (Ruckert,  1894.) 

Pyre'nin  (irvprjv,  the  stone  of  a  fruit ;  /.t\  relating  to  the  nucleus),  the  substance  of 
true  nucleoli.     Equivalent  to  the  paranuclein  of  Hertwig.     (Schwarz.  1887.) 

Pyre'noid  (-n-vprjv,  the  stone  of  a  fruit:  like  a  nucleus),  colourless  plastids  (leuco- 
plastids).  occurring  in  the  chromatophores  of  lower  plants,  forming  centres  for 
the  formation  of  starch.     (Schmitz,  1883.) 

Reduction,  the  halving  of  the  number  of  chromosomes  in  the  germ-nuclei  during 
maturation. 

Sarcode  {crapL  flesh).  The  protoplasm  of  unicellular  animals.  (Du  Jardin, 
1835.) 

Sertoli-cells.  the  large,  digitate,  supporting,  and  nutritive  cells  of  the  mammalian 
testis  to  which  the  developing  spermatozoa  are  attached.  (Equivalent  to  "sper- 
matoblast" as  originally  used  by  Von  Ebner,  1871.) 

Sper'matid  (airepixa,  seed),  the  final  cells  which  are  converted  without  further 
division  into  spermatozoa  ;  they  arise  by  division  of  the  secondary  spermatocytes 
or  "  Samenmlitterzellen.''     (La  Valete  St.  George,  1886.) 

Sper'matoblasts  (cnrepfxa,  seed :  /^Aacrro?,  germ),  a  word  of  vague  meaning, 
originally  applied  to  the  supporting  cell  or  Sertoli-cell,  from  which  a  group  of 
spermatozoa  was  supposed  to  arise.  By  various  later  writers  used  synonymously 
with  spermatid.     (VoN  Ebner,  1871.) 

Sper'matocyst  (a-rreppja.  seed :  KvarL'i,  bladder),  originally  applied  to  a  group  of 
sperm-producing  cells  ("spermatocytes"'),  arising  by  division  from  an  "  Ursa- 
menzelle"'  or  "spermatogonium."     (La  Valette  St.  George,  1876.) 

Sper'matocyte  (o-Trep/xa,  seed;  Kvroq,  hollow  (a  cell)),  the  cells  arising  from  the 
spermatogonia.  The  primary  spermatocyte  arises  by  growth  of  one  of  the  last 
generation  of  spermatogonia.  By  its  division  are  formed  two  secondary  sper- 
matocytes, each  of  which  gives  rise  to  two  spermatids  (ultimately  spermatozoa). 
(La  Valette  St.  George,  1876.) 


GLOSSARY 


447 


[Spermatogem'ma]  (cr7rep/xa,  seed;  geniuia.  bud),  nearly  equivalent  to  spermatn- 
cyst.  Differs  in  the  absence  of  a  surrounding  membrane.  [In  mammals.  La 
Valette  St.  Georcje.  1878.] 

Spermatogen'esis  {air^pfxa,  seed;  yeVcmi;.  origin),  the  phenomena  involved  in 
the  formation  of  the  spermatozoon.  Often  used  more  specifically  to  denote  the 
process  of  reduction  in  the  male. 

Spermatogo'nium  ("•  Ursamenzelle'')  (rr7rep/xa,  seed;  yovy.  generation),  the 
descendants  of  the  primordial  germ-cells  in  the  male.  Kach  ultimate  sper- 
matogonium typically  gives  rise  to  four  spermatozoa.  (La  \'ai.f:ite  St. 
George,  1876.) 

Sperniatome'rites  {cnripixa,  seed;  /xepos.  a  part),  the  chromatin-granules  into 
which  the  sperm-nucleus  resolves  itself  after  entrance  of  the  spermatozoon.  (In 
Petrojnyzo7i,  Bohm,  1887.) 

Sper'niatcsome  (a-n-epfxa,  seed;  crCijxa,  body),  the  same  as  spermatozoon.  (L.\ 
Valette  St.  George,  1878.) 

Spermatozo'id  (see  Spermatozoon),  the  ciliated  paternal  germ-cells  in  plants. 
The  word  was  first  used  by  von  Siebold  as  synonymous  with  spermatozoon. 

Spermatozoon  (a-rrepfxa,  seed  ;  ^(Joov,  animal),  the  paternal  germ-cell  of  animals. 
(Leeuwenhoek,  1677.) 

Sperm-nucleus,  the  nucleus  of  the  spermatozoon  ;  more  especially  apjilied  to  it  after 
entrance  into  the  egg  before  its  union  with  the  egg-nucleus.  In  this  sense 
equivalent    to    the    "male   pronucleus"'    of    Van    Beneden.       (O.    Hkktnvk;. 

1875.) 
Sper'mocentre,  the  sperm-centrosome  during  fertilization.     (Fol,  1891.) 

Spi'reme  {criT^.ip'qjxa,  a  thing  wound  or  coiled;  a  skein),  the  skein  or  ••  Knauel" 
stage  of  the  nucleus  in  mitosis,  during  which  the  chromatin  appears  in  the  form 
of  a  thread,  continuous  or  segmented.     (Flemmixg,  1882.) 

Spon'gioplasm  {(nroyytov,  a  sponge;  TrAaa/xa,  a  thing  formed),  the  cytoreticulum. 
(Leydig,  1885.) 

Ste'reoplasm  {(TT€.p(.6^,  solid),  the  more  solid  part  of  protoplasm  as  opposed  to  the 
more  fluid  ••  hygroplasm.''     (Nageli,  1884.) 

Substantia  hyalina,  the  protoplasmic  ground-substance  or  -hyaloplasm." 
(Leydig,  1885.) 

Substantia  opaca,  the  protoplasmic  reticulum  or  '-spongioplasm.''  (Leydig, 
1885.) 

Synap'sis  (crvvaTrTui,  to  fuse  together).  A  stage  in  the  nucleus  preceding  the  first 
maturation-division,  characterized  by  the  massing  of  the  chromatin  at  one  side 
of  the  nucleus.  From  it  the  chromatin-masses  emerge  in  the  reduced  number. 
(Moore,  1895.) 

Te'loblast  (reXo^,  end  ;  ^Aao-ro?,  a  germ),  large  cells  situated  at  the  growing  end 
of  the  embryo  (in  annelids,  etc.),  which  bud  forth  rows  of  smaller  cells.  (Whit- 
man, Wilson,  1887.) 

Telole'cithal  {riXo<i,  end;  AeV-i^o?.  yolk),  that  type  of  ovum  in  which  the  yolk  is 
mainly  accumulated  in  one  hemisphere.     (Balfour,  1880.) 

Telophases.  Telekinesis  (re'Ao?,  end),  the  closing  phases  of  mitosis,  during 
which  the  daughter-nuclei  are  re-formed.     (Heideniiain,  1894.) 

To'noplasts  {t6vo^,  tension;  TrAao-ro?.  form),  plast  ids  from  which  arise  the  vacuoles 
in  plant-cells.     (De  Vries,  1885. ) 

Trophoplasm  (rpoc^r;,  nourishment;  7rAaa-/xa).  i.  The  nutritive  or  vegetative 
substance  of  the  cell,  as  distinguished  from  the  idioplasm.  (.\A(iKi.i.  1884.) 
2.  The  active  substance  of  the  cytoplasm  other  than  the  "kinoplasm  •"  or  archo- 
plasm.     (Strasburger,  1892.) 

Tro'phoplasts  (rpo<^r/,  nourishment ;  TrAao-ros.  form),  a  general  term,  nearly  equiv- 


448 


GLOSSARY 


alent    to   the   "plastids"   of    Schimper,   including   "  anaplasts ''   (amyloplasts), 

"autoplasts"  (chloroplasts),  and  chromoplasts.     (A.  Meyer,  1882-83.) 
Yolk-nucleus,  a  word  of  vague  meaning  applied  to  a  cytoplasmic  body,  single  or 

multiple,  that  appears  in  the  ovarian  egg.     [Named  "  Dotterkern ''  by  Carus, 

1850.) 
Zy'gote  or  Zy'gospore  {t,vy6v,  a  yoke),  the  cell    produced   by  the  fusion  of  two 

conjugating  cells  or  gametes  in  some  of  the  lower  plants. 


GENERAL    LITERATURE-LIST 

The  following  list  includes  only  the  titles  of  works  actually  referred  to  in  the  text 
and  those  immediately  related  to  them.  For  more  complete  bibliographv  the  reader 
is  referred  to  the  literature-lists  in  the  special  works  cited,  especially  the  followinj^. 
For  reviews  of  the  early  history  of  the  cell-theory  see  Remak"s  Uutcrsucliuui^in 
('50-55).  Huxley  on  the  Cell-theory  ('53),  Sach's  History  of  Botany  and  Tyson's 
Cell-doctrine  ('78)-  An  exhaustive  review  of  the  earlier  literature  on  proto- 
plasm, nucleus,  and  cell-division  will  be  found  in  Flemming's  Zellsubstanz  ("82). 
and  a  later  review  of  theories  of  protoplasmic  structure  in  Butschli"s  Protoplasma 
('92)  and  in  Fischer's  Fixierung,  etc.,  des  Frotoplasmas  ('99).  The  earlier  work  on 
mitosis  and  fertilization  is  very  thoroughly  reviewed  in  Whitman's  Ckpsine  ('78), 
Fol's  Heiwgeiiie  ('79),  and  Mark's  Li)iiax  ('81).  For  more  recent  general  litera- 
ture-lists see  especially  Hertwig's  Zelle  iind  Gewebe  ("93, '98),  Yves  Delage  ('95), 
Henneguy's  Cellule  ("96),  Hacker's  Praxis  iind  Tlieorie  der  Zellen  und  Befnich- 
tiingslehre  ('99),  and  the  admirable  reviews  by  Flemming,  Boveri,  RUckert,  Sieves, 
Roux,  and  others  in  Merkel  and  Bonnet's  Ergebnisse  ('9i-'98). 

The  titles  are  arranged  in  alphabetical  order,  according  to  the  system  adopted  in 
Minot's  Human  Embryology .  Each  author's  name  is  followed  by  the  date  of  publi- 
cation (the  first  two  digits  being  omitted,  except  in  case  of  works  published  before 
the  present  century),  and  this  by  a  single  number  to  designate  the  paper,  in  case 
two  or  more  works  were  published  in  the  same  year.  For  example,  Boveri.  Th., 
'87,  2,  denotes  the  second  paper  published  by  Boveri  in  1887. 

In  order  to  economize  space,  the  following  abbreviations  are  used  for  the  journals 
most  frequently  referred  to  :  — 

ABBREVIATIONS 

A.  A.     Anatomischer  Anzeiger, 

A.  B.     Archives  de  Biologic. 

A.  A.  P.  Archiv  fiir  Anatomic  und  Physiologic. 

A.  m.  A.  Archiv  fiir  mikroscopische  Anatomic. 

A.  Entwni.  Archiv  fiir  Entwicklungsmcchanik. 

B.  C.  Biologisches  Centralhlatt. 

C.  R.  Comptes  Rend  us. 

J.  M.  Journal  of  Morphology. 

y.  w.  Bot.  Jahrhuch  fiir  wissenschaftlichc  Botanik. 

J.  Z.  Jenaischc  Zcitschrift, 

M.  A.  Muller's  Archiv. 

M.  J.  Morphologischcs  Jahrhuch, 

Q.J.  Quarterly  Journal  of  Microscopical  Science. 

Z.  A.  Zoologischer  Anzeiger. 

Z.  w.  Z.  Zcitschrift  fiir  wissenschaftlichc  Zoologie. 

ALBRECHT,  E.,  "98.  Untersuchungen  zur  Structur  des  Seeigeleies  :  Sitzb.  Gcs. 
Morph.  P/iys.  Munchen.,  3.  —  Altman,  R..  "86.  Studien  liber  die  Zelle.  I.  :  Leipzig. 
—  Id.,  "87.  Die  Genese  der  Zellen:  Leipzig.— \A.,'Q^.  f'ber  Nucleinsaure  :  A. 
A.  P.,  p.  524. —  Id.,  '90,  "94.     Die  Elementarorganismen  und  ihre  Beziehung  zu 

2  G  449 


450  GENERAL   LITERATURE-LIST 

den  Zellen  :  Leipzig. — Amelung,  E..  "93.     L'ber  mittlere  Zellgrosse  :  Flo;'a.\).  \j6. 

—  Andrews.  E.  A.,  98,  1.     Filose  Activities  in  Metazoan  Eggs:  Zool.  Bull..  II..  i. 

—  Id..  "98,  2.  Activities  of  Polar  Bodies  of  Cerebratulus  :  Arch.  Entium..  W.,  2.  — ■ 
Andrews.  G.  F.,  '97.  The  Living  Substance  as  Such  and  as  Organism  :  /.  /]/., 
XII.,  2,  Suppl.  — Arnold.  J..  '79.  Uber  feinere  Struktur  der  Zellen,  etc.  :  Virchow's 
Arch..,  1879.  (See  earlier  papers.) — Atkinson.  G.  F.,  '99.  Studies  on  Reduction 
in  Plants  :  Bot.  Gas.,  XXVIIl..  i.  2.  —  Auerbach.  L..  '74.  Organologische  Studien  : 
Ih'eslau.  —  Id..  '91.  Uber  einen  se.xuellen  Gegensatz  in  der  Chromatophilie  der 
Keimsubstanzen :  Sitz2i}is;sber.  der  K'onigl.  preitss.  Akad.  d.  IViss.  Berlin.  XXXV. 

—  Id.  "96.     Untersuchungen  Uber  die  Spermatogenese  von  Paludina  :  /.  Z..  XXX. 

VON  BAER.  C.  E.,  "28,  '37.  Uber  Entwickelungsgeschichte  der  Thiere.  Beo- 
bachtung  und  Reflexion:  I.  Konigsberg.  1828;  II.  1837.  —  Id.,  "34.  Die  Metamor- 
phose des  Eies  der  Batrachier  :  MYiller's  Arch.  — Balbiani,  E.  G..  "61.  Recherches 
sur  les  phenomenes  sexuels  des  Infusoires  :  Jonrn.  de  la  Phys..  IV.  —  Id..  "64.  Sur 
la  constitution  du  germe  dans  Toeuf  animal  avant  la  fecondation  :  C.  R..  LVIII. — 
Id.,  "76.  Sur  les  phenomenes  de  la  division  du  noyau  cellulaire :  C.  P..  XXX., 
October,  1876.  — Id..  '81.  Sur  la  structure  du  noyau  des  cellules  salivares  chez  les 
larves  de  Chironomus  :  Z.  A..  1881,  Nos.  99.  100.  —  Id..  "89.  Recherches  experi- 
mentales  sur  la  merotomie  des  Infusoires  cilies  :  Rec2ieil  Zool.  Suisse,  January.  1889. 

—  Id.,  '91,  1.  Sur  les  regenerations  successives  du  peristome  chez  les  Stentors  et 
sur  le  role  du  noyau  dans  ce  phenomene :  Z.  A..  372,  373.  —  Id.,  "91,  2.  Sur 
la  structure  et  division  du  noyau  chez  les  Spirochona  gemmipara :  Ann.  d. 
Micrographie.  —  Id., '93,  Centrosome  et  Dotterkern :  y.??/;';/.  de  Vanat.  et  de  la 
physiol..  XXIX. — Balfour,  F.  M.,  '80.  Comparative  Embryology:  I.  1880, — 
Ballowitz,  '88-'91.  Untersuchungen  liber  die  Struktur  der  Spermatozoen  :  i .  (birds) 
^.  ;;/.  ^..  XXXII.,  1888;  2.  (insects)  Z.w.Z..  LX,,  1890;  3.  (fishes,  amphibia, 
reptiles)  A.  m.  A.,  XXXVI.,  1890;  4.  (mammals)  Z.  w.  Z..  1891. — Id..  '89. 
Fibrillare  Struktur  und  Contractilitat :  Arch.  ges.  Phys.,  XLVL— Id.,  '91,  2. 
Die  innere  Zusammensetzung  des  Spermatozoenkopfes  der  Saugetiere  :  Centralb.  f. 
Phys..  V.  —  Id.,  '95.  Die  Doppelspermatozoa  der  Dytisciden  :  Z.  iu.  Z..  XLV..  3.  — 
Id..  "97.  Uber  Sichtbarkeit  und  Aussehen  der  ungetarbten  Centrosomen  in  ruhen- 
den  Gewebszellen :  Z.  w.  Mic.  XIV.  —  Id.,  '98.  Zur  Kenntniss  der  Zellsphare : 
Arch.  Anat.  Phys.,  "'98,  II..  III.  —  Van  Bambeke,  C,  "93,  Elimination  d'elements 
nucleaires  dans  I'oeuf  ovarien  de  Scorpaena  scrofa  :  A.  B.,  XIII.,  i. — Id.,  "96.  De 
Temploi  du  terme  Protoplasma  :  Bull.  Soc.  Beige.  Mic,  XXII.  — Id.,  '97.  A  propos 
de  la  delimitation  cellulaire:  Ibid.,  XXIII.  —  Id,,  '98.  Recherches  sur  I'oocyte 
de  Pholcus  phalangioides  :  A.  B.,  XV,  —  De  Bary,  "58.  Die  Conjugaten.  — Id..  '62. 
tJber  den  Bau  und  das  Wesen  der  Zelle  :  Flora,  1862.  — Id..  "64.  Die  Mycetozoa  : 
2d  Ed.,  Leipzig. — Barry,  M.  Spermatozoa  observed  within  the  Mammiferous 
Ovum:  Phil.  Trans..,  1843.  — Beale,  Lionel  S.,  '61.  On  the  Structure  of  Simple 
Tissues  of  the  Human  Body:  London.  — Bechamp  and  Estor,  "82.  De  la  consti- 
tution elementaire  des  tissues:  Montpellier. — Belajeff,  "W.,  '89.  Mittheilung 
Uber  Bau  und  Entwicklung  der  Spermatozoiden  :  Ber.  D.  Bot.  Ges. — Id..  "92.1. 
Uber  den  Bau  und  die  Entwicklung  der  Antherozoiden,  I.,  Characeen.  —  Id.,  '92.  2. 
Uber  die  Karyokinesis  in  den  Pollenmutterzellen  bei  Larix  und  Fritillaria :  Sitzb. 
Warsch.  Naturf.  Ges.  —  Id..  '94.  1.  Zur  Kenntniss  der  Karyokinese  bei  den 
Pflanzen  :  Flora.  1894,  Erganzungsheft. — Id.,  '94,2.  Uber  Bau  und  Entwicklung 
der  Spermatozoiden  der  Pflanzen:  Flora,  LIV.  —  Id.,  '97.  1.  Uber  den  Neben- 
kern  in  Spermatogenen  Zellen  und  die  Spermatogenese  bei  den  Farnkrauten :  Ber. 
D.  Bot.  Ges..,  XV.  —  Id.,  '97,  2.  Uber  die  Spermatogenese  bei  den  Schachtel- 
halmen :  Ibid.  —  Id.,  '97,  3.  Uber  die  Aehnlichkeit  einiger  Erscheinungen  in 
der  Spermatogenese  bei  Thieren  und  Pflanzen:  Ibid.  —  Id.,  '97,  4.     Einige  Streit- 


GENERAL  LITERATURE-LIST  45  I 

fragen  in  den  Untersuchungen  liber  die  Karyokinese :  Ibid.  — Id..  "98.  1.  ("'her 
die  Reductionstheilung  des  Pflanzenkerns :  Ibid.,  XVI. — Id..  98.  2.  i'ber 
die  Cilienbildner  in  den  spermatogenen  Zellen :  Ibid. — Id.,  '99.  ("ber  die 
Centrosomen  in  den  .spermatogenen  Zellen:  Ibid.,  XVII.,  6.  —  Benda.  C.  "87. 
Untersuchungen  liber  den  Bau  des  funktionirenden  Samenkenkanalchens  einigcr 
Saugethiere:  A.  m.  A.— Id..  "93.  Zellstrukturen  und  Zclltheilungen  des  Sala- 
manderliodens :  /V;-//.  d.  Anat.  Ges.,  1893.  —  Van  Beneden.  E..  "70.  Recher- 
ches  sur  la  composition  et  la  signification  de  Ta-ut":  JAV//.  tour,  dc  I'Al.  rov. 
d.  S.  de  Belgiqut\  1870.  —  Id.,  "75.  La  maturation  de  Toeuf,  la  fecondation  et  Tes 
premieres  phases  du  developpement  embryonnaire  des  mammifcres  d'aprcs  des 
recherches  faites  chez   le    lapin :    Bull.    Ac.    roy.   de  Belt^ique,  XI.  —  Id..    76.    1. 

Recherches    sur   les    Dicyemides :    Bull.    Ac.    roy.    Beli(iqut\   XLI.,    XLII. Id. 

"76,  2.     Contribution  a  Thistoire  de  la  vesicule  germinative  et  du  premier  novau 
embryonnaire  :  Ibid.,  XLI.  ;  also  Q.J.,  XVL  — Id..  '83.     Recherches  sur  la  matura- 
tion de  Tceuf,  la  fecondation  et  la  division  cellulaire  :  A.  B.,  W . — Van  Beneden 
and  Juliu.  "84.  1.     La  segmentation  chez  les  Ascidiens  et  ses  rapports  avec  Turgani- 
sation  de  la  larve :   Ibid.,V.  —  Id..    '84,   2.     La   spermatogenese   chez   TAscaride 
megalocephale :  Bull.  Ac.  roy.  Belgiquc,   3me  ser..  VII.  — Van   Beneden.  E..   et 
Neyt.  A.,  "87.     Nouvelles  recherches  sur  la  fecondation  et  la  division  mitosique 
chez  TAscaride  megalocephale:  Ibid..,  1887. — Bergh.  R.  S..  "89.     Recherches  sur 
les  noyaux  de  TUrostyla  :  A.  B.  IX.  — Id..  '94.     Vorlesungen  liber  die  Zelle  und  die 
einfachen  Gewebe  :    Wiesbaden.  —  Id., '95.     Uber  die  relativen  Theilungspotenzen 
einiger  Embryonalzellen :   A.  Ent?)i.,  II.,  2.  —  Bernard.  Claude.     Lc(;ons  sur  les 
Phenomenes  de  la  Vie:    ist  Ed.    1878,  2d  Ed.   1885.  Paris.  —  Berthold.  G..  "86. 
Studien  liber  Protoplasma-mechanik  :  Leipzig.  —  Bickford.  E.  E..  "94.     Notes  on 
Regeneration  and  Heteromorphosis    of  Tubularian    Hydroids :   /.    J/..    IX.,    3. — 
Biondi.  D..  "85.     Die  Entvvicklung  der  Spermatozoiden  :  A.  in.  A..  XX  \' .  —  Blanc. 
H.,   "93.      Etude  sur  la  fecondation  de  I'oeuf  de  la  truite  :  Ber.  iVaturf'orsc/i.  Ges. 
zu  Freiburg.,  VIII.  — Blochmann,  F.,  '87,  2.     I'ber  die  Richtungskorper  bei  Insek- 
teneiern  :  M.  J.,  XII. — Id.,  '88.     Uber  die  Richtungskorper  bei  unbefruchtet  sich 
entwickelnden  Insekteneiern  :    Vcrii.naturh.  nied.  I'er.  Heidelberg  \.  F..  I\'.,  2. — 
Id.,  '89.     Uber  die  Zahl  der  Richtungskorper  bei  befruchteten  und  unbefruchteten 
Bieneneiern  :  M.J.  — Id..  '94.     Uber  die  Kerntheilung  bei  Euglcna  :  />'.  C,  XIV.  — 
Bohm,  A..  "88.     Uber  Reifung  und  Befruchtung  des  Eies  von  Petromyzon  Planeri : 
A.  m.  A.,  XXXII.  —Id..  "91.     Die  Befruchtung  des  Forelleneies  :  Sit'z.-Iicr.  d.  Ges. 
f.  Morph.  u.  Phys.  Munchen,  VII.  —  Boll.  Fr..  '76.     Das  Princip  des  Wachsthums  : 
Berlin.  —  Bonnet,  C.  1762.     Considerations  sur  les  Corps  organi.ses  :  Amsterdam. 
—  Born.  G.,  '85.     Uber  den  Einfluss  der  Schwere  auf  das  Froschei :    ./.   ///.  .-/., 
XXIV.  —  Id.,    '94.       Die    Structur   des    Keimblaschens    im    Ovarialei    von    Triton 
taeniatus  :  A.  ;;/.  A.,  XLIII.  —  Bourne.  G.  C,  '95.     A  Criticism  of  the  Cell-theory  ; 
being  an  Answer   to   Mr.  Sedgwick's    Article  on  the  Inadequacy   of  the   Cellular 
Theory  of  Development :    <2.  /•.   XXXVIII..    i.— Boveri.    Th..    Se.     Uber   die 
Bedeutung  der  Richtungskorper:  Sitz.-Ber.   Ges.  Morpli.  u.  P/iys.  M'lhic/ien,  II. — 
Id.,  '87,  1.     Zellenstudien,  Heft  I. ;  /.  Z.,  XXI.  —Id..  '87.  2.      C'ber  die  Befruch- 
tung der  Eier  von  ^hYvrr/i- ///r<,7?^^v///r7  A/ ;  Sdz.-Per.  Ges.  MorpJi.  Phys.  M'tinchen, 
III. — Id.,  "87,  2.      t'ber  den  Anteil  des  Spermatozoon  an  der  Teilung  des  Eies: 
Sitz.-Ber.  Ges.  Morph.  Phys.  Miinchen,  III.,  3.  —  Id.,  "87,  3.      C'ber  Dif^erenzierung 
der  Zellkerne  wahrend  der  Furchung  des  Eies  von  Ascaris  meg.:   -/.  ./.,   1887. — 
Id.,  '88, 1.     Uber  partielle  Befruchtung  :  Sitz.-Ber.  Ges.  Morph.  Phys.  .Mian  hen.  I  \'., 
2.  — Id..  '88.  2.     Zellenstudien.  II.:  /.  Z..  XXII.— Id..  "89.     Ein  geschlechtlich 
erzeugter  Organismus  ohne  mlitterliche  Eigenechaften  :  Sitz.-Ber.  Ges.  Morph.  Phys. 
Munchen,  V.     Trans,  in.-^w.  Nat.,  March,  "93.  —Id..  "90.     Zellen.studien.  Heft  III.  : 
/.  Z.,  XXIV.  —  Id..  '91.     Befruchtung  :  Merkel  und  Bonnet's  Ergebnisse,  I.  —  Id., 


452 


GENERAL  LITERATURE-LIST 


•95.  1.  Uber  die  Befmchtungs-  und  Entwickelungsfaihigkeit  kernloser  Seeigel-Eier, 
etc.  :  A.  Entwiu.  II..  3.  —  Id.,  "95,  2.  tJber  das  Verhalten  der  Centrosomen  bei  der 
Befruchtung  des  Seeigeleies,  nebst  allgemeinen  Bemerkungen  liber  Centrosomen 
und  Verwandtes:  Verh.  d.  Fhysikal.-nied.  Gesellschaft  zu  IVih'zbiirg,  N.  F., 
XXIX.,  I. — Id..  "96.  Zur  Physiologie  der  Kern-  und  Zellteilung:  Sitzb.  Phys.- 
Med.  Ges.  W'urzburg.  —  Braem.  F..  "93.  Das  Prinzip  der  organbildenden  Keim- 
bezirke  und  die  entwicklungsmechanischen  Studien  von  H.  Driesch  :  B.  C  XIII., 

4.5. Brandt,  H..    11.     L'ber  Actinosphaerium  Eichhornii :  Dissertation.  Halle, 

1877. Brass.   A.,    "83-4.       Die    Organisation   der   thierischen    Zelle :    Halle.— 

Brauer,  A..  "92.  Das  Ei  von  Branchipus  Grubii  von  der  Bildung  bis  zur  Ablage : 
Abh.  preuss.  Akad.  IViss.,  92.  — Id.,  *93, 1.  Zur  Kenntniss  der  Reifung  des  par- 
thenogenetisch  sich  entwickelnden  Eies  von  Artemia  Salina :  A.  ni.  A.,  XLIII. — 
Id.,  "93,  2.  Zur  Kenntniss  der  Spermatogenese  von  Ascaris  megalocephala : 
A.  in.  A..,  XLII.  —  Id.,  "94.  Uber  die  Encystierung  von  Actinosph^erium  Eich- 
hornii:  Z.  iv.  Z.,  LVIII.,  2.  —  Braus.  "95.  Tber  Zellteilung  und  Wachstum  des 
Tritoneies:  /.  Z..  XXIX. —  Brooks.  W.  K..  "83.  The  Law  of  Heredity:  Balti- 
more. Brown.  H.  H.,  "85.  On  Spermatogenesis  in  the  Rat :  Q.  /.,  XXV.  — 
Brown.  Robert.  "33.  Observations  on  the  Organs  and  Mode  of  Fecundation  in 
Orchides  and  Asclepiadeae :  Trans.  Linn.  Sac.  1833.  Brucke,  C,  "61.  Die  Ele- 
mentarorganismen  :  Wiener  Sitzbcr.,  XLIV.,  i86r .  Brnnn.  M.  von,  '89.  Beitrage 
zur  Kenntniss  der  Samenkorper  und  ihrer  Entwickelung  bei  Vogeln  und  Sauge- 
thieren:  A.  in.  A..  XXXIII.  -De  Bruyne,  C,  '95.  La  sphere  attractive  dans  les 
cellules  fixes  du  tissu  conjonctif:  Bull.  Acad.  Sc.  de  Belgique,  XXX.— Burger.  O., 
'91.      Uber  Attractionsspharen  in  den  Zellkorpern  einer  Leibesflussigkeit :  A.  A.y 

VI. Id. ."92.      Was  sind  die  Attractionsspharen  und  ihre  Centralkorper?  A.  A.j 

1892. Butschli.  O..  "73.     Beitrage  zur  Kenntniss  der  freilebenden  Nematoden  : 

Nova  acta  ac ad.  Car.  Leopold,  XXX VI.  — Id.,  "75.  Vorlaufige  Mitteilungen  uber 
Untersuchungen  betrelTend  die  ersten  Entwickelungsvorgange  im  befruchteten  Ei 
von  Nematoden  und  Schnecken :  Z.  w.  Z.,  XXV.  — Id.,  '76.  Studien  liber  die 
ersten  Entwickelungsvorgange  der  Eizelle,  die  Zellteilung  und  die  Konjugation  der 
Infusorien:  Ab/i.  des  Senckenb.  Naturforscher-Ges..X.  —  Id.,  '85.  Organisations- 
verhaltnisse  der  Sog.  Cilioflagellaten  und  der  Noctiluca :  J/.  /..  X.  —  Id., '90. 
fber  den  Bau  der  Bakterien.  etc.:  Leipzig.  — ldi.,'^\.  Uber  die  sogenannten 
Centralkorper  der  Zellen  und  ihre  Bedeutung :  Verh.  N'aturhist.  Med.  Ver.  Heidel- 
berg 1891.  — Id.,  '92.  1.  t^ber  die  klinstliche  Nachahmung  der  Karyokinetischen 
Fi'^-uren:  Ibid.,  N.  F.,  V.  —  Id.,  '92.  2.  Untersuchungen  liber  mikroskopische 
Schaume  und  das  Protoplasma  (full  review  of  literature  on  protoplasmic  structure)  : 
Leipzig  {Engelniann).  —  1A..  '94.  Vorlaufige  Berichte  liber  fortgesetzte  Unter- 
suchungen an  Gerinnungsschaumen,  etc.  :  Verh.  Natnrhist.  Ver.  Heidelberg^  V. 
—  Id..  '96.  Weitere  Ausflihrungen  liber  den  Bau  der  Cyanophyceen  und  Bakterien  : 
Leipzig.  —Id.;  '98.     Untersuchungen  liber  Strukturen  :  Leipzig  {Engelniann). 

CALKINS.  G.  N..  '95,  1.  Observations  on  the  Yolk-nucleus  in  the  Eggs  of 
Lumbricus:  Trans.  N.Y.  Acad.  Sci..  June.  1895. —  Id., '95,  2.  The  Spermato- 
genesis of  Lumbricus  :  /.  M.,  XL,  2.  —Id.,  '97.  Chromatin-reduction  and  Tetrad- 
formation  in  Pteridophytes :  Bull.  Torrey  Bot.  Club.  XXI\\— Id..  '98,  1.  The 
Phylogenetic  Significance  of  Certain  Protozoan  Nuclei:  Ann.  N.  V.  Acad.  Sci..  XL, 
16.  —Id..  "98,  2.  Mitosis  in  Noctiluca  :  Ginn  &  Co.,  Boston,  also/.  J/.,  XV.,  3. — 
Calberla.  E.,  '78.  Der  Befruchtungsvorgang  beim  Ei  von  Petromyzon  Planeri : 
Z.w.Z.,  XXX.  — CampbeU.  D.  H.,  "88-9.  On  the  Development  of  Pilularia 
globulifera  :  Ann.  Bot..  II.  —  Carnoy,  J.  B.,  '84.  La  biologic  cellulaire  :  Lierre.  —■ 
Id.,  '85.  La  cytodierese  des  Arthropodes  :  La  Cellule.  I.  — Id..  "86.  La  cytodie'- 
rese  de  l"oeuf:  La  Cellule,  III.— Id.,  '86.      La  vesicule  germinative  et  les  globules 


GENERAL   LITERATURE-LIST 


453 


polaires  chez  qiielqiies  Nematodes:  La  CcUnlc,  III.  —  Id..  "86.  La  segmentation 
de  IVieuf  chez  les  Nematodes:  La  Cellule,  III.,  i.  —  Canioy  and  Le  Brun.  "97.  1. 
'98.  "99.  La  ve'sicule  genninative  et  les  globules  polaires  clic/.  1l-s  lialracicns  :  La 
Cellule.  XII,  XIV,  XVI.  — Id..  "97,  2.  La  fecondation  chez  TAscaris  megalo- 
cephala:  La  Cellule.  XIII. —  Castle.  W.  E.,  "96.  Tiic  Early  Kmbrvology  of  Ciona 
intestinalis  :  Bull.  Mus.  Coiitp.  Zo'dl.y  XXVIL,  7.  —  Chabiy.  L..  "87.  Conlrihu- 
tions  a  Tembryologie    normale  et  pathologique  des  ascidies  simples:  Paris.  1887. 

—  Child.  C.  M..  "97.  The  Maturation  and  Fertilization  of  the  Egg  of  Arenicola  : 
Trans.  iV.  V.  Acad.  Sci.,  X\'I.  —  Chittenden,  R.  H..  "94.  Some  Recent  Chemico- 
physiological  Discussions  regarding  the  Cell:  .liii.Nat..  X.W'IIl.,  P'eb.,  1894. — 
Chun.  C,  "90.  Uber  die  Bedeutung  der  direkten  Zelltheilung  :  .Sitzb.  .Silir.  Phvsik.- 
Okoii.  Ges.  I\dnigsberg,  1890.  —  Id.,  "92.  1.  Die  Di.ssogonie  der  Kippenquallen  : 
Festschr.  f.  Leuckart,  Leipsio;^  1892.  —  Id.,  "92.  2.  (In  Rou.\,  "92,  p.  55) :  \'erli.  d. 
Anat.  Ges..V\..,  1892.  —  Clapp.  C.  M..  "91.  Some  Points  in  the  Development  of 
the  Toad-Fish  :/.  J/.,  V.  —  Clarke.  J.  Jack.son.  "95.  Observations  on  various 
Sporozoa:  Q.J.,  XXXVIL,  3.  — Coe,  W.  R..  "99.  The  Maturation  and  Fertiliza- 
tion of  the  Egg  of  Cerebratulus  :  Zo'dl.  Jalirb.,  XII.  —  Cohn.  Ferd..  "51.  Xachtrage 
zur  Naturgeschichte  des  Protococcus  pluvialis  :  Nova  Acta.  XXII.  —  Conklin,  E.  G.. 
'94.  The  Fertilization  of  the  Ovum:  Biol.  Led..  Marine  Biol.  Lab..  Wood's  IIolL 
Boston.,  1894.  —  Id.,  "96.  Cell-size  and  Body-size:  Kept,  of  Am.  Morp/i.  Soc. 
Science,  III.,  Jan.  10,  1896. — Id.,  '97,  1.  Nuclei  and  Cytoplasm  in  the  Intestinal 
Cells  of  Land  Isopods  :  An/.  Nat..  Jan.  —  Id.,  '97,  2.  The  Embryology  of  Crepidula  : 
/.  J/.,  XIII.,  I.  — Id.,  "98.  Cleavage  and  Differentiation:  Wood's' LI  oil  Biol.  Lec- 
tures.—  Id..  '99.  Protoplasmic  Movement  as  a  Factor  in  Differentiation:  Wood's 
LL oil  Biol.  Lectures.  —  Crampton,  H.  E.,  '94.  Reversal  of  Cleavage  in  a  Sinistral 
Gasteropod:  Ann.  N.  V.  Acad.  Sci.,  March.  1894. —Id.,  "97.  The  Ascidian  Halt- 
Embryo:  Lbid.,  June  19. — Id..  '99.  The  Ovarian  History  of  the  Egg  of  .Molgula: 
/.  J/.,  XV.,  Suppl.  —  Crampton  and  Wilson,  "96.  E.xperimental  Studies  on 
Gasteropod  Development  (H.  E.  Crampton).  Appendi.v  on  Cleavage  and  Mosaic- 
Work  (E.  B.  Wilson)  :  A.  Entwni..  III.,  i.  — Czermak.  N..  "99.  C'ber  die  Desin- 
tegration  und  die  Reintegration  des  Kernkorperchens,  etc.:  A.  A..  X\'..  22. 

DARWIN,  F.,  '77.  On  the  Protrusion  of  Protoplasmic  Filaments,  e/c.  :  Q.J. 
XVII. — Davis,  B.  M.,  '99.  The  Spore-mother-cell  of  Anthoceros  :  Bot.  Gas., 
XXVIIL,  2.  —  Debski,  B.,  '97.  Beobachtungen  liber  Kerntheilung  bei  Chara : 
/.  w.  B.,  XXX.— Id.,  "98.     Weitere  Beobachtungen  an  Chara:  Ibid.,  XXXII..  4. 

—  Delage,  Yves,  "95.  La  Structure  du  Protoplasma  et  les  Theories  sur  rh<5reditc< 
et  les  grands  Problemes  de  la  Biologic  Generale  :  Paris,  1895.  —  Id..  "98.  Embry- 
ons  sans  noyau  maternel :  C.  P.,  CXXVIL,  15.— Id..  "99.  La  fecondation  m<^ro- 
gonique  et  ses  resultats :  C.  A'.,  Oct.  23.  —  Demoor,  J.,  "95.  Contribution  h 
Tetude  de  la  physiologic  de  la  cellule  (inde'pendance  fonctionelle  du  protoplasme  et 
du  noyau)  :  A.  B.,  XIIL  — Dendy.  A.,  "88.  Studies  on  the  Comparative  Anatomy 
of  Sponges:  Q.J.,  Dec,  1888.  — Dixon,  H.  H.,  "94.  Fertilization  of  P/nus :  Ann. 
Bot.,  VIII.  —  Id..  '96.  On  the  Chromosomes  of  Lilium  longlitli)rum  :  Proc.  P.  Ir. 
Ac,  III.  — Doflein,  F.  J.,  "97,  1.  Die  Eibildung  bei  Tubularia :  Z.  w.  Z.,  LXIL, 
I.  — Id.,  '97,  2.  Karyokinesis  des  Spermakerus  :  ./.  ///.  ./..  L.  2.  — Dogiel.  A.  S.. 
'90.  Zur  Frage  liber' das  Epithel  der  Harnblase  :  A.  ni.  A.,  XXW.  —  Driesch. 
H..  "92,  1.  Entwickelungsmechanisches  :  ./../..  VII.,  18.  — Id.  Enlwicklungs- 
mechanische  Studien,  L.^IL,  1892.  Z.  ic.  Z..  LIIL:  III. -VI..  1893,  Ibid.,  LV.  ; 
VII.-X.,  1893  :  JLdt.  Zool.  St.  Neapel,  XL,  2.  —Id..  "94.  Analytische  Theorie  der 
organischen  Entwicklung :  Leipcii^. —Id.,  '95.  1.  \'on  der  Entwickelung  einzelner 
Ascidienblastomeren  :  A.  Entwm..  L,  3-  — ^^i..  ^S-  2.  Zur  Analysis  der  Potenzen 
embryonaler   Organzellen :    Ibid.y    II.  — Id.,    "98,1.      I'bcr   den  Organisation  des 


454  GENERAL  LITERATURE-LIST 

Eies  :  Etitwm.,  IV. — Id..  ""SS,  2.  Von  der  Beendigung  morphogener  Elemen- 
tarprocesse:  Arch.  Entwfn..  VI. —- Id., '98.  3.  Ueber  rein-miitterliche  Charaktere 
an  Bastardlarven  von  Echiniden  :  Ibid.,  \'II.,  i. — Id.,  "99.  Die  Localisation  mor- 
phogenetischer  Vorgange  :  Ibid.,  VIII.,  i.  —  Driesch  and  Morgan,  "95,  2.  Zur 
Analysis  der  ersten  Entwickelungsstadien  des  Ctenophoreneies :  Ibid.,  II.,  2. — 
Druner,  L..  "94.  Zur  Morphologie  der  Centralspindel :  /.  Z.,  XXVIII.  (XXI.).  — 
Id.,  "95.  Studien  iiber  den  Mechanismus  der  Zelltheilung  :  Ibid.,  XXIX.,  2.  —  Dii- 
sing,  C,  "84.     Die  Regulierung  des  Geschlechtsverhaltnisses :  Jena,  1884. 

VON  EBNER,  V.,  '71.  Untersuchungen  iiber  den  Ban  der  Samencanailchen  und 
die  Entwicklung  der  Spermatozoiden  bei  den  Saugethieren  und  beim  Menschen : 
Inst.  PJiys.  It.  Hist.  Graz.,  187 1  {Leipzig).  —  Id.,  '88.  Zur  Spermatogenese  bei 
den  Saugethieren:  A.  ni.  A.,  XXXI.  —  Ehrlich.  P.,  '79.  Uber  die  specifischen 
Granulationen  des  Blutes :  A.  A.  P.  {FJiys.),  1879,  P-  573- — Eisen,  G.,  "97. 
Plasmocytes  :  Froc.  CaL  Acad.  Sci.,  I.,  i.  —  Id.,  "99.  The  Chromoplasts  and  the 
Chromioles  :  B.  C.  XIX..  4. — Eismond,  J..  '95.  Einige  Beitrage  zur  Kenntniss 
der  Attraktionsspharen  und  der  Centrosomen :  A.A.,X. — Endres  and  Walter, 
'95.  Anstichversuche  an  Eiern  von  Rana  fusca:  A.  Entwni.,  II.,  i.  — Engelniann, 
T.  "W.,  "80.  Zur  Anatomic  und  Physiologic  der  Flimmerzellen  :  Arch.  ges.  Phys., 
XXIII.  — von  Erlanger,  R.,  '96.  1.  —  Die  neuesten  Ansichten  iiber  die  Zelltheilung 
und  ihre  iMcchanik :  ZooL  Centralb.,  III..  2. — Id.,  '96,  2.  Zur  Befruchtung  des 
Ascariseies  nebst  Bemerkungen  iiber  die  Struktur  des  Protoplasmas  und  des  Centro- 
somas:  Z.  A..,  XIX.  —  Id..  "96.  3.  Neuere  Ansichten  iiber  die  Struktur  des  Proto- 
plasmas :  ZooL  Centralb.,  IIL,  8,9. — Id.,  '96.  4.  Zur  Kenntniss  des  feineren 
Baues  des  Regenwurmhodens.  etc.  :  A.  in.  A..  XLVII.  —  Id..  "96.  5.  Die  \'ersoni- 
sche  Zelle  :  ZooL  Centralb.,  III..  3.  — Id.,  "96.  6.  Die  Entwicklung  der  mannlichen 
Geschechtszellen  :  Ibid..  III..  12.  —  Id.,  "97,  1.  Uber  Spindelreste  und  den  echten 
Nebenkern,  etc.:  ZooL  Centralb.,  IV.,  i. — Id.,  '97,  2.  Uber  die  sogenannte 
Sphare  in  den  mannlichen  Geschlechtszellen  :  Ibid.,\\'.,^. — Id..  '97.  3.  Uber 
die  Chromatinreduktion  in  der  Entwicklung  der  mannlichen  Geschlechtszellen : 
Ibid.,  IV.,  8.  —  Id..  '97. 4.  Beitrage  zur  Kenntniss  des  Protoplasmas,  etc.  A.  ni.  A.., 
XLIX.  —  Id.,  '97,5.  Uber  die  Spindelbildung  in  den  Zellen  der  Cephalopoden 
Keimscheibe :  B.  C,  XVII.,  20. — Id.,  '98.  Uber  die  Befruchtung.  etc..  des 
Seeigeleies  :  B.  C  XVIII..  i.  — Errera.  '86.  Eine  fundamentale  Gleichgewichtsbe- 
dingung  organischen  Zellen:  Ber.  Dentsch.  Bot.  Ges.,  1886. — Id..  '87.  Zellformen 
und  Seifenblasen  :  Tagebl.  der  60  Versaminlnng  deutscher  Naturforscher  imd  Aerzte 
Z2i  Wiesbaden.,  1887. 

FAIRCHILD.  D.  G..  '97.  t'ber  Kerntheilung  und  Befruchtung  bei  Basidio- 
bolus:  JaJirb.  luiss.  Bot.,  XXX. — Farmer,  J,  B.,  '93.  On  nuclear  division  of  the 
pollen-mother-cell  of  Lilium  Martagon  :  Ann.  Bot.  VII..  27. — Id.,  '94.  Studies  in 
Hepaticae:  Ibid..,  VIII.,  29. — Id.,  '95,  1.  Uber  Kernteilung  in  Lilium-Antheren, 
besonders  in  Bezug  auf  die  Centrosomenfrage  :  Flora.  1895,  p.  57.  — Id.,  95,  2.  On 
Spore-formation  and  Nuclear  Division  in  the  Hepaticse  :  Ann.  Bot.,  IX. — Farmer 
and  Moore,  '95.  On  the  essential  similarities  existing  between  the  heterotype 
nuclear  divisions  in  animals  and  plants  :  A.  A.,  XL,  3. — Farmer  and  "Williams, 
'96.  On  Fertilization,  etc..  in  Fucus :  Ann.  Bot.,  X. — Fick.  R.. '93.  Uber  die 
Reifung  und  Befruchtung  des  Axolotleies  :  Z.  lu.  Z.,  LVI..  4. — Id..  '97.  Bemer- 
kungen zu  M.  Heidenhain's  Spannungsgesetz :  Arch.  Anat.  n.  Phys.  (Anat.). — 
Fiedler,  C.  "91.  Entwickelungsmechanische  Studien  an  Echinodermeneiern : 
Festschr.  N^'ds^eli  n.  Kolliker,  Zurich.  1891. — Field.  G.  "W..  '95.  On  the  Mor- 
phology and  Physiology  of  the  Echinoderm  Spermatozoon:  /.  J/..  XI. — Fischer, 
A., '94,  1.     Zur  Kritik  der  Fixierungsmethoden  der  Granula :   A.  A.,  IX.,  22. — 


GENERAL  LITERATURE-LIST  455 

Id.,  94,  2.— t'ber  die  Geisseln  einiger  Flagellaten :  /.  w.  //.  XW'II.  — Id., 
'95.  Neue  Beitrage  zur  Kritik  der  Fixiemngsmethoden  :  - /.  ^/.,  X.  —  Id..  '9?! 
Untersuchungen  liber  den  Bau  der  Cyanopliyceen  und  Baktcrien  :  Jena,  Fischer.  — 
Id.,  "99.  Fixierung,  Farbung  und  Bau  de.s  Protopia.sma.s  :  //^/^/.  —  Flemming.  'W., 
'75.  Studien  in  der  Entwicklungsge.schichle  der  Xajaden  :  Sitzb.  d.  k.  k.  Akad. 
Wiss.  Wien,  LXXI.,  3.  —  Id.,  '79,  1.  Beitrage  zur  Kenntni.s.s  der  Zelle  und  Hire 
Lebenserscheinungen,  I.  :  A.  m.  A.,  XVI.  —Id.,  '79,  2.  C'ber  das  Vcrlialten  des 
Kerns  bei  der  Zelltheilung,  etc.:  Virchow's  Arch.,  LXXVII.— Id..  80.  lieitrage 
zur  Kenntniss  der  Zelle  und  ihrer  Lebenserscheinungen.  II.  :  ./.  ///.  A.,  XIX.  —Id.. 
'81.  Beitrage  zur  Kenntniss  der  Zelle  und  ihrer  Lebenserscheinungen.  111.  :  IhuL, 
XX. — Id.,  '82.  Zellsubstanz,  Kern  und  Zellteilung  :  Lcipzii^,  1882.  — Id..  "87. 
Neue  Beitrage  zur  Kenntniss  der  Zelle:  .;.  ///.  ^.,  XXIX. —  Id.,  '88.  Wciicre 
Beobachtungen  uber  die  Entwickelung  der  Sperniatosomen  Ijei  Salamandra  maculosa  • 
Ibid.,  XXXI.  — Id.,  '91-97.  Zelle,  I.-VI.  :  Er^i^cbn.  Anat.  u.  Entioicklum^s^esch. 
{Merkel  and  Bonnet^,  1891-97.  —  Id.,  -91. 1.  Attraktionsspharen  u.  Centralklirper 
in  Gevvebs-  u.  Wanderzellen  :  A.  A.  —  Id.,  '91,  2.  Neue  Beitrage  zur  Kenntniss  der 
Zelle,  II.  Teil:  A.  m.  A.,  XXXVII.— Id.,  '95,  1.  C'ber  die  Struktur  der  Spinai- 
ganglienzellen  :  VerhandL  der  anat.  Gesellschaft  in  Basel,  i-j  Ajjril,  1895,  P-  '9-  — 
Id.,  '95,  2.  Zur  Mechanik  der  Zelltheilung:  A.  in.  A.,  XLVI.  —  Id.,  *97.  2.  Ucbcr 
den  Bau  der  Bindegewebszellen,  etc.:  Zeit.  iSV^/..  XXXIV. —Floderus,  M..  "96. 
Uber  die  Bildung  der  Follikelhlillen  bei  den  Ascidien  :  Z.  lu.  Z.,  LXI..  2.  —  Fol.  H.. 
'73.  Die  erste  Entwickelung  des  Geryonideies  :  /.  Z.,  VII.  —Id.,  75.  Etudes  sur 
le  de'veloppement  des  Mollusques.  — Id.,  '77.  Sur  le  commencement  de  Thcnogenie 
chez  divers  animaux  :  Arch.  Sci.  Nat.  et  Phys.  Genh>e,  LVIII.  See  also  Arch.  'zod/. 
Exp.,  VI.  —  Id.,  '79.  Recherches  sur'la  fecondation  et  la  commencement  de  rh<f- 
nogenie:  Mem.  de  la  Soc.  de  physique  et  dliist.  nat.,  Gentve,  XXVI.— Id..  "91. 
Le  Quadrille  des  Centres.  Un  episode  nouveau  dans  Thistoire  de  la  fecondation  : 
Arch,  des  sci.  phys.  et  nat.,  15  A-uril,  1891  ;  also,  A.  A.,  9-10,  1891. — Foot,  K.. 
'94.  Preliminary  Note  on  the  Maturation  and  Fertilization  of  .Allolobophora  :  /.  .1/.. 
IX.,  3,  94.  —  Id.,  '96.  Yolk-nucleus  and  Polar  Rings:  Ibid.,  XII.,  i.  — Id..  '97. 
The  Origin  of  the  Cleavage  Centrosomes  :  /.  J/.,  XII.,  3. — Francotte.  P..  "97. 
Recherches  sur  la  maturation,  etc.^  chez  les  Polyclades  :  Mem.  coiir.  Acad.  Sci.  Belt^. 

—  Frenzel,  J.,  '93.  Die  Mitteldarmdrlise  des  Flusskrebses  und  die  amitotische 
Zelltheilung:  A.  m.  A.,  XLI.  —  Fromman,  C,  '65.  Cber  die  Struktur  tier  Binde- 
substanzzellen  des  Rlickenmarks  :  Centrl. /.  mcd.  ll'iss..  III.,  6.  —  Id.,  '75.  Zur 
Lehre  von  der  Structur  der  Zellen  :  /.  Z.,  IX.  (earlier  papers  cited).  —  Id.,  '84. 
Untersuchungen  uber  Struktur,  Lebenserscheinungen  und  Reactionen  thierischer 
und  pflanzlicher  Zellen:  J.  Z.,  XVII. — Furst,  E..  "98.  l"bcr  Centrosomen  bei 
Ascaris :  A.  m.  A.,  LII. — Fulmer,  E.  L., '98.  Cell-division  in  Pine  Seedlings: 
Bot.  Gas.,  XXVI.,  4. 

GALEOTTI,  GINO,  '93.  Uber  experimentelle  Erzeugnng  von  Unregclmassig- 
keiten  des  karyokinetischen  Processes:  />V/.  zur  patlK^loi:;.  Anat.  11.  z.  Alli^.  i\ithol., 
XW .,  2,  Jena,  Fischer,  1893. — Gallardo.  Angel,  "96.  La  Carioquinesis  :  ./////. 
Soc.  Cientif.  Argentina,  XLII.  —  Id..  '97.  Significado  Dinamico  de  las  Figuras 
Cariocineticas  :  Ibid.,  XLIV.  —  Gardiner.  E.  G..  "98.  The  (irowth  of  the  Ovum, 
etc.,  in  Polychoerus  :  /.  M.,  XV.,  r.  —  Gardiner.  W..  "83.  Continuity  of  Proto- 
plasm in  Vegetable  Cells:  Phil.  Trans.,  CLX.XIW  -  Garnault.  88.  Q%.  Sur  les 
phenomenes  de  la  fecondation  chez  Helix  aspera  et  .Arion  empiricorum  :  Zool.  Anz., 
XL,  XII.  —  Geddes    and    Thomp.5on.     The  Evolution  of  Sex:  London,  1899. — 

—  Gegenbaur.  C,  "54.  Beitriige  zur  naheren  Kenntniss  der  Sch\vimmpolyj)en  : 
Z.  w.  Z.,  V.  —  Van  Gehucliten.  A.,  "90.  Recherches  histologiques  sur  Tapparei! 
digestif  de  la  larva  de  la  Ptychoptera  contaminata  :  La  Cellule,  VL  —  Giard,  A..  "77. 


456  GENERAL  LITERATURE-LIST 

Sur  la  signification  morphologique  des  globules  polaires  :  Revue  scientifique,  XX. — 
Id.,  '90.  Sur  les  globules  polaires  et  les  homologues  de  ces  elements  chez  les  infu- 
soires  cilies  :  BuUeiin  scieiitifique  de  la  France  et  de  la  Belgiqiie,  XXII. — God- 
lewsky,  E.,  *97.  1,  Uber  niehrfache  bipolar  Mitose  bei  der  Spermatogenese  von 
Helix:  Ans.  Akad.  W^iss.  Krakaii. — Id.,  "97,  2.  Weitere  Untersuchungen  liber 
die    Umwandlung  der  Spermatiden.  etc.  :   Anz.  Akad.  IViss.  K7-akau..  Nov.,  '97. 

—  Goroschanktin,  J.,  "83.  Zur  Kenntniss  der  Corpuscula  bei  den  Gymnosper- 
men:  Bot.  Zeit..  LXI.  —  Graf.  A.,  "97.  The  Individuality  of  the  Cell:  N.  Y.  State 
Hosp.  Bidl.^  April.  —  Grdgoire.  V..  "99.  Les  cineses  polliniques  dans  les  Liliacees  : 
Bot.  Ccntb..XX..  i;  La  Cellule,  XVI.,  2.  — Griffin,  B.  B.,  ^96.  The  History  of 
the  Achromatic  Structures  in  the  Maturation  and  Fertilization  of  TJialassoiia  :  Traits. 
N.  Y.  Acad.  Sci.  —  Id..  "99.  Studies  on  the  Maturation.  Fertilization,  and  Cleavajre 
of  Thalassema  and  Zirphaea  :  /.  M..  XV.  —  Gierke.  H..  '85.  Farberei  zu  mikro- 
skopischenZwecken  :  Zeit.  IViss.  Mik..  II.  —  Grobben.  C,  '78.  Beitrage  zur  Kennt- 
niss der  mannlichen  Geschlechtsorgane  der  Dekapoden :  Arb.  Zool.  Inst.    Wien.  I. 

—  Gruber.  A..  '84.  Uber  Kern  und  Kerntheilung  bei  den  Protozoen  :  Z.  iv.  Z., 
XL. — Id..  '85.  Uber  klinstliche  Teilung  bei  Infusorien  :  B.  C,  I\\,  23;  V.,  5. — 
Id.,  '86.  Beitrage  zur  Kenntniss  der  Physiologie  und  Biologic  der  Protozoen  :  Ber. 
Naturf.  Ges.  Freiburg,  I.  —  Id.,  '93.  Mikroscopische  Vivisektion  :  Ber.  d.  N'aturf. 
Ges.  su  Freiburg,  WW.,  i.  —  Id.,  "97,  Weitere  Beobachtungen  an  vielkernigen 
Infusorien  :  Ber.  Naturf.  Ges.  Freiburg,  III.  —  Guignard,  L.,  '89.  Developpement 
et  constitution  des  Antherozoides  :  Rev.  gen.  Bot.,  I.  —  Id.,  "91,  1.  Nouvelles  etudes 
sur  la  fecondation  :  Ann.  d.  Sciences  Nat.  Bot.,  XIV.  — Id..  "91,  2.  Sur  Texistence 
des  "  spheres  attractives ""  dans  les  cellules  vegetales  :  C.R.,  9  Mars. — Id., '98,  1. 
Les  centres  cinetiques  chez  les  vegetaux  :  Ann.  Sci.  N'at.  Bot.,  (VIII.)  V. ;  also,  Bot. 
Gaz.,  XXV.  —  Id.,  '98,  2.  Le  developpement  du  pollen  et  la  reduction  chromatique 
dans  le  Nais  niajor :  Arch.  Anat.  Mik.,  II.,  4.  —  Id.,  '99.  Sur  les  antherozoides  et 
la  double  copulation  sexuelle  chez  les  vegetaux  angiospermes :   C.  R.,  CXXV^III.,  14. 

HABERLANDT,  G..  '87.  Uber  die  Beziehungen  zwischen  Funktion  und  Lage 
des  Zellkerns :  Fischer,  1887.  —  Hackel.  E..  "66.  Generelle  Morphologic. — 
Id.,  '91.  Anthropogenic,  4th  ed.,  Leipzig,  1891.  —  Hacker.  "V.,  "92.  1.  Die  Fur- 
chung  des  Eies  von  ^quorea  Forskalea  :  A.  ni.  A.,  XL.  —  Id.,  "92,  2.  Die  Eibil- 
dung  bei  Cyclops  und  Canthocamptus :  Zool.  Jahrb.,  V.  —  Id.,  '92.  3.  Die 
heterotypische  Kerntheilung  im  Cyclus  der  generativen  Zellen  :  Ber.  naturf.  Ges. 
Freibitrg,V\. — Id., '93.  Das  Keimblaschen,  seine  Elemente  und  Lageverander- 
ungen :  A.  in.  A.,  XLI.  —  Id..  "94.  Uber  den  heutigen  Stand  der  Centrosomen- 
frage  :  Verhandl.  d.  deutschen  Zool.  Ges..  1894.  p.  11.  —  Id..  "95.  1.  Die  Vorstadien 
der  Eireifung  :  A.  in.  A.,  XLV.,  2. — Id.,  "95.  2.  Zur  frage  nach  dem  Vorkommen 
der  Schein-Reduktion  bei  den  Pflanzen :  Ibid.,  XL\'I.  Also  Ann.  Bot.,  IX. — 
Id..  '95.  3.  Uber  die  Selbstandigkeit  der  vaterlichen  und  mlitterlichen  Kernsbe- 
standtheile  wahrend  der  Embryonalentwicklung  von  Cyclops:  A.  in.  A..  XLVI.,  4. 

—  Id.,  '97, 1.  Die  Keimbahn\'on  Cyclops  :  A.  in.  A.,  XLIX.  — Id.,  '97,  2.  Uber 
weitere  Ubereinstimmungen  zwischen  den  Fortpflanzungsvorgangen  der  Thiere 
und  Pflanzen :  B.  C  XVII.  —  Id.,  "98.  t^ber  vorbereitende  Theilungsvorgange 
bei  Thieren  und  Pflanzen:  Verh.  d.  Zool.  Ges.,  VIII. — Id.,  '99.  Praxis  und 
Theorie  der  Zellen  und  Befruchtungslehre  :  fena,  Fischer.  —  Hallez.  P..  "86.  Sur 
la  loi  de  I'orientation  de  I'embryon  chez  les  insectes :  C.  R..  103,  1886.  —  Hallibur- 
ton. W.  D..  "91.     A  Text-book  of  Chemical  Physiology  and  Pathology:  London. 

—  Id.,  "93.  The  Chemical  Physiology  of  the  Cell:  {Gouldstonian  Lectures)  Brit. 
Med.  fount.  —  Hammar,  J.  A., '96.  Uber  einen  primaren  Zusammenhang  zwi- 
schen den  Furchungszellen  des  Seeigeleies  :  A.  in.  A.,  XLVIL,  i.  —  Id.,  '97.  Uber 
eine  allgemein  vorkommende  primare  Protoplasmaverbindung  zwischen  den  Bias- 


GENERAL  LITER  A  TV  RE- LI  ST 


457 


tomeren:  A.  m.  A.,  XLIX.  —  Hammarsten.  O..  "94.  Zur  Kcnntniss  der  Nucleo- 
proteiden :  Zcif.  P/iys.  Clicm.,  XIX. —Id..  "95.  Lehrhuch  der  phy.siolo^i.schen 
Chemie,  36  Au.sgabe  :  Wiesbaden^  1895.  —  Hansemann.  D..  "91.  Karyokinese  und 
Cellularpathologie :  Bcrl.  Klin.  ll'oc/ioiscJnift^  No.  42.  —  Id..  "93.  Spe/iricitat. 
Altmismu.s  und  die  Anaplasie  der  Zellen  :  Berlin,  1893.  —  Hansteiu.  J.,  "80.  Das 
Protoplasma  als  Trager  der  pflanzlichen  und  thierischen  Lcljensverrichtungen. 
Hcidclbeyg.  —  Harper,  R.  A..  "96.  Uber  das  \'erhalten  der  Kerne  W\  der 
Fruchtentwickelung  einiger  A.scomyceten :  Jahrb.  luiss.  Hot..  XXIX.  — Id..  97. 
Kernteiluug    und    freie    Zellbildung    im    Ascu.s :    Ibid..   XXX.  —  Hardy,   W.   B.. 

"99.     On    the   Structure   of   Cell-protoplasm:  Joitr.    /'//vs..   XX I \'..    2. Harvey. 

Wm..  1651.  Exercitationes  de  Generatione  Animalium :  Loudon.  Trans,  in 
Sydcn/iani  Sac.  X..  1847. — Hartog,  M.  M.,  "91.  Some  Problems  of  Reproduc- 
tion, etc. :  (2-/-'  XXXIII.  —  Id.,  "96.  The  Cytology  of  Saprolegnia  :  ./////.  /lot.. 
IX.  —  Id.. '98.  Nuclear  Reduction  and  the  Function  of  Chromatin:  Xat.Sci., 
XII.  —  Hatschek.  B..  *87.  Uber  die  Bedeutung  der  geschlechtlichen  Forti)rian- 
zung:  Prager  Med.  Woc/iensc/irift.  XL\T.  —  Id..  '88.  Lehrbuch  der  Zooloj^ie. — 
Heath.  H..  "99.  The  Development  of  Ischnochiton  :  y<v/^7.  Fisc/ier.  —  Heiden- 
hain,  M..  "93.  Uber  Kern  und  Protoplasma  :  Feste/ir.  z.  30-Jii/ir.  Doctorjub.  von 
V.  Kolliker :  Leipzig.  —  Id.,  "94.  Neue  Untersuchungen  liber  die  Centralkdrper  und 
ihre  Beziehungen  zum  Kern  und  Zellenprotoplasma  :  ./.  ///.  A..  XLIII.  —  Id..  "95. 
Cytomechanische  Studien  :  A.  Entiuni.^  I.,  4.  —  Id.,  '96.  1.  Ein  neues  .Modell  zum 
Spannungsgesetz  der  centrirten  Systeme  :  Ver/i.  anat.  Ges.  —  Id.,  "96.  2.  \'\n-v 
die  Mikrocentren  mehrkerniger  Ricsenzellen.  etc.:  Morp/i.  Arb..  \\\..  i.  — Id..  99. 
Uber  eine  eigenthlimliche  Art  Knospung  an  Epithelzellen.  etc. :  A.  m.  A..  Ll\'.. 
I. — Heidenhain  and  Cohn,"  97.  Uber  die  Mikrocentren  in  den  Geweben  des 
Vogelembryos,  etc. :  iMorp/i.  Arb..  VII.  —  Heitzmann,  J..  "73.  Untersuchungen 
uber  das  Protoplasma  :  Sits.  d.  Jz.  Acad.  IViss.  // Vtv/.,  LXV'Il.  —  Id..  "83.  .Mikro- 
scopische  ^Morphologic  des  Thierkorpers im  gesunden  und  kranken  Zustande  :  W'icn. 
1883.  —  Henking.  H.  Untersuchungen  liber  die  ersten  Entwicklungsvorgange  in 
den  Eiern  der  Insekten,  I.,  II..  III.:  Z.  iu.  Z..  XLIX..  LI.,  LI\'..  1890-02.— 
Henle.  J..  "41.  Allgemeine  Anatomic:  Leipzig.  —  Henneguy.  L.  F..  '91.  .NOu- 
velles  recherches  sur  la  division  cellulaire  indirecte :  Jonrn.  .Inot.  et  /'/tysio/.. 
XXVII.  —  Id..  93.  Le  Corps  vitellin  de  Balbiani  dans  Tteuf  des  \'^rtebres  :  Ib/d.. 
XXIX.  —  Id..  "96.  Legons  sur  la  cellule:  Paris.  —  Id..  "98.  Sur  k-s  rapports  des 
cils  vibratils  avec  les  centrosomes  :  Arc/i.  Anat.  .Ui/c.  I.—  Hensen.  V..  "81.  Phy- 
siologic der  Zeugimg  :  Hermann's  P/iysiologie.  VI. — Herbst,  C.  Expt-rimenteile 
Untersuchungen  liber  den  Eint^uss  der  veranderten  chemischen  Zusammen.sct/ung 
des  umgebenden  Mediums  auf  die  Entwicklung  der  Thiere.  I.  :  Z.  iu.  Z..  L\'..  1802  : 
\\..Mitt.Zool.  St.Neapel.XX.,  1893  ;  III.-VI.,  Arc/i.  Entwni..  11.,  4,  1896.-  Id..  94. 
■95.  Uber  die  Bedeutung  der  Reizphysiologie  flir  die  Kausale  Ai.ffa.ssung  von  \'or- 
gangen  in  der  tierischen  Ontogenese :  Biol.  Centralb..  XI\'.,  X\'..  181)4,  i8t)5. — 
Herla,  V.,  '93.  Etude  des  variations  de  la  mitose  chez  I'a.scaride  megal()cei)hale  : 
A.  B.,  XIII.  —  Herlitzka.  A.,  "95.  Contribute  alio  studio  della  capncit.H  cvolutiva 
dei  due  primi  blastomeri  nell"  uove  di  Tritone  :  . /.  /inficni..  11.,  3.  —  Hermann.  F., 
'89.  Beitrage  zur  Histologic  des  Hodens  :  ./.  ///.  -  /.,  XXXIV.  —  Id..  "91.  lieitnig 
zur  Lehre  von  der  Ent.stehung  der  karyokinetischen  Spindel :  Ibid..  XXXVI 1.— 
Id..  "92.  Urogenitalsvstem.  Struktur  und  Histiogene.se  der  Spermatozoen  :  .Merkel 
und  Bonnet's  Ergebn'isse.  II.  — Id..  "97.  Beitrage  zur  Kenntniss  der  Spermato- 
genese:  A.  ni.  A..  L. —  Hertwig.  O..  "75.  Beitrage  zur  Kenntniss  der  Bildung. 
Befruchtung  und  Teilung  des  tierisciien  Eies,  I. :  J/.  /.,!.  —  Id..  '77.  Ik^itrage.  etc.. 
II. ;  Ibid.,  in.  —Id.,  "78.  Beitrage.  etc..  III. :  Ibid..  IV.  — Id.,  84.  Das  Problem 
der  Befruchtung  und  der  Isotropic  des  Eies.  eine  Theorie  der  \'ererbung : /.  Z.. 
XVIII.  — Id.,  "90,  1.     Vergleich  der  Ei- und  Samenbildung  bei  Nematoden.     Eine 


458  GENERAL  LITERATURE-LIST 

Grundlage  fiir  cellulare  Streitfragen  :  A.  in.  A.,  XXXVI.  — Id..  "90,  2.  Experi- 
mentelle  Studien  am  tierischen  Ei  vor.  waihrend  und  nach  der  Befruchtung : /.  Z.. 
1890. —  Id..  "92.  1.  —  Urmund  und  Spina  Bifida:  A.  ///.  A.,  XXXIX.  — Id.,  92,  2. 
Aeltere  und  neuere  Entwicklungs-theoneen :  Berlin. — Id.,  "93.  1.  Cber  den 
Werth  der  ersten  Furchungszellen  fiir  die  Organbildung  des  Embryo:  A.  ni.  A., 
XLIL — Id..  93.  2.  Die  Zelle  und  die  Gewebe :  Fischer,  Jena,  1893.  1898. — 
Id..  "94.  Zeit  und  Streitfragen  der  Biologie  :  Berlin.  —  Hertwig.  O.  and  R..  "86. 
Experimentelle  Untersucbungen  liber  die  Bedingungen  der  Bastardbefruchtung : 
/.  Z.,  XIX.  —  Id.,  "87.  Uber  den  Befruchtungs-  und  Teilungsvorgang  des  tierischen 
Eies  unter  dem  Eintluss  ausserer  Agentien  :  Ibid.,  XX.  —  Hertwig.  R..  "77.  Uber 
den  Bau  und  die  Entwicklung  der  Spirochona  gemmipara :  Ibid..  XI. — Id..  "84. 
Die  Kerntheilung  bei  Actinosphaerium  Eichhorni : //;/V/..  XVII.  —  Id.,  "88.  Uber 
Kernstruktur  und  ihre  Bedeutung  fiir  Zellteilung  und  Befruchtung :  Ibid..  IV.,  1888. 
— -Id..  '89.  Uber  die  Konjugation  der  Infusorien  :  Abh.  der  bayr.  Akad.  d.  PFiss., 
II..  CI.,  XVII.  —  Id.,  "92.  liber  Befruchtung  und  Conjugation  :  Ver/i.  dentsch.  Zool. 
Ges.y  Berlin.  —  Id..  '95.  tJber  Centrosoma  und  Centralspindel :  Siiz.-Ber.  Ges. 
Morph.  und  Phys.,  Miific/ien,  1805,  Heft  I. — Id.,  '96.  Uber  die  P2ntwicklung  des 
unbefruchteten  Seeigeleies,  etc. :  Festchr.  f.  Gegejibaiir. — Id.,  "97,  1.  Uber  die 
Bedeutung  derNucleolen  :  Siizb.  Ges.  Morph.  Fhys.  Munchen,  1898,  I.  —  Id.,  "97.  2. 

—  Uber  Karyokinese  bei  Actinosphaerium :  Sitzb.  Ges.  MorpJi.  Fhys.  Mwichen,  XIII., 

1.  —  Id..  "98.  Kerntheilung,  Richtungskorperbildung  und  Befruchtung  von  Acti- 
nosphaerium: Abh.  K.  bayer.  Akad.  M7ss.,  XIX,  2. — Heuser.  E..  "84.  Beobacli- 
tung  liber  Zelltheilung  :  BoL  Cent.  —  Hill.  M.  D..  '95.  Notes  on  the  Fecundation 
oiXh^^ggoi  SphcEr  echinus  granular  is  2.xvA  on  the  Maturation  and  Fertilization  of 
the  Egg  of  Fhallusia  niannnillata :  Q.  J.,  XXXVIII.  —  Hirase.  S.,  '97.  Unter- 
sucbungen liber  das  erhalten  des  Pollens  von  Gingko  biloba :  BoL  Centb.,  LXIX., 

2,  3.  —  Id..  '98.  Etudes  sur  la  fecondation  et  Tembryogenie  der  Gingko : /<9//;-. 
Coll.  Sci.,  Fokio.  XII.  —  His,  "W.,'74.  Unsere  Korperform  und  das  physiologische 
Problem  ihrer  Entstehung  :  Leipzig.  —  Hofer,  B..  "89.  Experimentelle^  Untersucb- 
ungen liber  den  Einfluss  des  Kerns  auf  das  Protoplasma :  /.  Z.,  XXIV. — Hoff- 
man. R.   "W..  "98.     Uber  Zellplatten  und  Zellplattenrudimente :  Z.  w.  Z.,  LXIII. 

—  Hofmeister.  "67.  Die  Lehre  von  der  Pflanzenzelle :  Leipzig,  1867. — Holl, 
M.,  '90.  Uber  die  Reifung  der  Eizelle  des  Huhns :  Sitzb.  Acad.  Wiss.  Wien, 
XCIX.,  3.  — Hooke.  Robt.,  1665.  Mikrographia.  or  some  physiological  Descrip- 
tions of  minute  Bodies  by  magnifying  Glasses  :  London.  —  Hoyer.  H..  "90.  Uber 
ein  fiir  das  Studium  der  '•  direkten ''  Zelltheilung  vorzuglich  geeignetes  Objekt :  A. 
A.,  v.  — Hubbard.  J.  W..  "94.  The  Yolk-Nucleus  in  Cymatogaster  :  Froc.  Am. 
Fhil.  Soc,  XXXIII.— Huie.  L.,  "97.  Changes  in  the  Cell-organs  oi  Drosera 
produced  by  Feeding  with  Egg-albumen:  g- /••  XXXIX.  —  Humphrey.  J.  E., 
"94.  Nucleolen  und  Centrosomen  :  Ber.  dentschen  bot.  Ges.,  XII..  5. — Id..  "95. 
On  some  Constituents  of  the  Cell:  Ann.  Bot..,  IX. — Huxley.  T.  H..  "53.  Review 
of  the  Cell-theory:  Brit,  and  Foreign  Med.-Chir.  Review,  XII. — Id..  '78.  Evo- 
lution in  Biology,  Enc.  Brit.,  9th  ed.,  1878  ;  Science  and  Culture,  N.  Y.,  1882. 


IKENO,  S.,  '97.  Vorlaufige  Mitth.  liber  die  Spermatozoiden  bei  Cycas :  Bot. 
Centb.,  LXIX.,  i.  —  Id., '98.  1.  Zur  Kenntniss  des  sogenannten  centrosomahn- 
lichen  Korpers  im  Pollenschlauche  der  Cycaden  :  Flora,  LXXXV.,  i. — Id.,  '98,  2. 
Untersucbungen  liber  die  Entwickeiung  der  Geschlechtsorgane,  etc.,  bei  Cycas :  Jahrb. 
wiss.  Bot.,  XXXII.,  4.  —  Ishikawa,  M.,  '91.  Vorlaufige  Mitteilungen  liber  die 
Konjugationserscheinungen  bei  den  Noctiluceen  :  Z.  A.,  No.  353,  1891. — Id.,  '94. 
Studies  on  Reproductive  Elements:  \\.,  Noctiluca  miliaris  Sur.,  Its  Division  and 
Spore-formation:  Joiirn.   College  of  Sc.  Lmp.   Univ.  Japan,  VI.  —  Id.,  '97.      Die 


GENERAL   UTERATURE-UST  459 

Entwickelung  der  Pollenkorner  von  Allium:  Journ.  Coll.  Sci.  Tokyo,  X.,  2.— Id 
'99.     Further  Observations  on  the  Nuclear  Division  of  Noctiluca  :  Ibid.,  XII..  4. 

JENNINGS,  H.  S..  96.  The  Early  Development  of  Asplanchna:  Hull  Mus 
Camp.  ZooL,  XXX.— Jensen.  O.  S..  SS.  Recherches  sur  la  spermatogenese : 
A.  B..  IV.  — Johnson,  H.  P.,  92.  Amitusis  in  the  embryonal  envelopes  of  the 
Scorpion:  Bull.  JJus.  Conip.  ZooL,  XXII.,  3. —Jordan.  E.  O..  -93.  Tin-  Habits 
and  Development  of  the  Newt:  /.  J/..  Vlll.,  2.  — Jordan  and  Eycleshymer.' -94 
On  the  Cleavage  of  Amphibian  Ova  :  /.  J/.,  IX..  3,  1894.  —Juel.  H.  O..  -97.  Die 
Kerntheilungen  in  den  Pollenmutterzellen,  etc.  :  Jahrb.  7u/ss.  Bo/.,  XXX.  —  Julin.  J., 
'93,  1.  Structure  et  developpement  des  glandes  se.xuelle.s.  ovogdn6se,  spermatoge- 
nese et  fecondation  chez  Styleopsis  grossularia  :  Bull.  Si.  de  France  ft  de  /ielt^i^ue, 
XXIV.  —Id.,  *93,  2.  Le  corps  vitellin  de  Balbiani  et  les  dlcments  des  Metazoaires 
qui  correspondent  au  Macronucle'us  des  Infusoires  cilies :  /did..  XXI  \'. 

KARSTEN,  G.,  "96.  Untersuchungen  iiber  Diatomeen  :  Flora.  LXXXII.— 
Keuten,  J..  "95.  Die  Kerntheilung  von  Eiis^lena  viridis  Ehr :  Z.  u\  Z..  L.\'.— 
Kienitz-Gerloff,  F.,  -91.  Review  and  Bibliography  of  Researches  on  Pn)ti)plasmic 
Connection  between  adjacent  Cells:  in  Bot.  Zeitinnr,  XLIX. -Kingsbury.  B.  F., 
'99.  The  Reducing  Divisions  in  the  Spermatogenesis  of  Desmognathus :  Zool. 
Bull.,  II.,  5.  —  Klebahn,  '90.  Die  Keimung  von  Closterium  und  Cosmarium  :  Jahrb. 
wiss.  Bat.,  XXII.  — Id.,  "92.  Die  Befruchtung  von  CEdigonium  :  Jahrb.  /'.  iciss. 
Bot.,  XXIV.  —  Id.,  *96.  Beitrage  zur  Kenntniss  der  Auxosporenbildung,  I.,  Rho- 
palodia  :  Jahrb.  wiss.  Bot.,  XXIX.  —  Klebs.  G..  "83.  t'ber  die  Organisation  einiger 
Flagellaten-Gruppen,  etc.  :  Bot.  Inst.  Tubingen,  I.,  i.  —  Id..  "84.  ('berdie  neueren 
Forschungen  betrelTs  der  Protoplasmaverbindungen  benachbarter  Zellen  :  Hot.  Zett., 
188.4  — Id.,  "87.  Uber  den  Einfiuss  des  Kerns  in  der  Zelle  :  B.  C.  \'II.  -  Klein, 
E.,  ■78-'79.      Observations  on  the  Structure  of  Cells  and  xXuclei :  Q.J..  X  Vlll..  Xl.\. 

—  Klinckowstrom,  A.  v.,  *97.  Beitrage  zur  Kenntniss  der  Eireife  und  Befruch- 
tung bei  Prosthecer^us  :  A.  in.  A..  XLVIII.  —  von  KoUiker,  A..  "41.  Beitriiiie  /ur 
Kenntniss  der  Geschlechtsverhaltnisse  und  der  Samenfliissigkeit  wirbelloser  Tiere : 
Berlin.  — Id.,  '44,  Entwicklungsgeschichte  der  Cephalopoden  :  Ziirieh.  —  Id..  '85. 
Die  Bedeutung  der  Zellkerne  fur  die  Vorgange  der  Vererbung:  Z.  10.  Z..  XLII. — 
Id.,  "86.  Das  Karyoplasma  und  die  Vererbung,  eine  Kritik  der  Weismann'schen 
Theorie  von  der  Kontinuitat  des  Keimplasmas  :  Ibid.,  XLIII.  — Id.,  '89.  Handbuch 
der  Gewebelehre,  6th  ed.  :  Leipzig. — Id..  '97.  Die  Energiden  \o\\  Sachs,  etc.: 
Verh.  Phys.  Med.  Ges.,  ll'ursburg,  XXXI..  5. — Korff.  "99.  Zur  Histogenese  der 
Spermien  von  Helix  :  A.  ;//.  A..  LIV.  Korschelt.  E..  "89.  Beitrage  zur  .Mor- 
phologic und  Physiologic  des  Zell-kernes  :  Zool.  Jahrb.  .Inat.  it.  Ontog.,  1\'.—  Id., 
'93.  iiber  Ophryotrocha  puerilis  :  Z.  2l>.  Z.,  lAX . — Id..  "95.  Tber  Kerntheilung, 
Eireifung  und  Befruchtung  bei  (9/'///"jv//7^t7/rf  puerilis:  Ibid.,  LX.  —  Id.. '96.  Ktrn- 
structuren  und  Zellmembranen  in  den  Spinndriisen  der  Rau]jen  :  .1.  ///.  .1..  XlA'Il. 

—  Id..  '97.     t'ber  den  Bau  der  Kerne  in  den  Spinndrii.sen  der  Raupen  :   Ibid.,  XLIX. 

—  Kossel,  A..  '91.  t^ber  die  chemische  Zusammensetzung  der  /elie  :  .Irch.  .Inat. 
u.  Phys. — Id..  "93.  C'ber  die  Nucleinsaure  :  Ibid.,  1893. — ^^  ■  ^^  ''^tT  f^'»* 
basischen  Stofife  des  Zellkernes  :  Zeit.  Phys.  Cheiii.,  XXII.  —  von  Kostanecki.  K  . 
'91.  f^ber  Centralspindelkorperchen  bei  karyokinetischer  Zellteilung  :  .\nat.  Hefte, 
1892,  dat.  91. — Id.,  "96.  (ber  die  Gestalt  der  Centro.somen  im  befmchtcten  See- 
igelei :  Ibid..  \TI.,  2.  —  Id..  "97.  1.  ('ber  die  Bedeutung  der  Polstrahlung,  etc.: 
A.  ni.  A..  LXIX.  —  Id.,  '98.  Die  Befruchtung  des  Eies  von  .llyc/'sti'/na  :  Ibid., 
LI.  —  Kostanecki  and  Siedlecki.  "96.  t'bcr  das  \'erhalton  der  Centrosomen 
zum  Protoplasma:  Ibid.,  XLIX.  —  Kostanecki  and  "Wierzejski.  "96.  I'ber  das 
Verhalten  der  sos^enannten  achromatischen  Substanzen  im  befruchleten  Ei  :  Ibid.. 
XLII.,  2.  — Kiihne,  "W..  "64.     Lhitersuchungen  liber  das  Protoplasma  und  die  Con- 


460  GENERAL   LITERATURE-LIST 

tractilitat. — Kupffer,  C,  '75.  tJber  Differenzierimg  des  Protoplasma  an  den 
Zellen  thieiischer  Gewebe :  Schr.  nafiir.  Ver.  Schles.-Hoht.,  I.,  3. — Id.,  "90.  Die 
Entwicklung  von  Petromyzon  Planeri  :  A.  in.  A.,  XXXV. — Id.,  '96.  Uber  Ener- 
giden  und  paraplastische  Bildungen  :  Rcktoratrede.,  Miinchoi,  1896. 

LAMEERE,  A.,  "90.  Recherches  sur  la  reduction  karyogamique  :  Briixelks.  — 
Lauterborn.  R.,  '93.  Uber  Bau  und  Kerntheilung  der  Diatomeen :  Vcrh.  d. 
N'aturh.  Med.  Ve)-.  in  Heidelberg.  1893. —Id., '95.  Protozoenstudien,  Kern-  und 
Zellteilung  von  Ceratium  hirundinella  O.  F.  M.  :  Z.  w.  Z.,  XLIX.  —  Id.,  '96.  —  La 
"Valette  St.  George, '65.  Uber  die  Genese  der  Samenkcirper :  A.  ni.  A..  I. — 
Id.,  "67.  Uber  die  Genese  der  Samenkorper,  II.  (Terminology):  Ibid.,  III. — 
Id., '76.  Die  Spermatogenese  bei  den  Amphibien  :  Ibid..,  XII. — Id., '78.  Die 
Spermatogenese  bei  den  Saugethieren  und  dem  Menschen  :  Ibid..,  XV.  —  Id.,  ■85-"87. 
Spermatologische    Beitrage.    I.-V.  :    Ibid.,   XXV.,  XXVII.,  XXVIII.,  and   XXX. 

—  Lankester,  E.  Ray,  "77.  Notes  on  Embryology  and  Classification:  London. — 
Lavdovsky.  M.,  "94.  Von  der  Entstehung  der  cbromatischen  und  achromatischen 
Substanzen  in  den  tierischen  und  pflanzlichen  Zellen :  Merkel  und  Bonnefs  Anat. 
Hefte,  IV.,  13.  —  Lawson,  A.  A.,  "98.  Some  Observations  on  the  Development 
of  the  Karyokinetic  Spindle,  etc. :  Proc.  Cal.  Acad.  Sci.,  I.,  5.— Lazarus,  A.,  '98. 
Die  An^emie:  IVien. — Lee,  A.  Bolles,  '96.  Sur  le  Nebenkern,  etc.,  chez  Helix: 
La  Cellule,  XI.  — Id.,  '97.  Les  cineses  spermatogenetiques  chez  Helix:  Ibid., 
XIII.  —von  Lenhossek,  M.,  '95.  Centrosom  und  Sphare  in  den  Spinalganglien 
des  Frosches:  A.  ni.  A.,  XLVI.— Id.,  '98,  1.  Uber  Flimmerzellen  :  Ver h.  An. 
Ges.,  XII.  — Id.,  '98,  2.  Untersuchungen  liber  Spermatogenesis  :  A.  m.  A.,,  LI. — 
Id.,  '99.  Das  Mikrocentrum  der  glatten  Muskelzellen :  A.  A.,  XVI.,  13,  14. — 
Leydig.  Fr.,  '54.  Lehrbuch  der  Histologic  des  Menschen  und  der  Thiere :  Frank- 
fitrt.—16...'Q5.  Zelle  und  Gewebe,  i?^;/;/. —Id-,  '89.  Beitrage  zur  Kenntniss 
des  thierischen  Eies  im  unbefruchteten  Zustande  :  SpengeVs  Jahrb.  Anat.  Ont.,  III. 

—  Lilienfeld,  L.,  '92,  '93.  Uber  die  Verwandtschaft  der  Zellelemente  zu  gewissen 
Farbstoffen:  Verh.  Phys.  Ges.,  Berlin,  1892-93.  —  Id.,  '93.  Uber  die  Wahlver- 
wandtschaft  der  Zellelemente  zu  Farbstoffen:  A.  A.  P.,  1893.— Lillie,  F.  R.,  '95. 
The  Embryology  of  the  Unionidae :  /.  M.,  X.— Id.,  '96.  On  the  Smallest  Parts 
of  Stentor  capable  of  Regeneration:  /.  M.,  XII.,  i.— Id.,  "97.  On  the  Origin  of 
the  Centres  of  the  First  Cleavage-spindle  in  Unio  :  Science,  V. — Id.,  '98.  Centro- 
some  and  Sphere  in  the  Egg  of  Unio  :  Zool.  Bull.,  I.,  6.  — Id.,  '99.  Adaptation  in 
Cleavage  :  Wood^s  Holl  Biol.  Led.  —  List,  Th.,  '96.  Beitrage  zur  Chemie  der  Zelle 
und  Gewebe,  I. :  Mitth.  Zool.  St.  Neap.,  XII.,  3.  — Loeb,  J.,  '91-92.  Untersuch- 
ungen zur  physiologischen  Morphologic.  I.  Heteromorphosis :  Wiirzburg,  1891. 
II.  Organbildung  und  Wachsthum  :  Ibid.,  1892. —  Id., '92.  Experiments  on  Cleav- 
age :  /.  il/.,  VII. — Id.,  '93.  Some  Facts  and  Principles  of  Physiological  Mor- 
phology: Wood's  Holl  Biol.  Lectures,  1893.  — Id.,  '94.  Uber  die  Grenzen  der 
Theilbarkeit  der  Eisubstanz  :  A.  ges.  P.,  LIX.,  6,  7.— Id., '95.  Uber  Kernthei- 
lung ohne  Zelltheilung  :  Arch.  Entwni.,  II.  — Id.,  '99,  1.  Warum  ist  die  Regenera- 
tion kernloser  Protoplasmastlicken  unmoglich,  etc.:  Ibid.,  VIII..  4.  —  Id.,  '99,  2. 
On  the  Nature  of  the  Process  of  Fertilization  and  the  Artificial  Production  of  Nor- 
mal Larv£e,  etc. :  Am.  Journ.  Phys.,  III.,  3.  —  Lowit,  M.,  '91.  Uber  amitotische 
Kerntheilung:  B.  CXI.  —  Lukjanow, '91.  Grundzuge  einer  allgemeinen  Patho- 
logic der  Zelle:  Leipzig.— liW&ti^  and  Galeotti,  "93.  Cytologische  Studien  uber 
pathologische  menschliche  Gewebe :  Beitr.  Path.  Anat.,  XIV. 

MACALLUM,  A.  B.,  '91.  Contribution  to  the  Morphology  and  Physiology  of 
the  Cell:  Trans.  Canad.  Inst.,  I.,  2.  —  McClung,  C.  E.,  '99.  A  Peculiar  Nuclear 
Element  in  the  Male  Reproductive  Cells  of  Insects  :  ZooL  Bull.,  II.,  4-  — MacFar- 


GENERAL   TITERATURE-LIST  46  I 

land,  F.  M.,  '97.  Gellulare  Studien  an  Molluskeneiern  :  Zo'ol.  Jahrh.  Anat.,  X. — 
McGregor,  J.  H.,  '99.  The  Spermatogenesis  of  .Amphiiima :  J.  J/.,  XV'..  Suppl. 
—  McMvirrich.  J.  P.,  '86.  A  Contribution  to  the  Enibryoh>gy  of  the  Prosobranch 
Gasteropods  :  Studies  Biol.  Lab.  Johns  Hopkins  Unii'..  III. — Id.,  "95.  Embry- 
ology of  the  Isopod  Crustacea:  /.  J/.,  XL,  i. — Id..  "96.  The  Volk-Lobe  and  the 
Centrosome  of  Fulgur  :  A.  A..  XII.,  23.  — Id.,  "97.  The  Epithchum  of  the  .Midgut 
of  the  Terrestri-1  Isopods  :  /.  JA,  XIV.,  i.  —  Maggi,  L.,  "78.  I  plastitluli  nei 
ciliati  ed  i  plastiduli  liberamente  viventi :  Atti.  Soc.  Ital.  Sc.  Nat.  Milano,  X.Xi. 
(also  later  papers).  —  Malfatti,  H.,  '91.  Beitrage  zur  Kenntniss  der  Nucleine: 
Zeit.\Phys.  Chein..,  XVI.  —  Mark,  E.  L.,  "81.  Maturation,  P'ecundation.  and  Seg- 
mentation of  Limax  campestris  :  Bull.  Mus.  Conip.  /.ool.  Harvard  Collct^cW. — 
Mathews.  A.  P.,  "97,  1.  Internal  Secretions  considered  in  Relation  to  \'ariation 
and  Development:  Science.,  V.,  122. — Id..  "97.  2.  Zur  Chemie  der  Six-rmatozoen  : 
Zeit.  Phys.  CJieui.,  XXIII.,  4,  5. — Id.,  "98.  A  Contribution  to  the  Chemistry  of 
Cytological  Staining:  Am.  Journ.  P/iys.,  I..  4.  —  Id..  "99,  1.  The  Origin  of  Fibri- 
nogen: Ibid.,  III.— Id.,  "99.2.  The  Metabolism  of  the  Pancreas  Cell:  /.  .1/., 
XV.,  Suppl.  —  Maiipas,  M.,  "88.  Recherches  experimentales  sur  la  multiplication 
des  Infusoires  cilies :  Arch.  Zool.  Exp.,  2me  serie,  VI. — Id..  "89.  Le  rejeunisse- 
ment  karyogamique  chez  les  Cilies:  Ibid.,  2me  serie,  VII.  —  Id.,  "91.  Sur  h-  deter- 
minisme  de  la  sexualite  chez  THydatina  senta :  C.  /?.,  Paris.  —  Mayer.  P.,  "91, 
Uber  das  Farben  mit  Carmin,  Cochenille  und  Hamatein-Thonerde:  Mitth.  Zool.  St. 
Neapol.,  X.,  3.  —  Id.,  "97.  Beruht  die  Farbung  der  Zellkerne  auf  einem  chem- 
ischen  Vorgang  oder  nicht?:  A.  A.,  XIII.,  12.  — Mead.  A.  D..  "95.  Some  Obser- 
vations on  Maturation  and  Fecundation  in  Chaetopteruspergamentaceus  Cuv.  :  /. .)/., 
X.,  I. —Id.,  "97,  1.  The  Origin  of  the  Egg-centrosomes  :  Ibid.,  .Xll.  — Id..  "97.  2. 
The  early  Development  of  marine  Annelids:  Ibid.,  \. — Id..  "98.  1.  Tlic  ()r;^in 
and  Behaviour  of  the  Centrosomes  in  the  Annelid  Egg  :  Ibid.,  Xl\'..  2.  —  Id.,  98.  2. 
The  Rate  of  Cell-division  and  the  Function  of  the  Centrosome  :  Wood's  I/oll  Bwl. 
Lectures. — Merkel.  F.,  "71.  Die  Stiitzzellen  des  menschlichen  Hodens  :  Mi'illers 
Arch.  —  Mertens,  H.,  "93.  Recherches  sur  la  signihcation  du  corps  vitcllin  de 
Balbiani  dans  lovule  des  Mammiferes  et  des  Oiseaux  :  A.  />'..  XIll.  —  Metschui- 
koff,  E.,  "66.  Embryologische  Studien  an  Insecten  :  Z.Il.Z.,  XVL  — Meves, 
F..  "91.  Uber  amitotische  Kernteilung  in  den  Spermatogonien  des  Salamanders, 
und  das  Verhalten  der  Attraktionsspharen  bei  derselben  :  ./.  ./.,  1891,  No.  22.— 
Id.,  '94.  Uber  eine  Metamorphose  der  Attraktionssphiire  in  den  Spermatogonien 
von  Salamandra  maculosa:  A.  ui.  A.,  XLIV.— Id.,  '95.  t'ber  die  Zelh-n  des 
Sesambeines  der  Achillessehne  des  Frosches  {Rana  teniporaria)  und  iiber  ihre  Cen- 
tralkorper:  //;/V/.,  XLV.  —  Id.,  "96.  Uber  die  Entwicklung  der  mannlichen  (ie- 
schlechtszellen  von  Salamandra:  Ibid.,  XLVIIL— Id.,  '97.  1.  Zur  Struktur  der 
Kerne  in  den  Spinndriisen  der  Raupen :  Ibid.,  XlA'lIl.  —  Id..  '97.  2.  Uber 
Struktur  und  Histiogenese  der  Samenfiiden  von  Salamandra  :  Ibid.,  L.  -  Id  .  "97.  3. 
Uber  den  Vorgang  der  Zelleinschnlirung :  Arch.  Pntu'/n.,  V.,  2.— Id..  97.  4. 
Zelltlieilung:  Jlerkel  u.  Bonnet,  Er^.,  VI.— Id..  '97.  5.  Cher  Cc-ntralkiirpt-r  in 
mannlichen  Geschlechtszellen  von  Schmetterlingen  :  .-/.  ./..  Xl\' .  i.  — Id..  '98. 
t'ber  das  Verhalten  der  Centralkorper  bei  der  Histogenese  der  Samenfaden  vom 
Men.sch  und  Ratte :  I'erh.  An.  c;.'^..  XIV. —  Id., '99.  Cber  Struktur  und  Histo- 
genesis der  Samenfaden  des  Meerschweinschens  :  .-/.  ///.  .-/..  LI\'.-  Meyer,  A.,  '96. 
Die  Plasmaverbindungen.  etc.:  Bat.  Zeit.,  ir,  12.— Meyer.  O..  95.  Celhilar- 
Untersuchungen  an  Nematodeneiern  :  /.  Z.,  XXiX.  (XXII.  ).  -  Michaelis.  L..  -97. 
Die  Befruchtung  des  Tritoneies :  ./.  w.  -/•.  XL\'I1I.  —  Miescher.^  F..  ^96. 
Physiologisch-chemische  Untersuchungen  iiber  die  Lachsmilch  :  .Irch.  Exp.  Path. 
u.'Pharni..  XXXVII.  —  Mikosch,  "94.  C'ber  Struktur  im  ptianzlichen  Proto- 
plasma:    Verhandl.  d.  Ges.  deutscher  Xaturf.  und  . /rr/c-.  1 894 :  Abteil  f.  Pjlanzcn- 


462  GENERAL   LITERATURE-LIST 

physiologie  ii.  P/lansenanaiojm'e.  —  Minot,  C.  S.,  '77.  Recent  Investigations  of 
Embryologists  :  Froc.  Bost.  Soc.  Nat.  Hist..,  XIX.  —  Id.,  '79.  Growth  as  a  Function 
of  Cells  :  Ibid.,  XX.  —  Id.,  *82.  Theorie  der  Genoblasten  :  B.  C,  II.,  12.  See  also 
Am.  Nat.,  February,  1880,  and  Froc.  Bost.  Soc.  A'at.  Hist.,  XIX.,  1877. — ^^-^  "91- 
Senescence  and  Rejuvenation  :  Joiini.  /V/jj'.,  XII.,  2.  —  Id. ,"92.  Human  Embryol- 
ogy: New  York.~^o\\  Mohl  Hugo,  '46.  Uber  die  Saftbevvegung  im  Innern  der 
Zellen  :  Bot.  Zeitinig.  —  Moll.  J.  W.,  "93.  Observations  on  Karyokinesis  in  Spiro- 
gyra :  Verh.  Kon.  Akad.,  Amsterdam,  No.  9.  —  Montgomery,  Th.  H..  "98,  1. 
The  Spermatogenesis  of  Pentatoma,  etc. :  Zo'dl.  Jahrb.  —  Id.,  '98,  2.  Comparative 
Cytological  Studies,  witli  Especial  Reference  to  the  Morphology  of  the  Nucleolus : 
J.  M.,  XV.,  2. — Moore,  J.  E.  S.,  '93.  Mammalian  Spermatogenesis:  A.  A., 
VIII.  —  Id.,  "95.  On  the  Structural  Changes  in  the  Reproductive  Cells  during  the 
Spermatogenesis  of  Elasmobranchs :  Q.  /.,  XXXVIII.  —  Morgan,  T.  H.,  "93. 
Experimental  Studies  on  Echinoderm  Eggs:  A.  A.,  IX.,  5,  6.  —  Id.,  '95,  1. 
Studies  of  the  "  Partial "'  Larvae  of  Sphaerechinus  :  A.  Entivm.,  II.,  i.  — Id.,  '95,  2. 
Experimental  Studies  on  Teleost-eggs  :  A.  A.,  X.,  19. — Id.,  '95,  3.  Half-embryos 
and  Whole-embrvos  from  one  of  the  first  two  Blastomeres  of  the  Frost's  Esfor ; 
Ibid.,  X.,  19. — Id.,  '95.  4.  The  Fertilization  of  non-nucleated  P^ragments  of 
Echinoderm-eggs  :  Arch.  Entiviu.,  II.,  2.  — Id.,  '95,  5.  The  Formation  of  the  Fish- 
embryo  :  /.  M.,  X.,  2.  —  Id.,  "96,  1.  On  the  Production  of  artificial  archoplasmic 
Centres:  Kept,  of  the  Am.  Morph.  Soc,  Science,  III.,  January  10,  1896. — Id.,  '96, 
2.  The  Number  of  Cells  in  Larvae  from  Isolated  Blastomeres  of  Amphioxus : 
Arch.  Entwm.,  III.,  2. — Id..  "96.  3.  The  Production  of  Artificial  Astrosphccres  : 
Arch.  Entwm.,  III. — Id.,  "98.  1.  Experimental  Studies  of  the  Regeneration 
of  Planaria  maculata :  Ibid.,  VII.,  2.  3. — Id..  "98,  2.  Regeneration  and  Liability 
to  Injury:  Zo'dl.  Bull.,  I.,  6.  —  Id.,  '99,  1.  The  Action  of  Salt-solutions  on  the 
Unfertilized  and  Fertilized  Eggs  oi  Arbacia  and  other  Animals:  Arch.  Efitii'm., 
VIII.,  3.  —  Id.,  '99.  2.  A  Confirmation  of  Spallanzani's  Discovery,  etc.:  A.  A., 
XV.  21.  —  Mottier,  D.  M.,  "97,  1.  Uber  das  Verhalten  der  Kerne  bei  der  Entwick- 
lung  des  Embryosacs,  etc. :  Jahrb.  wiss.  Bot.,  XXXI.  —  Id.,  '97,  2.  Beitraige  zur 
Kenntniss  der  Kerntheilung  in  den  Pollenmutterzellen. (?/t\  ;  Ibid.,  XXX.  —  Id.,  '98. 
Das  Centrosoma  bei  Dictyota :  Ber.  D.  Bot.  Ges.,  XVI.,  5.  —  Miiller,  E.,  "96. 
tJber  die  Regeneration  der  Augenlinse  nach  Exstirpation  derselben  bei  Triton : 
A.  m.  A.,  XLVIL.  i.  — Munson,  J.  P.,  '98.  The  Ovarian  Egg  of  Limulus,  etc.  : 
J.  M.,  XV.,  2.  —Murray,  J.  A.,  "98.  Contributions  to  a  Knowledge  of  the  Neben- 
kern  in  the  Spermatogenesis  of  Pulmonata:  Zo'ol.  Jahrb.,  XL,  14. 

NADSON,  G.,  '95.  Uber  den  Bau  des  Cyanophyceen-Protoplastes :  Script. 
Botan.  Horti.  Petropol.,  W .  —  Nageli,  C,  "84.  Mechanisch-physiologische  Theorie 
der  Abstammungslehre  :  M'unchen  u.  Leipzig,  1884.  — Nageli  und  Schwendener, 
'67.  Das  Mikroskop.  (See  later  editions.)  Leipzig. — Nawaschin.  "99.  Neue 
Beobachtungen  liber  Befruchtung  bei  Fritillaria  und  Lilium :  Bot.  Centb., 
LXXVIL,  2.— Nemec,  B.,  "97.  Uber  die  Stmktur  der  Diplopodeneier,  A.  A., 
XIII.,  10,  II. — Id.,  '99.  Uber  die  karyokinetische  Kerntheilung  in  den  WUr- 
zelspitzen  von  Allium :  J.  w.  B.,  XXVIII,  2.  —  Newport.  G.  On  the  Impregnation 
of  the  Ovum  in  the  Amphibia:  Fhil.  Trans.,  1851,  1853,  1854. — Norman.  "W. 'W., 
'96.  Segmentation  of  the  Nucleus  without  Segmentation  of  the  Protoplasm  :  Arch. 
Entwm.,  III.  —  Nussbaum,  M..  "80.  Zur  Differenzierung  des  Geschlechts  im  Tier- 
reich  :  A.  m.  A.,  XVIIL  — Id.,  '84,  1.  Uber  Spontane  und  Kunstliche  Theilung 
von  Infusorien  :  Verh.  d.  naturh.  Ver.  preus..  Rheinland,  1884.  —  Id.,  "84,2.  Uber 
die  Versinderungen  der  Geschlechtsproducte  bis  zur  Eifurchung :  A.  m.  A.,  XXIII. 
—  Id.,  "85.      Uber  die  Teilbarkeit  der  lebendigen  Materie,  I.  :  A.  ?n.  A.,  XXVI.  — 


GENERAL   LITERATURE-UST  463 

Id.,  ^4.  Die  mit  der  Entwickeluno-  fortschreitende  DifTcrenziemng  der  Zellen  : 
Sitz.-Ber.  d.  niedcrrhein.  Gescllschaft  f.  Natiir-  u.  Hcill:undt\  Honn,  5  Nov.,  1894; 
also  B.  C,  XVI.,  2,  1896.  —Id..  97.  Die  Entstehung  des  Geschlechts  bei  Hyda- 
tina:  A.  7n.  A.,  XLIX. 

OBST.  P..  "99.  Untersuchungen  liber  das  Verhalten  der  Nucleolen,  etc.  :  Z.  w.  Z. 
LXVI.,  2.  —  Ogata.  "83.  Die  Veranderungen  der  Fancreas/.ellen  bei  der  Secre- 
tion: A.  A.  P.  —  Oppel.  A..  92.  Die  Befruchtung  des  Reptilieneies  :  A.  w.  A. 
XXXIX.— Osterhout.  W.  J.  V..  "97.  f'ber  Entstehung  der  karyoi<inetischen 
Spindel  bei  Equisetum  :  Jahrb.  wiss.  Bot.^  XXX. — Oltmanns.  F..  '95.  I'ber  die 
Entwickelung  der  Sexualorgane  bei  VaucJieria:  Flora.  —  Overtou.  C.  E..  "88. 
Uber  den  Conjugationsvorgang  bei  Spirogyra:  Ber.  deutsch.  Bot.  Ucs...  X'l.^Id.. 
'89.  Beitrag  zur  Kenntniss  der  Gattung  Volvox  :  Bot.  Ccntralb.,  XXXIX.  —  Id.,  "93. 
liber  die  Reduktion  der  Chromosomen  in  den  Kernen  der  Pflanzen  :  /  'ierteljalirschr. 
iiatiirf.  Ges.  Zurich.,  XXXVIII.     Also  Ann.  Bot..,  VII.,  25. 

PALADINO,  G..  "90.  I  ponti  intercellulari  tra  1'  novo  ovarico  e  le  cellule  t'olli- 
colari.  etc.  :  A.  A..,  V.  —  Paulmier,  F,  C,  '98.  Chromatin  Reduction  in  the 
Hemiptera :  A.  A.,  XIV. — Id..  '99.  The  Spermatogenesis  of  Anasa  tristis: 
y.  il/.,  XV.,  Suppl.  —  Peter,  K.,  "99.  Das  Centrum  fur  die  Flimmer-  und  Gei.s.sel- 
bewegung :  A.  A.,  XV.,  14,  15. — Pfeffer,  "W.,  99.  Uber  die  Erzeugung  und  die 
physiologische  Bedeutung  der  Amitose :  Ber.  k'dm'gl.,  sac/is.,  Ges.  Hiss.  Leipzig.., 
July  3.  —  Pfitzner.  "W.,  "82.  Uber  den  feineren  Bau  der  bei  der  Zelltheiking 
auffretenden  fadenformigen  Differenzierungen  desZellkerns  :  M.  J.,  \'1I.  —  Id..  "83. 
Beitrage  zur  Lehre  vom  Baue  des  Zellkerns  und  seinen  Theilungserscheinungen  : 
A.  ?n.  A.jXXll.  —  Pfluger,  E.,  '83.  Uber  den  Einfluss  der  Schwerkraft  auf  die 
Theilung  der  Zellen  :  \.,Arc/i.ges.  Phys.,  XXXI.;  II.,  Hud..  XXXIl.:  abstract  in 
BioL  Centb.,  III.,  1884.  —  Id.,  '84.  Uber  die  Einwirkung  der  Schwerkraft  und 
anderer  Bedingiingen  auf  die  Richtung  der  Zelltheilung:  Arch.  ges.  Fhys.,  XXX  I\'. 
—  Id..  "89.  Die  allgemeinen  Lebenserscheinungen  :  Bonn.  —  Platner.  G..  '86.  1 
Zur  Bildungder  Geschlechtsproduktebei  den  Pulmonaten  :  A.  )n.  A.,  XX\'I.  —  Id.. 
'86.2.  —  tiber  die  Befruchtung  von  Arion  enipiricoriim:  A.  m.  A.,  XX\'II.— 
Id..  "89.  1.  Uber  die  Bedeutung  der  Richtungskorperchen  :  B.  C  \'III.  — Id.. 
'89.  2.  Beitrage  zur  Kenntniss  der  Zelle  und  ihrer  Teilungserscheinungen.  I.-\"I.  : 
A.  m.  A..  XXXIII.  —  Poirault  and  Raciborski.  '96.  C'ber  konjugate  Kerne  und 
die  konjugate  Kerntheilung :  B.  C.,  XVI.,  i.  — Prenant,  A..  '94.  Sur  le  corpus- 
cule  central:  Bid/.  Soc.  Sci.,  Nancy,  1894.  — Id.,  98.  '99.  Sur  le  protoplasma 
superieure  (archoplasme,  kinoplasme,  ergastoplasme)  :  Jour.  Anat.  Phys.,  .\.\.\'1\  .. 
XXXV.— Preusse,  F.,  '95.  Uber  die  amitotische  Kerntheilung  in  den  Ovarien 
der  Hemipteren  :  Z.  w.  Z.,  LIX.,  2*.  —  Provost  and  Dumas.  '24.  X.uivelle  tht^orie 
de  la  generation:  Ann.  Sci.  Nat.,  I.,  II.  —  Pringsheim.  N..  '55.  t'ber  die 
Befruchtung  der  Algen  :  Monatsb.  BcrL  Akad.,   1855-56. 

RABL,  C,  '85.  Uber  Zellteilung :  J/./.,  X.  — Id.,  -89.  1.  iMx-r  Zelltheil- 
ung: A.  A.,  IV.  — Id.,  '89,  2.  Uber  die  Prinzipien  der  Histologie:  I'crh.  Anat. 
Ges.,  III.  —vom  Rath,  O,  '91.  Uber  die  Bedeutung  der  amitotischen  Kernteilung 
im  Hoden:  ZooL  Anz.,  XIV.  — Id.,  '92.  Zur  Kenntniss  der  Sperm.-\togene.se  von 
Gryllotalpa  vulgaris:  A.  m.  A.,  XL. —Id.,  '93.  Beitrage  zur  Spermatogenese 
von  Salamandra :  Z.  7U.  Z.,  LVII.  — Id.,  '94.  Vhw  die  Konstanz  der  Chromo- 
somenzahl  bei  Tieren  :  B.  C,  XIV.,  13.— Id.,  95,  1.  Neue  Beitrage  zur  Frage 
der  Chromatinreduction  in  der  Samen-  und  Eireife  :  A.  m.  .4..  XL\'I.  —  Id..  95.  2. 
tiber  den  feineren  Bau  der  Drusenzellen  des  Kopfes  von  Anilocra,  etc.  :  Z.  w.  Z., 
LX.,  I.  —  Rauber,  A.,  83.     Neue  Grundlegungen  zur  Kenntni.ss  der  Zelle:  M.J., 


464  GENERAL   LITERATURE-LIST 

VIII. Rawitz.  B..  '95.     Centrosoma  und  Attraktionsphare  in  der  ruhenden  Zelle 

des  Salamanderhodens :  A.  ;//.  A.,  XLIV..  4. —Id..  *97.  Bemerkungen  uber 
Mikrotomschneiden,  etc.:  A.  A.,  XIII.  —  Reinke.  Fr..  '94.  Zellstudien,  I., 
A.  m.  A.,  XLIII.  :  III.,  Ilmt.,  XLIV..  1894.  — Id..  '95.  Untersuchungen  liber 
Befruchtung  und  Furchung  des  Eies  der  Echinodermen  :  Sitz.-Ber.  Akad.  d.  IViss. 
Berlin,  1895,  June  20. — Reinke  and  Rodewald.  "81.  Studien  liber  das  Proto- 
plasma:  Untersiich.  aiis.  d.  bot .  Inst.  Gotlmgen,  II.— Remak.  R..  '41.  Uber 
Theilung  rother  Blutzellen  beim  Embryo:  Med.  Ver.  Zeit.,  1841.  — Id.,  '50-'55., 
Untersuchungen  liber  die  Entwicklung  der  Wirbelthiere :  Berlin^  1850-55. — Id., 
"58.  Uber  die  Theilung  der  Blutzellen  beim  Embryo:  Muller's  Arch.,  1858. — 
Retzius,  G.,  '89.  Die  Intercellularbrlicken  des  Eierstockeies  und  der  Follikelzellen  : 
Verli.  Anat.  Ges.,  1889. — Rhumbler.  L.,  "93.  Uber  Entstehung  und  Bedeutung 
der  in  den  Kernen  vieler  Protozoen  und  im  Keimblaschen  von  Metazoen  vorkom- 
menden  Binnenkorper  (Nucleolen)  :  Z.  w.  Z..  LVL  — Id.,  '96.  Versuch  einer 
mechanischen  Erklarung  der  indirekten  Zell-  und  Kerntheilung :  Arch.  Entwm.^ 
in. — Id..  "97.  Stemmen  die  Strahlen  der  Astrosphare  oder  ziehen  sie?  Arch. 
Eniivni.,  IV.  —  Rompel,  '94.  Kentrochona  Nebaliee  n.  sp.,  ein  neues  Infusor 
aus  der  Familie  der  Spirochoninen.  Zugleich  ein  Beitrag  zur  Lehre  von  der  Kern- 
teilung  und  dem  Centrosoma  :  Z.  w.  Z.,  LVIII.,  4.  —Rosen,  "92.  Uber  tinctionelle 
Unterscheidung  verschiedener  Kernbestandtheile  und  der  Sexual-kerne :  Cohn's 
Beitr.  z.  Biol.  d.  P/lanzen,V.  —  Id.,  *94.  Neueres  uber  die  Chromatophilie  der 
Zellkerne  :  Schles.  Ges.  v'dterl.  Kult.,  1894.  — Roux,  'W.,  ;83,  1.  Uber  die  Bedeu- 
tung der  Kernteilungsfiguren  :  Leipzig.  — 16..,  "83,  2.  Uber  die  Zeit  der  Bestim- 
mung  der  Hauptrichtungen  des  Froschembryo :  Leipzig.  —  Id.,  '85.  Uber  die 
Bestimmung  der  Hauptrichtungen  des  Froschembryos  im  Ei,  und  liber  die  erste 
Theilung  des  Froscheies  :  Breslaner  iirtzl.  Zeitg.,  1885.— Id.,  '87.  Bestimmung 
der  medianebene  des  Froschembryo  durch  die  Kopulationsrichtung  des  Eikernes 
und  des  Spermakernes  :  A.  in.  A.,  XXIX.  — Id.,  *88.  Uber  das  klinstliche  Hervor- 
bringen  halber  Embryonen  durch  Zerstorung  einer  der  beiden  ersten  Furchungskugeln, 
etc.:  Virchow's  Archiv,  \\\. — Id., '90.  Die  Entwickelungsmechanik  der  Organ- 
ismen.  IVienjiSgo. — Id., '92,  1.  Entwickelungsmechanik:  Merkel  and  Bonjiet, 
Erg.,  II. — Id.,  '92,  2.  tJber  das  entwickelungsmechanische  Vermogen  jeder  der 
beiden  ersten  Furchungszellen  des  Eies:  l/erh.  Anat.  Ges.,  VI. — Id.,  '93,  1. 
Uber  Mosaikarbeit  und  neuere  Entwickelungshypothesen  :  An.  Hefte,  Feb.,  1893. — 
Id..  '93.  2.     Uber  die  Spezifikation   der  Furchungzellen,  etc.  :  B.  C,  XIII.,  19-22 

—  Id.,  "94,  1.  Uber  den  "  Cytotropismus  "  der  Furchungszellen  des  Grasfrosches  : 
Arch.  Entwni.,  I.,  i,  2. — Id.,  '94,  2.  Aufgabe  der  Entwickelungsmechanik,  etc.: 
Arch.  Entwm.,  I.,  i.  Trans,  in  BioL  Lectures,  Wood's  Noll,  1894.  —  Ruckert.  J., 
'91.  Zur  Befruchtung  des  Selachiereies :  y^.  ^.,  VI.  — Id.. '92,  1.  Zur  Entwick- 
lungsgeschichte  des  Ovarialeies  bei  Selachiern  :  A.  A.,  VII.  — Id..  '92.  2.  Uber  die 
Verdoppelung  der  Chromosomen  im  Keimblaschen  des  Selachiereies:  Lbid.,  VIII. 

—  Id.. '93.  2.  Die  Chromatinreduktion  der  Chromosomenzahl  im  Entwicklungs- 
gang  der  Organismen  :  Merkel  and  Bonnet,  Erg.,  III. — Id.,  '94.  Zur  Eireifung 
bei  Copepoden:  An.  Hefte.—16.:,  '95.  1.  Zur  Kenntniss  des  Befruchtungs- 
vorganges:  Sitsb.  Bayer.  Akad.  Wiss.,  XXVI.,  i.  — Id.,  '95,  2.  Zur  Befruchtung 
von  Cyclops  strenuus :  A.  A.,  X.,  22.  — Id.,  '95,  3.  Uber  das  Selbstandigbleiben 
der  vaterlichen  und  mlitterlichen  Kernsubstanz  wahrend  der  ersten  Entwicklung  des 
befruchteten  Cyclops-Eies :  A.  in.  A.,  XLV.,  3.  — Ruge,  G..  '89.  Vorgange  am 
Eifollikel  der  WirbeUhiere  :  y^/./.,  XV. —  Ryder.  J.  A.,  '83.  The  Microscopic 
Sexual  Characteristics  of  the  Oyster,  etc.,  Bull.  U.  S.  Eish.  Comm.,  March  14,  1883. 
Also,  Ann.  Mag.  Nat.  Hist.^  XII.,  1883. 


GENERAL   UTERATURE-I.IST  465 

SABASCHNIKOFF.  M..  -97.  Heitrage  7Air  Kenntniss  der  Chromatinreduk- 
tion  in  der  Ovogenesis  von  Ascaris  :  Bull.  Sac.  Nat .,  Mosanu,  i.  — Sabatiei.  A., 
"90.  De  la  Spermatogdnese  chez  les  Locustides :  Coinptes  RnuL,  CXI..  90.— 
Sachs,  J..  "82.  Vorlesungen  liber  FHanzen-physiologie  :  Lcipzii^.  —  Id.  lM)er  die 
Anordnung  der  Zellen  in  jungsten  Prian/.entheile  :  Arb.  Hot.  Inst.  U'urchnr^,  II.— 
Id..  "92.  Physiologische  Notizen.  I!..  Beitrage  zur  Zellentheorie :  Flora,  1892. 
Heft  I.  —  Id.,  '93.  Stoff  und  Form  der  Pflanzen-organe  :  Gcsatnmelte  Ahhamiluti^en, 
II.,  1893.— Id.,  "95.  Physiologische  Notizen,  IX.,  weitere  Hetrachtungen  Uber 
Energiden  und  Zellen:  Flora,  LXXXI.,  2.  —  Sala.  L.,  "95.  E.xpcrimentelle  L'nter- 
suchungen  uber  die  Reifung  und  Befruchtung  der  Y^\kix  hit\  Ascaris  mej^aloitphala : 
A.  111.  A.,  XL.  —  Sargant,  Ethel,  "95.  Some  details  of  the  first  nuclear  Division 
in  the  Pollen-mother-cells  oi  Lilium  inartaQ^on:  Journ.  Roy.  Mic.  Soc .,  1S95,  jjart  3. 

—  Id.,  "96.  The  Formation  of  the  Sexual  Nuclei  in  Lilium,  I.,  Oogenesis:  ,/;/;/. 
Bot.^  X. — Id.,  "97.  Same  tide.  II.,  Spermatogenesis  :  Ibid.,  XL  — Schafer.  E.  A., 
'91.     General  Anatomy  or  Histology:  in  (2uains  Anatomy,  L.  2.  loth  ed.,  London. 

—  Schaffner,  J.  H.,  "97,  1.     The  Life-history  of  Sagittaria  :   Hot.  Gas.,  XXIII..  4. 

—  Id.,  "97,  2.  The  Division  of  the  .Macrospore  Nucleus  (in  Lilium)  :  Ibid.,  .\XII1., 
6.  —  Id.,  "98.  Karyokinesis  in  Root-tips  of  Allium  :  Ibid.,  XXV'L,  4. — Schaudinn. 
F.,  '95.  Uber  die  Theilung  von  Ania'ba  binucleata  (/ruber:  Sita.-Ber.  cJts.  \iitur- 
forsch.  Freunde,  Berlin,  Jahrg.  1895,  No.  6.  —  Id.,  "96,  1.  t'ber  den  Zeugungs- 
kreis  von  Paraniceba  Eilhardi :  Sitz.-Ber.  Akad.  Wiss.,  Berlin,  1896,  Jan.  16. — 
Id.,  "96,  2.  Uber  die  Copulation  von  Actinophrys  Sol:  Ibid. — Id..  "96.  3. 
Uber  das  Centralkorn  der  Heliozoen  :  Verh.  D.  Zool.  Ges.  —  Schewiakoff.  "W., 
'88.     Uber  die  karyokinetische  Kerntheilung  der  ^'/ziV^'/Z/rt  alveolata:   .1/. ./.,  .MIL 

—  Id.,  "93.  Uber  einen  neuen  Bakterienahnlichen  Organismus :  Hab.  Sclinft, 
Heidelberg,  Winter.  —  Schiefferdecker  and  KosseL  "91.  Die  (iewehe  des 
Menschlichen  Korpers  :  BraunscJiiveig.  —  Schiinper.  '85.  Untersuchungen  uber 
die  Chlorophyllkorper,  etc.:  Zeitsch.  luiss.  Bot.,  X\T.  —  Schleicher,  "W.,  "78.  Die 
Knorpelzelltheilung.  Ein  Beitrag  zur  Lehre  der  Theilung  von  Gewebezellen : 
Centr.  nied.  IViss.  Berlin,  1878.  Also  A.  ni.  A.,  X\T.,  1879.  —  Schleiden.  M.  J.. 
'38.  Beitrage  zur  Phytogenesis  :  M'ullers  Arc/iiiu  1838.  [Trans,  in  Syden/iam 
Soc,  XII.:  London,  1847.] — Schloter,  G.,  "94.  Zur  Morphologic  der  Zelle : 
A.  m.  ^.,XLI\^.,  2.  —  Schmitz.  "84.  Die  Chromatophoren  der  Algen.  —  Schnei- 
der, A.,  '73,  Untersuchungen  uber  Plathelminthen  :  Jalirb.  d.  ober/iess.  LJes.  f. 
Natur-Heilkunde,  XIV\,  Giessen.  —  Schneider.  C  "91.  Untersuchungen  uber  die 
Zelle:  Arb.  Zool.  Inst.  IVien,  IX.,  2.  —  Schottlander.  J..  "88.  Cber  Kern  und 
Zelltheilungsvorgange  in  dem  Endothel  der  entzundeten  Hornhaut  :  .  /.  ///.  ,-/., 
XXXI.  —  Schottlander,  P..  "93.  Beitrage  zur  Kenntniss  des  Zellkerns,  t'tc: 
Colin' s  Beitriis^e.,  W.  — Schultze.  Max.  "61.  ('ber  .Muskelkorperchen  und  da'^  was 
man  eine  Zelle  zu  nennen  hat:  Arch.  Anat.  P/iys.,  1861.  —  Schultze.  O..  07. 
Untersuchungen  liber  die  Reifung  und  Befruchtung  des  Amphibicn-cics :  Z.ic.Z., 
XLV.  — Id.,  '90.  i'ber  Zelltheilung :  Sitcb.  pliys.  nied.  Ges.  ll'tircbnrg. —  Id.. 
'94.  Die  klinstliche  Erzeugung  von  Doppelbildungen  bei  F'roschlarvon,  <•/<-. :  Arc/i. 
Fntwni.,  I.,  2.  —  Schwann,  Th..  "39.  .Mikroscopische  L^nter.suchungen  liber  die 
i'bereinstimmung  in  der  Structur  und  dem  W'achsthum  der  Thiere  und  Pflanzen  : 
Berlin.  [Trans,  in  Sydenham  Soc,  XII.:  London.  1847.]  —  Schwarz.  Fr..  "87. 
Die  Morphologische  und  chemische  Zusammensetzung  des  Protopl.isnias :   Bresiau. 

—  Schweigger-Seidel.  O..  "65.  C'ber  die  SamenkJirpcrchcn  und  ihre  Entwick- 
elung:  A.  m.  A.,  I.  —  Sedgwick,  A..  "85  "88.  The  Development  of  the  Cape 
Species  of  Peripatus,  I.-VI. :  Q./.,  XX\'.-XX\'III.  — Id..  "94.  On  the  Inade(iu.icy 
of  the  Cellular  Theory  of  Development,  etc.:  Ibid.,  X.X.W'IL.  i.  — Seeligei.  O., 
'94.  Giebt  es  geschlechtlicherzeugte  Organismen  ohne  miitterliche  Eigenschaften? : 
A.  Ent.,  I.,  2. — Selenka,  E.,  "83.      Die  Keimbliitter  der  Echinodermen  :  Studien 

2  H 


466  GENERAL   LITERATURE-LIST 

iiber  Entwkk.,  II.,  Wiesbaden,  1883.  —  Sertoli,  E.,  "65.  Dell' esistenza  di  parti- 
colari  cellule  ramificate  dei  canaliculi  seminiferi  del  testicolo  umano :  II  Morgagni. 

—  Shaw,  "W.  R.,  "98,  1.  I'ber  die  Blepharoplasten  bei  Onoclea  und  Marsilia : 
Ber.  D.  Bot.  Ges.,  X\'I.,  y.  —  16..,  '98.  2.  The  Fertilization  of  Onoclea  :  Ann.  Bot., 
XII.,  47.  —  Siedlecki,  M.,  "95.  Uber  die  Struktur  und  Kerntheilungsvorgange  bei 
den  Leucocyten  der  Urodelen  :  Auz.  Akad.  IV/ss.,  /Crakau,  1895.  — Id.,  "99.  fitude 
cytologique  et  cycle  evolutif  de  Adelea :  Ann.  Inst.  Pasteur.,  XIII.  —  Sobotta,  J., 
"95.  Die  Befruchtung  und  Furchung  des  Eies  der  Maus  :  A.  ni.  A.,  XLV. — Id., 
'97.  Die  Reifung  und  Befruchtung  des  Eies  von  Amphioxus  :  Id/d.,  L.  —  Solger, 
B.,  "91.  Die  radiaren  Strukturen  der  Zellkorper  im  Zustand  der  Ruhe  und  bei  der 
Kerntheilung:  Bert.  Klin,  irochenschr.,  XX.,  1891.  —  Spallanzani,  1786.  Expe- 
riences pour  servir  k  I'histoire  de  la  generation  des  animaux  et  des  plantes  :  Generia. 

—  Spitzer,  '97.  Die  Bedeutung  gewisser  Nucleoproteide  flir  die  oxydative  Leistung 
der  Zelle :  Ajxh.  ges.  Phys.^  LXVII.  —  Stevens.  "W.  C,  '98.  tHDer  Chromoso- 
mentheilung  bei  der  Sporenbildung  der  Fame:  Ber.  D.  Bot.  Ges.,  XVI.,  8.  —  Ste- 
vens, F.  L..  '99.     The  compound  Oosphere  of  Albugo:   Bot.  Gas.,  XXVIII.,  3,  4. 

—  Strasburger,  E.,  "75.  Zellbildung  und  Zelltheilung  :  ist  ed.,  Jena,  1875. — Id., 
'77.  Uber  Befruchtung  und  Zelltheilung:  /.  Z.,  XL  — Id.,  '80.  Zellbildung  und 
Zelltellung:  3d  ed. — Id.,  "82.  Uber  den  Theilungsvorgang  der  Zellkerne  und  das 
Verhaltniss  der  Kerntheilung  zur  Zelltheilung:  A.  ni.  A..,  XXI.  —  Id.,  '84,  1.  Die 
Controversen  der  indirecten  Zelltheilung:  Ibid.,  XXIII.  —  Id.,  "84,  2.  Neue  Unter- 
suchungen  iiber  den  Befruchtungsvorgang  bei  den  Phanerogamen,  als  Grundlage  flir 
eine  Theorie  der  Zeugung  :  Jena,  1884.  — Id.,  '88.  Uber  Kern-  und  Zellteilung  im 
Pflanzenreich,  nebst  einem  Anhang  iiber  Befruchtung:  Jena.  —  Id.,  '89.  Uber  das 
Wachsthum  vegetabilischer  Zellhaute  :  Hist.  Bei.,  \\.,Jena.  —  Id.,  '91.  Das  Proto- 
plasma  und  die  Reizbarkeit :  Rektoratsrede,  Bonn,  Oct.  18,  1891.  Jena,  Fischer. — 
Id.,  '92.  Histologische  Beitrage,  Heft  IV.  :  Das  Verhalten  des  Pollens  und  die 
Befruchtungsvorgange  bei  den  Gymnospermen,  Schwarmsporen,  pflanzliche  Sperma- 
tozoiden  und  das  Wesen  der  Befruchtung:  Fischer,  Jena,  1892. — Id.,  "93,  1.  Uber 
die  Wirkungssphare  der  Kerne  und  die  Zellengrosse  :  Hist.  Beitr.,  W  —  Id.,  "93,  2. 
Zu  dem  jetzigen  Stande  der  Kern-  und  Zelltheilungsfragen  :  A.  A.,  VIII.,  p.  177. — 
Id.,  "94.  Uber  periodische  Reduktion  der  Chromosomenzahl  im  Entwicklungsgang 
der  Organismen:  B.  C,  XIV. — Id.,  "95.  Karyokinetische  Probleme  :  Jahrb.  f. 
wiss.  Botanik,  XXVIII.,  i .  —  Id.,  "97, 1.  Kerntheilung  und  Befruchtung  bei  Fucus  : 
Jahrb.  wiss.  Bot.,  XXX.— Id..  '97,  2.  Uber  Befruchtung:  Ibid.—IA.,  '97,  3. 
Uber  Cytoplasmastrukturen,  Kern-  und  Zelltheilung:  Ibid. — Id.,  "98.  Die  Pflanz- 
lichen  Zellhaute:  Ibid.,  XXXI.  —  Strasburger  and  Mottier,  "97.  Uber  den 
zweiten  Theilungsschritt  in  Pollenmutterzellen  :  Ber.  D.  Bot.  Ges.,  XV.,  6.  —  Van 
der  Stricht,  O.,  '92.  Contribution  a  l"etude  de  la  sphere  attractive:  A.  B.,  XII., 
4- — Id..  "95.  1.  La  maturation  et  la  fecondation  de  I'oeuf  d'Amphioxus  lanceolatus  : 
Bull.  Acad.  Roy.  Belgiqiie,  XXX.,  2.  — Id..  "95,  2.  De  I'origine  de  la  figure  achro- 
matique  de  Tovule  en  mitose  chez  le  Thysanozoon  Brocchi :  Verhandl.  d.  anat. 
Versamnd.  in  Strassbiirg,  1895,  p.  223.  — Id.,  "95,  3.  Contributions  a  Tetude  de  la 
forme,  de  la  structure  et  de  la  division  du  noyau :  BiilL  Acad.  Roy.  Sc.  Belgiqne, 
XXIX. — Id..  "98,  1.  La  formation  des  globules  polaires,  etc.,  chez  Thysanozoon: 
Arch.  Biol.,  XV.  —  Id.,  "98.  2.  Contribution  a  Tetude  du  noyau  vitellin  de  Balbiani : 
Verh.  An.  Ges.,  XII. — Strieker,  S..  "71.  Handbuch  der  Lehre  von  den  Geweben  : 
Leipzig.  —  Stuhlmann.  Fr.,  '86.  Die  Reifung  des  Arthropodeneies  nach  Beobach- 
tungen  an  Insekten,  Spinnen,  Alyriopoden  und  Peripatus  :  Ber.  Naturf.  Ges.  Frei- 
burg, 1.  —  Suzuki,  B.,  '98.  Notiz  iiber  die  Entstehung  des  Mittelstuckes  von  Sela- 
chiern:  A.  A.,  XV.,  8.  —  Swaen  and  Masquelin.  "83.  Etude  sur  la  Spermato- 
genese:  A.  B.,  IV.  — Swingle,  "W.  T.,  "97.  Zur  Kenntniss  der  Kern-  und 
Zellteilungen  bei  den  Sphacelariaceae :  /.  w.  B.,  XXX. 


GENERAL   LITERATURE-LIST  467 

THOMA,  R.,  "96.  Text-book  of  General  Pathologv  and  Pathological  Anatomy  : 
Trans,  by  A.  Bruce,  /.rv/^/^w.  —Thomson.  Allen.  Article  -  (;eneration  '  in  Todd's 
Cyclopaedia. —Id.  Article  "Ovum"  in  Todd's  Cyclopa-dia.  —  Townsend.  C.  O., 
"97.  Der  Einfluss  des  Zellkerns  auf  die  liildung 'der  Zellhaut :  Jahrb.  ;.■/.,.  Hot., 
XXX.— Treat,  Mary.  -73.  Controlling  Sex  in  Butterflies:  .////.  Xnt .,  VII.— 
Trow,  A.  H..  "95.  The  Karyology  of  Saprolegnia :  .-/////.  Hot.,  IX.  — Tyson, 
James,  '78.      The  Cell-doctrine  :    2d  ed.,  Pliiladelpliia. 

UNNA,  P.,  '95..  tJber  die  neueren  Protoplasmathcorien,  und  das  Spongio- 
plasma:  Deutsche  Med.  Zeit.,  1895,  98-100. — Ussow.  M..  "81  Untersuchungen 
Uber  die  Entwickelung  der  Cephalopoden  :  Arch.  JhoL,  11. 

VEJDOVSKY.  F..  *88.  Entwickelungsgeschichtliche  Untersuchungen.  Heft  I. : 
Reifung,  Befruchtung  und  Furchung  des  Rhynchelmis-Eies :  Pra^,  1888.  Vej- 
dovsky  and  Mrazek.  "98.  Centrosom  und  Periplast :  Sitzber.bdhm.  Gcs.  ll'iss.  — 
Verworn,  M..  "88.  Biologische  Protisten-studien  :  Z.  a-.  Z.,  XLVI.  —  Id.. '89. 
Psychophysiologische  Protisten-studien:  /eua.  —  Id.,  "91.  Die  phvsiologi>the 
Bedeutung  des  Zellkerns  :  Pfl'i'iger's  Arch.  f.  d.  ges.  Physiol.,  II.— Id. ,"95.  Allge- 
meine  Physiologie  : /.?;/^.  —  Virchow,  R.,  "SS.  Cellular-Pathologie  :  Arch.  I\ith. 
Anat.  Phys.,  VHI.,  i.— Id.,  "58.  Die  Cellularpathologie  in  ihrer  Begriinduncj  auf 
physiologische  und  pathologische  Gewebelehre  :  Berlin,  1858.  — De  Vries.  H..  '89. 
Intracellulare  Pangenesis  :  Jena. 

"WAGER,  H.,  "96.  On  the  StiTicture  and  Reproduction  of  Cvstopus.  .-/;///.  />V)/., 
X.  —  Waldeyer,  W..  '70.  Eierstock  und  Ei :  Leipzij^.—l^.'.  87.  Bau  und  Ent- 
wickelung der  Samenfaden  :  Verh.  A71.  Ges.  Leipzig,  1887.  —  Id..  "88.  ('ber  Karvo- 
kinese  und  ihre  Beziehungen  zu  den  Befruchtungsvorgangen :  ./.  m.  .7.,  X.XXII. 
[Trans,  in  Q.  J.~\  —  Id.,  "95.  Die  neueren  Ansichten  liber  den  Bau  und  das  Wesen 
der  Zelle  :  Deutsch.  Med.  Wochenschr.,  No.  43,  flf.,  Oct.  ff.,  1895.  —  Warneck.  N, 
A.,  '50.  Uber  die  Bildung  und  Entwickelung  des  Embryos  bei  Gasteropoden : 
Bidl.  Soc.  Imp.  N'at.  Moscou,  XXIII.,  i.  —  "Watase.  S..  "91.  Studies  on  Cephaio- 
pods;  I.,  Cleavage  of  the  Ovum:  /.  M.,  IV.,  3.  —  Id..  92.  On  the  Phenomena  of 
Sex-differentiation:  Ibid.^  VL,  2,  1892.  —  Id..  "93.  1.  On  the  Nature  of  Cell- 
organization:  Wood's  Hall  Biol.  Lectures,  1893. — Id..  '93.  2.  Homology  of  the 
Centrosome  :  J.  M.,  VIII.,  2.  —  Id.,  "94.  Origin  of  the  Centrosome  :  Biological  Lec- 
tures, Wood'' s  LI  all,  1894.  "Webber,  H.  J,  "97.  1.  Peculiar  Structures  occurring 
in  the  Pollen-tube  of  Zamia:  Bot.  Gazette,  XXII I.,  6.  — Id..  "97.  2.  The  Develop- 
ment of  the  Antherozoids  of  Zamia:  Ibid.,  XX1\'..  i.  —  Id..  "97.  3.  Notes  on  the 
Fecundation  of  Zamia  and  the  Pollen-tube  Apparatus  of  liingko:  Ibid.,  XXIV'.,  4. 
—  Weismann,  A.,  "83.  Uber  Vererbung:  Jena. — Id..  "85.  Die  Kontinuit.iit  des 
Keimplasmas  als  Grundlage  einer  Theorie  der  \'ererbung  :  Jena.  —  Id..  '86.  1. 
Richtungskorper  bei  parthenogenetischen  Eiern  :  Zool.  Anz.,  No.  233.  Id..  86.  2. 
Die  Bedeutung  der  sexuellen  Fortpflanzung  fiir  die  Sclcktionstheorie :  Jeita.  — 
Id.,  '87.  i'^ber  die  Zahl  der  Richtungskorper  und  uber  ihre  Beileutung  fiir  die 
Vererbung  : /i?;/ (^7. — Id..  "91.  1.  Essays  upon  Heredity.  First  Series:  O.vford. — 
Id..  "91,  2.  Amphimixis,  Oder  die  \'ermischung  der  Inilividuen  :  A*//,/.  Fischer.— 
Id., '92.  Essays  upon  Heredity.  Second  Series:  O.x/ord,  1S92.  —  Id..  "93.  The 
Germ-plasm:  A^e^u  y^ork.  —  16...  "94.  .Vu.s.sere  Einfiii.sse  als  Entwicklungsreize : 
Jena.  —  Id..  "99.  Regeneration:  Xat.  Sci.,  XIV ..  6.  [See  al.so  A.  A..  1899.] 
Wheeler.  W.  M..  "89.  The  Embryology  of  />la/ta  Gcrmauica  and  Doryphora 
decemlineata:  J.  M.,  111. —  Id..  "93.  .A.  Contribution  to  Insect-embryology:  Ibid., 
VIII.,  I.  —  Id.,  "95.  The  Behaviour  of  the  Centrosomes  in  the  Fertilized  Egg  of 
Myzostoma  glabruni:  Ibid.,  X.  — Id..   "96.     The   Sexual   Phases  of   .Myzostoma : 


/ 

I 

468  GENERAL  LITERATUKE-LIST 

Mitth.  Zool.  St.  Neapel.  Xll.,  2. — Id.,  "97,  The  Maturation,  Fecundation,  and 
early  Cleavage  in  Myzostoma :  Arc/i.  Biol.,  XV. -r. Whitman.  C.  O.,  '78.  The 
Embryology  of  Clepsine:  (2- J-^  X\TII.  —  Id.,  '87.  The  Kinetic  Phenomena  of 
the  Egg  during  Maturation  and  Fecundation  :  /.  J/.,  I.,  2.  — Id..  "88.  The  Seat  of 
Formative  and  Regenerative  Energy:  Ibid.,  II.  —Id..  "93.  The  Inadequacy  of  the 
Cell-theory  of  Development :  Wood's  H oil  Biol.  Lectures,  1893.  —  Id.,  "94.  Evolu- 
tion and  Epigenesis  :  Lbid.,  1894. — Wiesner.  J..  "92.  Die  Elementarstruktur  und 
das  Wachstum  der  lebenden  Substanz  :  ll'ieu. — Wilcox.  E.  "V..  "95.  Spermato- 
genesis of  Caloptenus  and  Cicada  :  Bull,  of  tJie  ALuseuui  of  Co)np.  Zool.,  Liarvard 
College,\' o\.XXX\\.,'^Q.  i.  —  Id.,  "96.  Further  Studies  on  the  Spermatogenesis 
of  Caloptenus  :  Bull.  ALus.  Comp.  Zool.,  XXIX. — "Will,  L..  "86.  Die  Entstehung 
des  Eies  von  Colymbetes :  Z.  w.  Z.,  X  LI  1 1. —"Wilson,  Edm.  B..  "92.  The  Cell- 
lin»eage  of  Nereis :  f.  M.,  VI.,  3.  — Id..  "93.  Amphioxus  and  the  Mosaic  Theory 
of  Development :  Ll?id.,  \TII.,  3.  —  Id.,  "94.  The  Mosaic  Theory  of  Development: 
Wood's  Noll  Biol.  Led.,  1894.  —  Id..  "95.  1.  Atlas  of  Fertilization  and  Karyo- 
kinesis :  Ne7i>  York,  Maonillau. — Id.,  '95,  2.  Archoplasm,  Centrosome,  and 
Chromatin  in  the  Sea-urchin  Egg:  /.  J/.,  XI. — Id.,  "96.  On  Cleavage  and 
Mosaic-work.  [Appendix  to  Crampton  and  Wilson,  "96.]  :  A.  Eiitwni.,  III.,  i. — 
Id.,  "97.  Centrosome  and  Middle-piece  in  the  Fertilization  of  the  Egg.  Science-, 
Vol.  v.,  No.  114. — Id..  "98.  Considerations  on  Cell-lineage  and  ancestral  Remi- 
niscence: Ann.  JV.  Y.  Acad.  Sci.,  XI.  See  also  JVood's  Moll  Biol.  Lectures,  '99. 
—  Id..  "99.  On  protoplasmic  Structure  in  the  Eggs  of  Echinoderms  and  some 
other  Animals  :/.  J/.,  XV.  Suppl. — "Wilson  and  Mathews.  "95.  Maturation, 
Fertilization,  and  Polarity  in  the  Echinoderm  Egg:  /.  J/.,  X.,  i.  —  "Wolff,  Caspar 
Friedrich,  1759.  Theoria  Generationis.  —  "Wolff,  Gustav,  "94.  Bemerkungen 
zum  Darwinismus  mit  einem  experimentellen  Beitrag  zur  Physiologic  der  Entwick- 
lung:  B.  C,  XIV.,  17.  —  Id.,  '95.  Die  Regeneration  der  Urodelenlinse :  Arc/i. 
Entu)}n.,  I.,  3.  —  "Wolters.  M.,  "91.  Die  Conjugation  und  Sporenbildung  bei 
Gregarinen:  A.  m.  A.,  XXXVII.  —  Woltereck,  R.,  "98.  Zur  Bildung  und  Ent- 
wicklung  des  Ostrakoden-Eies  :  Z.  w.  Z.,  LXIV. 

■yUNG.  E..  "81.  De  Tinfluence  de  la  nature  des  aliments  sur  la  sexualite : 
C.  R.,  XCIII ;  also  Arc/i.  Exp.  Zool.,  2d,  I.,  1883. 

ZACH ARIAS.  O.,  "85.  Uber  die  amoboiden  Bewegungen  der  Spermatozoen 
von  Polyphemus  pediculus  :  Z.  w.  Z.,  XLI. — Zacharias,  E..  "93.  1.  t'ber  die 
chemische  Beschaffenheit  von  Cytoplasma  und  Zellkern :  Ber.  deutsch.  Bot.  Ges., 
II.,  5.  — Id..  "93.  2.  Uber  Chrimatophihe  :  Lbid.,  1893. —  Id..  "95.  Uber  das 
Verhalten  des  Zellkerns  in  wachsenden  Zellen :  Flora,  %i,  1895. — Id..  "94.  t^ber 
Beziehungen  des  Zellenwachstums  zur  Beschaffenheit  des  Zellkerns  :  Berichte  der 
deutschen  botan.  Gesellschaft,  XII.,  5.  — Id..  "98.  t'ber  Nachweis  und  Vorkommen 
von  Nuclein  :  Ber.  d.  Bot.  Ges.,  XVI.,  7.  —  Ziegler.  E.,  "88.  Die  neuesten  Arbeiten 
liber  Vererbung  und  Abstammungslehre  und  ihre  Bedeutung  fur  die  Pathologic : 
Beitr.  zur  path.  Anat.,  IV. — Id.,  "89.  Uber  die  Ursachen  der  pathologischen 
Gewebsneubildungen  :  /;//.  Beitr.  zur.  iviss.  Med.  Festschrift,  R.  Virchow,  II. — 
Id..  "92.  Lehrbuch  der  allgemeinen  pathologischen  Anatomic  und  Pathogenese,  7th 
ed.,  fena.  —  Ziegler.  H.  E..  "87.  Die  Entstehung  des  Blutes  bei  Knochenfischen- 
embryonen :  A.  in.  A.  —  Id.,  '91.  Die  biologische  Bedeutung  der  amitotischen 
Kerntheilung  im  Tierreich  :  B.  C,  XI.  — Id.,  "94.  Uber  das  Verhalten  der  Kerne 
im  Dotter  der  meroblastischen  Wirbelthiere  :  Ber.  N'aturf.  Ges.  Freiburg,  1894. — 
Id.,  '95.  Untersuchungen  liber  die  Zelltheilung :  I'erhandl.  d.  deutsch.  Zool.  Ges., 
1895. — ^^-j  ^^-  Einige  Betrachtungen  zur  Entwicklungsgeschichte  der  Echino- 
dermen  :    Verh.  d.  Zool.  Ges.  —  Id.,  "98.     Experimentelle  Studien  uber  die  Zellthei- 


GENERAL   UrERATUKE-LIST  469 

lung,  I.,  II.:  Arch.  Entwm.,  VI.,  2.  — Ziegler  and  vom  Rath.  Die  amitotische 
Kerntheilung  bei  den  Arthropoden  :  />'.  C"..  XI.  -  Zimmermann,  A..  93.  1.  Hei- 
trage  zur  Morphologic  und  Phy.siologie  der  Frian/.en/.elle :  Tubiui^nt. —  1^  .  94 
Sammelreferate  aus  dem  Gesammtgebiete  der  Zellenlehre :  Hot.  Lnith.  lialufte, 
1894.  Zimmermann.  K.  W..  93.  2.  Studien  iiber  i'igmentzellen,  etc.  :  ./.  //;../.] 
XLI.— Id..  "98.  Beitrage  zur  Kenntni.s.s  einiger  Drusen  und  Kpithelzellcn  :  .-/.  m. 
A..LU.  —  Zoja.  R..  "95.1.  Sullo  .sviluppo  dei  bla.stomeri  isolati  dalle  uova  di 
alcunemeduse:^.  ^;//w///.,I.,4;  II.,  i  :  II.,  IV. -Id.  95.  2.  .Sulla  independenza 
della  cromatina  paterna  e  materna  nel  nucleo  delle  cellule  embrionali :  .-/.  .-/..  XI., 
10.  Id.. '97.  Stato  attuale  degli  Studii  sulla  Fecondazione  :  />W/.  .SV/.  di  Pavta, 
XVIII.,  XIX.— Zur  Strassen.  O..  -98.  C'ber  die  Rie-senbildung  bei  Ascaris- 
Eiern  :  Arch.  E?itwm.,  \'II.,  4. 


INDEX    OF    AUTHORS 


Albrecht,  nuclei,  32. 

Altmann,  granule-theory,  25,  27,  290 ;    nu- 

clein,  332. 
Amici,  pollen-tube,  218. 
Andrews,  spinning  activities,  61. 
Apathy,  nerve-cells,  48. 
Aristotle,  epigenesis,  8. 
Arnold,  fibrillar   theory  of  protoplasm,  23; 

leucocytes,   117;    nucleus  and   cytoplasm, 

303. 
Atkinson,  reduction,  269. 

Auerbach,    6;      double    spermatozoa,    142; 

staining-reactions,  176;    fertilization,  i8i. 

Von  Baer,  cleavage,  lO;  cell-division,  64; 
egg-axis,  378;   development,  396. 

Balbiani,  scattered  nuclei,  40;  spireme- 
nuclei,  36;  mitosis  in  Infusoria,  88;  chro- 
matin-granules,  112;  yolk-nucleus,  155- 
156;   regeneration  in  Infusoria,  343. 

Balfour,  polar  bodies,  243;  rate  of  division, 
366;    unequal  division,  371. 

Ballowitz,  structure  of  spermatozoa,  139, 
140;    double  spermatozoa,  142. 

Van  Bambeke,  deutoplasm  and  yolk-nucleus, 
156-160;    elimination  of  chromatin,  155. 

Barry,  fertilization,  181. 

De  Bary,  protoplasm,  4,  5,  20;  conjugation, 
181;   cell-division  and  growth,  393. 

Beale,  cell-organization,  291. 

Bechamp  and  Estor,  microsome-theory,  290, 
291. 

Belajeff,  spermatozoids,  172-175;  reduction 
in  plants,  267. 

Benda,  spermatogenesis,  163;  Sertoli-cells, 
284. 

Van  Beneden,  cell-theory,  i,  6,  7;  proto- 
plasm, 23;  nuclear  membrane,  ^S;  cen- 
trosome  and  attraction-sphere,  51,  74,  77, 
310,  323;  cell-polarity,  55;  cell-division, 
64,  74;  origin  of  mitotic  figure,  74-77! 
theory  of  mitosis,  100;  division  of  chromo- 
somes, 112;  fertilization  oi  Ascoris,  7,  1 82; 
continuity     of     centrosomes,     75;     germ- 


nuclei,  205;  centroNonie  in  fertilization, 
208;  theory  of  sex,  243;  j)arlhcnogt:niMs, 
281;  nucleus  and  cyt<^plasni,  303;  nuckar 
microsomes,  302 ;  promorphology  of  cleav- 
age, 381 ;   germinal  localization,  399. 

Van  Beneden  and  Julin,  first  cleavage-plane, 
380. 

Bergmann,  cleavage,  lO;   cell,  17. 

Bernard,  Clauile,  nucleus  and  cytoplasm.  ^41 ; 
organic  synthesis,  431. 

Berthold,     protoplasm,     42  ;     cell-division, 

376- 

Bickford,  regeneration  in  ctelenierates,  392, 
429. 

Biondi,  Sertoli-cells,  284. 

Biondi-Ehrlich,  staining-tluid,  157. 

Bischoff,  cell,  17. 

Bizzozero,  cell-bridges,  60. 

Blanc,  fertilization  of  trout,  210. 

Blochmann,  insect-egg,  132;  budding  of  nu- 
cleus, 155;  polar  bodies,  281 ;  bilaterality 
of  ovum,  3S3. 

Bohm,  fertilization  in  fishes,  192. 

Bolsius,  ne[)hridial  cells,  47. 

Bonnet,  theory  of  development.  S,  432. 

Born,  chromosomes  in  7>/A)//-cgg,  338; 
gravitation-experiments,  386. 

Boveri,  centrosome.  named.  51 :  a  i>ermancnl 
organ,  51,  74;  in  fertilization,  192,  Jii. 
215,  230;  structure,  309:  functions, 
archoplasm,  69,  318;  irigin  of  inuouc 
figure.  74,  77,  319:  varieties  of  Asmrif, 
87;  theory  of  mitosis.  lOi.  loS:  division 
of  chromosomes,  1 1 2 ;  origin  of  gcrm-ccUs. 
147;  fertilization  of  Asraris,  182;  of 
rurotroihen,  1 84;  of  Echinus,  102:  the- 
ory of  fertilization,  KK3,  211;  of  partheno- 
genesis. 281 ;  partial  fertilization,  I90,  I94; 
retiuction,  233;  maturation  in  Ascitris^ 
23S;  tetrads,  238;  centriole,  309:  attrac- 
tion-sphere, 324;   egg-fragments,  353. 

Braeni,  cell-division,  377. 

Brandt,  symbiosis,  53;  regeneration  in  Tro- 
tozoa,  342. 


47 


472 


INDEX   OF  AUTHORS 


Brauer,  bivalent  chromosomes,  82;  mitosis 
in  rhizopod,  96;  fission  of  chromatin- 
granules,  113;  deutoplasm,  153;  fertiliza- 
tion in  Bra7ichipus,  192;  parthenogenesis 
in  Artemia,  281 ;  spermatogenesis  in  Asca- 
ris,  255;    intra-nuclear  centrosome,  304. 

Braus,  81. 

Brogniard,  pollen-tube,  218. 

Brooks,  heredity,  12;   variation,  179. 

Brown,  Robert,  cell-nucleus,  18;  pollen- 
tube,  218. 

Briicke,  cell-organization,  289. 

Von  Brunn,  spermatozoon,  141. 

Biihler,  astral  systems,  318. 

Biitschli,  6;  protoplasm,  25,  36,  50;  diffused 
nuclei,  40;  artifacts,  42;  asters,  48,  316; 
cell-membrane,  54;  mitosis,  109,  no; 
centrosome  in  diatoms,  51;  rejuvenes- 
cence, 178;    polar  bodies,  238. 

Calberla,  micropyle,  200. 

Calkins,  nuclei  of  flagellates,  40;  mitosis  in 
A'octiluca,  92;  yolk-nucleus,  157;  origin 
of  middle-piece,  165;   reduction,  253,  257. 

Campbell,  fertilization  in  plants,  216. 

Carnoy,  nucleus,  40;  muscle-fibre,  48;  cen- 
trosome, no;  amitosis,  115,  117;  germ- 
nuclei,  184;    asters,  305,  317. 

Carnoy  and  Le  Brun,  nucleoli,  130;  fertiliza- 
tion, 211;   reduction,  263. 

Castle,  egg-axis,  379;   fertilization,  193. 

Chittenden,  organic  synthesis,  341. 

Chmielewski,  reduction  in  Spirogyj'a,  280. 

Chun,  amitosis,  117;  partial  development  of 
ctenophores,  418. 

Clapp,  first  cleavage-plane,  381. 

Coe,  fertilization,  194,  213;    centrosome,  321. 

Cohn,  cell,  17, 

Conklm,  size  of  nuclei,  71 ;  union  of  germ- 
nuclei,  204;  centrosome  in  fertilization, 
210;  centrosome  and  sphere,  323;  un- 
equal division,  373;  protoplasmic  cur- 
rents, 377;  cell-size  and  body-size,  388; 
types  of  cleavage,  423. 

Corda,  pollen-tube,  218. 

Crampton,  yolk-nucleus,  158;  reversal -of 
cleavage,  368;  experiments  on  snail,  419, 
421 ;   on  tunicates,  419. 

Crato,  protoplasm,  50. 

Darwin,  evolution,  2,  5;  inheritance,  12,  396; 
variation,  ii;  pangenesis,  12,  290;  gem- 
mules,  290. 

Darwin,  F.,  protoplasmic  fragments,  346. 

Dendy,  cell-bridges,  60. 


Dogiel,  amitosis,  118. 

Driesch,  dispermy,  198;  fertilization  of  egg- 
fragments,  200,  353;  pressure-experiments, 
375,410;  regeneration,  393;  isolated  blas- 
tomeres,  409;  theory  of  development,  394, 
415;  experiments  on  ctenophores,  418; 
ferment-theory,  427. 

Driiner,  spindle-fibres,  79;  central  spindle, 
105;   aster,  321,  326. 

Von  Ebner,  Sertoli-cells,  284. 

Ehrlich,  tar-colours,  335. 

Eismond,  structure  of  aster,  48. 

Elssberg,  plastidules,  291. 

Endres,  experiments  on  frog's  egg,  399,  419. 

Engelmann,  ciliated  cells,  44;  rejuvenes- 
cence, 179, 

Von  Erlanger,  asters,  48,  316;  spindle,  81; 
elimination  of  chromatin,  155;  Xebenkern, 
163,165;  fertilization,  194,  212,  213;  cen- 
troplasm,  324. 

Eycleshymer,  first  cleavage-plane,  381. 

Farmer,  reduction  in  plants,  275. 

Fick,  fertilization  of  axolotl,  192,  212. 

F'ield,  staining-reactions,  176. 

Fischel,  ctenophores,  419. 

Fischer,  nucleus,  40;  artifacts,  42;  staining- 
reactions,  335. 

Flemming,  protoplasm,  25,  27 ;  chromatin,  2,3 ', 
centrosome,  51 ;  cell-bridges,  60,  61 ;  cell- 
division,  64,  70;  splitting  of  chromosomes, 
70;  mitotic  figure,  79;  heterotypical  mito- 
sis, 86;  leucocytes,  102;  theory  of  mitosis, 
106;  division  of  chromatin,  1 13;  amitosis, 
117,285;  nucleoli,  127;  rotation  of  sperm- 
head,  188;  spermatogenesis,  259-262; 
astral    rays,    317;     germinal    localization, 

399- 
Floderus,  follicle-cells,  150. 

Fol,  I,  6,  64;  amphiaster,  68;  theory  of  mi- 
tosis, 108;  sperm-centrosome,  191 ;  poly- 
spermy, 192;  attraction-cone,  198;  vitel- 
line membrane,  199;    asters,  316. 

Foot,  yolk-nucleus  and  polar  rings,  156,  202; 
fertilization  in  earthworm,  187;  entrance- 
funnel,  201;    fertilization-centrosome,  212. 

Foster,  cell-organization,  somacules,  291. 

Francotte,  polar  bodies,  235 ;  centrosome, 
306;    sphere,  312,  325. 

Frommann,  protoplasm,  23;  nucleus  and 
cytoplasm,  303. 

Gaieotti,  pathological  mitoses,  97. 
Gallardo,  mitosis,  109. 


INDEX   OF  AUrilOKS 


473 


Galton,  inheritance,  9. 

Gardiner,  cell-bridges,  59;    chroniatin-elimi- 

nation,  276;    sphere,  325, 
Garnault,  fertilization  in  Ai'ion,  207. 
Geddes  and  Thompson,  theory  of  sex,  124. 
Van  Gehuchten,  spireme-nuclei,  36;  nuclear 

polarity,  36;   muscle-tibre,  48. 
Giard,  polar  bodies,  235,  238. 
Gierke,  staining-reactions,  335. 
Gilson,  spireme-nuclei,  36. 
Godlewski,  spermatogenesis,  168. 
Graf,  nephridial  cells,  47. 
Gregoire,  reduction,  267. 
Griffin,  fertilization,  centrosomes  in  Thalas- 

sevia,    193,     194,     213;     reduction,    259; 

structure  of  centrosome,  314;   aster-forma- 
tion, 321. 
Grobben,  spermatozoa,  141. 
Gruber,  diffused  nuclei,  40;   regeneration  in 

Stentor,  342. 
Guignard,  mitosis  in  plants,  82;   fertilization 

in  plants,  218,  221;    reduction,  263,  267. 

Haberlandt,  position  of  nuclei,  346. 

Hackel,  inheritance,  7;  epithelium,  56;  cell- 
state,  58. 

Hacker,  polar  spindles,  276;  bivalent  chro- 
mosomes, 88;  nucleolus,  125,  128;  primor- 
dial germ-cells,  148;  germ-jnuclei,  208, 
299;    reduction  in  copepods,  249. 

Hallez,  promorphology  of  ovum,  384. 

Halliburton,  proteids,  331 ;   nuclein,  I'i^'}^. 

Hamm,  discovery  of  spermatozoon,  9,  181. 

Hammar,  cell-bridges,  60. 

Hammarsten,  proteids,  331. 

Hansemann,  pathological  mitoses,  97. 

Hanstein,  metaplasm,  19. 

Hardy,  artifacts,  42. 

Harper,  mitosis,  82. 

Hartsoeker,  spermatozoon,  9. 

Harvey,  inheritance,  7;    epigenesis,  8. 

Hatschek,  cell-polarity,  56;  fertilization,  179. 

Heidenhain,  nucleus,  36;  basichromatin  and 
oxychromatin,  38,  337;  cell-polarity,  55; 
position  of  centrosome,  57;  leucocytes, 
102;  theory  of  mitosis,  105;  amitosis,  116; 
staining-reactions,  337;  nuclear  micro- 
somes, 303;  microcentrum,  311;  asters, 
311,  317;  origin  of  centrosome,  315;  po- 
sition of  spindle,  377. 

Heider,  insect-egg,  132. 

Heitzmann,  cell-bridges,  59;  nucleus  and 
cytoplasm,  303. 

Henking,  fertilization,  187;  insect-egg,  96; 
spermatogenesis,  165,  248,  253,  27 lo 


llenle,  granules,  289. 

Henneguy,  dcutoplasm,    153;    yolk-nucleus, 

1 60;    centrosome,  356. 
Hensen,  rejuvenescence,  179. 
Herbst,  development  and  environment,  428. 
Herla,  independence  of  chromosomes,  208, 

299. 

Hermann,  central  spindle,  78,  105;  division 
of  chromatin,  112;  spermatozoon,  165, 
166;   staining-reactions,  176. 

Hertwig,  O.,  i,  7,  9;  bivalent  chromosomes, 
88;  pathological  mitoses,  97;  rejuvenes- 
cence, 178;  fertilization,  iSi;  middle- 
piece,  187;  polyspermy,  199;  paths  of 
germ-nuclei,  204;  maturati(.n,  241 ;  polar 
bodies,  238;  inheritance,  1 82;  laws  ..f 
cell-division,  364;   theory  of  development, 

415- 

Hertwig,  O.  and  R.,  197;  cgg-fragmenls, 
199;    polyspermy,  199. 

Hertwig,  K.,  mitosis  in  Protozoa,  <>o;  gern)- 
cells  in  Sagi/ta,  146;  amphiaslers  in  un- 
fertilized eggs,  306;  conjugation,  222; 
reduction  in  Infusoria,  277;  in  .Utnto- 
sp/urriui/i,  278;  origin  of  centrosome,  315; 
cell-division,  391. 

Hill,  fertilization,  187,  193. 

Hirase,  spermatozoids,  144;  ferlili/alion, 
218. 

His,  germinal  localization,  398. 

Hofer,  regeneration  in  A/na/>a,  343, 

Hoffman,  micropyle,  200. 

Hofmeister,  cell-divismn  and  growtii,  393. 

Holmes,  cleavage,  368. 

Hooke,  R.,  cell,  17. 

Hoyer,  amitosis,  115. 

Huie,  Drosern,  350. 

Huxley,  protoplasm,  5;  germ,  7,  396;  fer- 
tilization, 1 78,  231 ;  evolution  and  epi- 
genesis, 432. 

Ikeno,  cell-bridges,  150:  blepharoplasts.  17;; 
fertilization,  221. 

Ishikawa,  Noctilucu,  mitosis,  92;  conjuga- 
tion, 227;  reduction,  267;    tiagcllum.  171. 

Jennings,  cleavage,  377. 

Jorilan,  deutoplasm    and   yolk-iuKKu>,   153, 

156;    first  cleavage-jdane.  3S1. 
Tulin,  fertilization  in  Sfy/i<^/>sis,  192. 

Keuten,  mitosis  in  Eugirna,  91. 
Klebahn,  conjugation  and  reduction  in  des- 
mills  and  diatoms,  2S0. 


474 


IXDEX   OF  AUTHORS 


Klebs,  pathological  mitosis,  97,  98;  cell- 
membrane,  346. 

Klein,  nuclear  membrane,  38;  theory  of 
mitosis,  100;  amitosis,  118;  nucleus  and 
cytoplasm,  303;   asters,  316. 

Klinckowstrom,  fertilization,  213;  reduction, 

259- 
Von  KolHker,  i,  6,  9,  10,  27;  epithelium,  56; 

cell-division,  63;    spermatozoon,    9,    134; 

inheritance,  182;   development,  413. 

Korff,  spermatogenesis,  163,  168,  173. 

Korschelt,  nucleus,  37;  amitosis,  115;  move- 
ments and  position  of  nuclei,  125,  349, 
387;  nurse-cells,  151 ;  fertilization,  193; 
tetrads  in  Ophryotrocha,  258;  physiology 
of  nucleus,  348;    polarity  of  egg,  387. 

Kossel,  chromatin,  336;  nuclein,  334;  or- 
ganic synthesis,  340. 

Kostanecki,  fertilization,  193;  astral  rays,  318. 

Kostanecki  and  Wierzejski,  fertilization  of 
Physa,  193,  210,  212;  continuity  of  cen- 
trosomes,  211. 

Kupffer,  energids,  30;   cytoplasm,  41. 

Lamarck,  inheritance,  12. 

Lamarle,  minimal  contact-areas,  361. 

Lankester,  germinal  localization,  398. 

Lauterborn,  mitosis  in  diatoms,  95;  origin 
of  centrosome,  315. 

Leeuwenhoek,  spermatozoon,  8;  fertiliza- 
tion, 181. 

Von  Lenhossek,  nerve-cell,  21,  47;  sperma- 
togenesis, 169,  315;  centrosome,  314,  356. 

Leydig,  cell,  19;  protoplasm,  20;  cell-mem- 
brane, 54;  spermatozoa,  142;  elimination 
of  chromatin,  159. 

Lilienfeld,  staining-reactions  of  nucleins,  336. 

Lillie,  fertihzation,  196,  213;  centrosome 
and  aster,  312,  326,  327;  regeneration  in 
Stentor,  343  ;   cleavage,  360,  369,  377. 

Loeb,  chemical  fertilization,  215,  392;  re- 
generation in  coelenterates,  392;  theory 
of  development,  427;  environment  and 
development,  430. 

Lustig  and  Galeotti,  pathological  mitoses, 
98;   centrosome,  51. 

Maggi,  granules,  290. 

Malfatti,  staining-reactions  of  nucleins,  335. 

Mark,  germ-nuclei,  204;    polar  bodies,  235; 

polarity  of  ovum,  387, 
Mathews,  pancreas-cell,  44;  aster-formation, 

iio;  fertilization  of  echinoderms,  192,212; 

origin  of  centrosome,   125;    nucleic  acid, 

334;   staining-reactions,  337. 


Maupas,  sex  in  Rotifers,  145;  rejuvenes- 
cence, 179;   conjugation  of  Infusoria,  223. 

Mayer,  staining,  335. 

McClung,  spermatogenesis,  271. 

MacFarland,  spindle,  79;  fertilization,  213, 
214;  centrosome  and  sphere,  312,  314, 
321. 

]\IcGregor,  spermatogenesis,  167;  reduction, 
261. 

McMurrich,  gasteropod  development,  152; 
metamerism  in  isopods,  390. 

Mead,  fertilization  of  Chcetopterns,  192,  194, 
215;  sperm-centrosome,  215;  centrosomes 
de  novo,  212,  306;   cell-division,  391. 

Merkel,  Sertoli-cells,  284. 

Mertens,  yolk-nucleus  and  attraction-sphere, 
156,  159. 

Metschnikoff,  insect-egg,  383. 

Meves,  amitosis,  119,  285;  spermatogenesis, 
167,  169;   reduction,  260;    cilia,  357. 

Meyer,  energids,  30;    cell-bridges,  60. 

Miescher,  nuclein,  332. 

Mikosch,  protoplasm,  44. 

Minot,  rejuvenescence,  179;  cyclical  divi- 
sion, 222;  theory  of  sex,  243;  Sertoli- 
cells,  284;   parthenogen  sis,  280. 

Von  Mohl,  cell-division,  9;   protoplasm,  17. 

Montgomery,  nucleolus,  34;  spermatogene- 
sis, 257,  271. 

Moore,  spermatozoon,  167,  171 ;  reduction, 
263. 

Morgan,  centrosomes,  307;  fertilization  of 
egg-fragments,  353;  cell-division,  391; 
effect  of  fertilization,  201 ;  numerical  rela- 
tions of  cells,  389;  regeneration,  393,  394; 
isolated  blastomeres,  410;  polarity,  417; 
experiments  on  ctenophores,  418;  on 
frog's  egg,  422. 

Mottier,  mitosis,  83;  fertilization,  221;  re- 
duction, 266;   asters,  305. 

Munson,  yolk-nucleus,  156. 

Nageli,  development,  I ;  cell-organization, 
micelloe,  289,  291;  polioplasm,  41 ;  idio- 
plasm-theory, 401. 

Nawaschin,  fertilization,  218. 

Nemec,  mitosis,  82;   yolk-nucleus,  159. 

Newport,  fertilization,  181 ;  first  cleavage- 
plane,  380. 

Nissl,  chromophilic  granules,  48. 

Nussbaum,  germ- cells,  122;  sex,  145;  re- 
generation in  Infusoria,  342;  nucleus,  426. 

Obst,  nucleoH,  130;    follicle-cells,  151. 
Osterhout,  spindle,  82;    tetrads,  253. 


INDEX   OF  AUTHORS 


475 


Overton,  germ-cells  of  Volvox,  134;  conju- 
gation of  Spirogyra,  229;    reduction,  274, 

275- 
Owen,  germ-cells,  122. 

Paladino,  cell-bridges,  60. 

Paulmier,  spermatozoon,  165;  reduction,  252, 
271. 

Peremeschko,  leucocytes,  117. 

Peter,  cilia,  357. 

Pfeffer,  hyaloplasm,  41;  amitosis,  1 19; 
chemotaxis  of  germ-cells,   197. 

Pfitzner,  cell-bridges,  60;  chromatin-gran- 
ules,  112. 

Pfluger,  position  of  spindle,  375;  first  cleav- 
age-plane, 3S0;  gravitation-experiments, 
386;   isotropy,  378. 

Plateau,  minimal  contact-areas,  366. 

Platner,  mitosis,  iio;  egg-centrosome,  125; 
formation  of  spermatozoon,  163;  fertiliza- 
tion of  ^rz^«,  207;   maturation,  241. 

Pouchet  and  Chabry,  development  and  en- 
vironment, 428. 

Prenant,  spermatozoon,  162;  archoplasm, 
322. 

Preusse,  amitosis,  1 19. 

Prevost  and  Dumas,  cleavage,  lO. 

Pringsheim,  Hautschicht,  41 ;  fertilization, 
181. 

Purkinje,  protoplasm,  17. 

Rabl,  nuclear  polarity,  36;  cell-polarity,  56; 
centrosome  in  fertilization,  210;  individu- 
ality of  chromosomes,  294;  astral  systems, 

317- 
Ranvier,  blood-corpuscles,  54. 

Vom  Rath,  bivalent  chromosomes,  88;  ami- 
tosis, 118,  225;  early  germ-cells,  149; 
reduction,  249. 

Rauber,  cell-division  and  growth,  393. 

Rawitz,    amitosis,     116;     staining-reactions, 

335- 
Redi,  genetic  continuity,  290. 

Reichert,  cleavage,  10,  64. 

Reinke,  pseudo-alveolar  structure,  50;  nu- 
cleus, 38,  303;  oedematin,  36;  asters,  305; 
nucleus  and  cytoplasm,  303. 

Remak,  cleavage,  i,  10,  361;  cell-division, 
64;   egg-axis,  378. 

Retzius,  muscle-fibre,  48;  cell-bridges,  60; 
end-piece,  140. 

Rhumbler,  105. 

Robin,  germinal  vesicle,  64. 

Rosen,  staining-reactions,  220. 

Roux,  245,  301,  351 ;  meaning  of  mitosis,  244, 


301,  351,  405;  position  of  spindle,  377; 
first  clcavagc-plane,  3S0;  frog-experi- 
ments, nnjsaic  thct^ry,  39<;;  theory  <jf  de- 
velopment, 405;    post-generation,  408. 

Ruckert,  pseudo-reduction,  248-  fertilization 
of  Cyclops,  193;  independence  ol'-g-rm- 
nuclei,  2o8,  209;  reduction  in  copejKjds, 
249,  251 ;  early  history  of  germ  nuclei, 
273;  reduction  in  selachians,  257;  history 
of  germinal  vesicle,  338. 

Riige,  amitosis,  1 17. 

Ryder,  staining-reactions,  175. 

Sabaschnikoff,  tetrads,  256. 

Saljatier,  amitosis,  116. 

Sachs,  energid,  19,  30;  laws  of  cell-division, 
362;  cell-division  and  growth,  393;  de- 
velopment, 427. 

St.  George,  La  \'alette,  spermatozoon,  lo, 
134;   spermatogenesis  (terminology;.  161. 

Sala,  polyspermy,  199. 

Sargant,  reduction  in  plants,  267. 

Schafer,  protoplasm,  29. 

Scharff,  budding  of  nucleus,  155. 

Schaudinn,  mitosis  in  Protozoa,  92,  94,  102; 
polar  bodies,  278. 

Schewiakoff,  mitosis  in  Euglypha,  yi. 

Schimper,  plastiils,  290. 

Schleicher,  karyokinesis,  64. 

Schleiden,  cell-theory,  i ;  cell-division,  9; 
nature  of  cells,   17;    fertilization,  218. 

Schloter,  granules,  I'i,  303. 

Schmitz,  jdastids,  290;   conjugation,  jio. 

Schneider,  discovery  of  mitosis,  64. 

Schottlander,  multipolar  mitosis,  99. 

Schultze,  M.,  cells,  i,  19;    protoplasm.  20. 

Schultze,  O.,  mitosis,  31 S;  gravitation-ex- 
periments, 422;    double  embryos,  422. 

Scliwann,  cell-thet)ry,  I;  the  egg  a  cell,  8; 
origin  of  cells,  9;  nature  of  cells,  17:  -t- 
ganization,  58;   adaptation,  433. 

Schwarz,  protoplasm,  42;  limn,  33;  chemis- 
try of  nucleus,  41 ;  nuclei  of  growing  cells, 

340- 
Schweigger-Seidel,  speriualo/oon,  o,  134. 

Setigwick,  cell-bridges,  (X). 

Seeliger,  egg-fragments,  353;   egg-axis,  379. 

Selenka,  doul)le  spermatozoa,  142. 

Shaw,  spermatozoids,  175. 

Siedlecki,  jiolar  bodies,  2S0. 

Sobotta,  ferlili/alion,  1S5,  211. 

Solger,  pigmeiU-cells,  102;  attraction-';t>hrri-. 

51- 
Spallanzani,   spermatozoa,  9;    regeneration, 

393- 


476 


INDEX   OF  AUTHORS 


Spencer,  physiological  units,  289;  develop- 
ment, <32. 

Stauffacher,  egg-centrosome,  125. 

Stevens,  fertilization,  217. 

Strasburger,  i,  7;  cytoplasm,  20;  Korner- 
plasma,  41 ;  centrosphere,  68,  356,  324; 
membranes,  55;  origin  of  amphiaster,  82; 
multipolar  mitoses,  99;  theory  of  mitosis, 
105,  no;  spermatozoids,  173;  kinoplasm, 
27,  82,  322;  staining-reactions  of  germ- 
nuclei,  220;  fertilization  in  plants,  216, 
219,  221;  reduction,  265,  269;  theory  of 
maturation,  275;  organization,  289;  in- 
heritance, 7,  182,  351;  action  of  nucleus, 
426. 

Zur  Strassen,  giant-embryos,  296;  germ- 
cells,  148. 

Van  der  Stricht,  spindle,  79;  amitosis,  116; 
fertilization,  210;  reduction,  259;  centro- 
some  and  sphere,  312,  325. 

Strobe,  multipolar  mitoses,  99. 

Stuhlmann,  yolk-nucleus,  156. 

Suzuki,  spermatogenesis,  168. 

Swingle,  mitosis,  82. 

Tangl,  cell-bridges,  59. 

Thiersch  and  Boll,  theory  of  growth,  392. 

Townsend,  cell-bridges,  61,  346. 

Treat,  sex,  145. 

Treviranus,  variation,  179. 

Unna,  protoplasm,  27. 

Ussow,  micropyle,  133;   deutoplasm,  153. 

Vejdovsky,  centrosome,  76;  fertilization  in 
Rhynchelmis,  192,  194;  metamerism  in 
annelids,  390. 

Verworn,  cell-physiology,  6;  regeneration  in 
Protozoa,  344;    inheritance,  359,  431. 

Virchow,  I;  cell-division,  lO,  63;  proto- 
plasm, 25;    cell-state,  58. 

De  Vries,  organization,  pangens,  291,  327, 
406;  tonoplasts,  53;  plastids,  229;  chro- 
matin, 43 1;    development,  404. 

Waldeyer,  nucleus,  38;   cytoplasm,  41 ;  cell- 
membrane,  54. 
Walter,  frog-experiments,  419. 
Watase,  theory    of   mitosis,   106;     staining- 


reactions  of  germ-nuclei,  176;  nucleus  and 
cytoplasm,  292;  asters,  305;  theory  of 
centrosome,  315;  astral  rays,  321;  cleav- 
age of  squid,  381 ;  promorphology  of  ovum, 
383,  386. 

Webber,  spermatozoids,  144,  173;  fertiliza- 
tion, 221. 

Weismann,  inheritance,  12;  cell-organiza- 
tion, biophores,  291;  somatic  and  germ 
cells,  122;  amphimixis,  179;  maturation, 
243-246;  constitution  of  the  germ-plasm, 
245;  parthenogenesis,  281;  theory  of  de- 
velopment, 404,  407,  432. 

Went,  vacuoles,  53. 

Wheeler,  amitosis,  115;  insect-egg,  132; 
egg  of  Myzostotna,  151;  fertilization  in 
Myzostoma,  208;  bilaterality  of  ovum,  383. 

Whitman,  on  Harvey,  7;  polar  rings,  202; 
cell-division  and  growth,  393;  polarity, 
384;   theory  of  development,  400,  416. 

Wiesner,  cell-organization,  290,  291. 

Wilcox,  sperm-centrosome,  165;    reduction, 

257- 
Will,  chromatin-elimination,  135. 

Wilson,  protoplasm,  27,  44;  mitosis,  106; 
fertilization  in  sea-urchin,  187,  212;  paths 
of  germ-nuclei,  202;  origin  of  linin,  303; 
astral  rays,  28;  centrosphere  and  centro- 
some, 314;  dispermy,  355;  rudimentary 
cells,  372;  pressure-experiments,  41 1; 
experiments  on  Amphioxus,  410;  theory 
of  development,  415. 

Von  Wittich,  yolk-nucleus,  155. 

Wolff,  C.  F.,  epigenesis,  8. 

Wolff,  G.,  regeneration  of  lens,  433. 

Wolters,  polar  bodies  in  gregarines,  278. 

Yung,  sex,  144. 

Zacharias,  E.,  nucleoli,  34;  ofmeristem,  37; 
staining-reactions,  176;  nuclein  in  grow- 
ing-cells, 340. 

Zacharias,  O.,  amoeboid  spermatozoa,  142. 

Ziegler,  artificial  mitotic  figure,  108;  amito- 
sis, 117;    sphere,  324. 

Zimmerman,  pigment-cells,  102;  centrosome, 

356. 
Zoja,  independence    of  chromosomes,   299; 

isolated  blastomeres,  410. 


INDEX    OF   SUBJECTS 


Acanthocystis,  94,  304,  306. 

Achromatic    figure    (see    Amphiaster),    69; 

varieties  of,  78;   nature,  316. 
AcJiromatium,  39. 
Actinophrys,  92,  278. 
Actinosphceriiiin,  mitosis,  90,  94;    reduction, 

278;   regeneration,  342. 
^quorea,  metanucleus,  128. 
Albugo,  217. 
Albumin,  331. 
Allium,  83,  253,  267. 
Allolobophora,  teloblasts,  374. 
Alveoli,  25, 
Amitosis,  114;   biological  significance,  116; 

in  sex-cells,  285. 
Amoeba,    5;     mitosis,    91;    experiments  on, 

343- 

Amphiaster,  68;  asymmetry  of,  70,  373; 
origin,  72,  74,  316;  in  amitosis,  116;  in 
fertilization,  187,  213;  nature,  316;  posi- 
tion, 375. 

Amphibia,  spermatozoa,  I40;   sex,  145, 

Atnphioxus,  fertilization,  210;  polar  body, 
236,  277;  cleavage,  370;  dwarf  larvK, 
389,  410;   double  embryos,  410. 

Amphipyrenin,  41. 

Amphiiitna,  167,  261. 

Amyloplasts,  53;   in  plant-ovum,  133, 

Anaphases,  70;    in  sea-urchin  egg,  106. 

A>iasa,  sperm-formation,  165,  271 ;  reduc- 
tion, 272. 

Ancylus,  368. 

Anilocra,  gland-cells,  nuclei,  36;  amitosis, 
116. 

Anodonia,  ciliated  cells,  43,  357. 

Antipodal  cone,  loi. 

Aphis,  281. 

Arbacia,  192,  215,  307. 

Archoplasm,  69;  in  developing  spermatozoa, 
171;   nature  of,  318. 

Archosome,  52. 

Argonauta,  micropyle,  133. 

Aricia,  rudimentary  cells,  372. 

Arion,  spindle,  81 ;   germ-nuclei,  207. 

Ariscema,  269. 


Ajtetnia,  chromosomes,  89;  parthcnogcnclic 
maturation,  281. 

Artifacts,  in  protoplasm,  42. 

Ascaris,  chromosomes,  87,  301 ;  n»it..sis.  So, 
loi;  primordial  germ-cells,  146;  fcrtili/a- 
tion,  182,  211;  polyspermy,  199;  polar 
bodies,  238;  spermatogenesis,  241,  253; 
individuality  of  chromosomes.  295;  in- 
tranuclear centrosome,  304;  centrosome, 
31 1;  attraction-sphere,  323;  supernumer- 
ary centrosome,  355. 

Aster,  68;  asymmetry,  70;  structure  and 
functions,  loi;  in  amitosis,  116;  in  fertili- 
zation, 187,  213;  nature  of,  316;  hncr 
structure,  326;    relative  size,  70,  373. 

Asterias,  spermatozoa,  176;  sperm-aster, 
187;    fertilization,  192,  210. 

Astrocentre,  324. 

Astrosphere,  324. 

Attraction-cone,  198. 

Attraction-sphere,  51.  72;  in  amitosis,  1 15; 
of  the  ovum,  125;  of  the  spermatid,  163; 
in  resting  cells,  323;   nature  of,  ^2y 

Axial  filament,  136;    origin  of,  165. 

Axis,  of  the  cell,  55;  of  the  nucleus.  ^6.  204: 
of  the  ovum,  378,  386. 

Axolotl,  fertilization,  192. 

Bacteria,  nuclei,  31,  39. 

Basichromatin,  38;   staining-reaclions,  33S. 

Bioblast,  290. 

Biogen,  291. 

Biophore,  245,  291. 

Birds,  blood-cells,  57;  spermatozoa,  13S; 
young  ova,  155. 

Blastomeres,  displacement  of,  366;  indi- 
vidual history,  378;  prospective  value, 
415;  rhythm  of  division,  366,  389;  «Ic- 
velopment  of  single,  409,  418;  in  normal 
development.  423. 

BUunitis,  pigment-cells,  loj. 

Blepharoplastoids,  175. 

HIepharoplasts,  173,  221. 

Branchipus,  yolk,  153;  sperm-aster,  192; 
reduction,  256. 


477 


478 


INDEX   OF  SUBJECTS 


Calavus,  tetrads,  250. 
Caloptenus,  165,  257. 
Cambium,  376. 
Cancer-cells,  mitosis,  98. 
Canthocamptus,    reduction,     251 ; 


ovarian 


eggs, 


-/J- 


Cell,  in  general,  4;  origin,  9;  name,  17; 
general  sketch,  19;  polarity  of,  55;  as  a 
structural  unit,  58;  structural  basis,  23, 
293;  physiology  and  chemistry,  330;  size 
and  numerical  relations,  389;  in  inheri- 
tance, 9,  430;  differentiation  of,  413,  426; 
independence  of,  427. 

Cell-bridges,  59. 

Cell-division  (see  Mitosis,  Amitosis),  general 
significance,  lo,  63;  general  account,  65; 
types,  64;  Remak's  scheme,  63;  indirect, 
65;  direct,  114;  cyclical  character,  178, 
223;  equal  and  reducing  or  qualitative, 
405 ;  relation  to  development,  388,  405, 
410,  427;  Sachs's  laws,  362;  rhythm,  366, 
389;  unequal,  370;  of  teloblasts,  371; 
energy  of,  388;  relation  to  metamerism, 
390;  causes,  391;  relation  to  growth, 
388;   and  differentiation,  427. 

Cell-membrane,  53. 

Cell-organization,  289. 

Cell-organs,  52;  nature  of,  291;  temporary 
and  permanent,  292. 

Cell-plate,  71. 

Cell-state,  58. 

Cell-theory,  general  sketch,  1-14. 

Central  spindle,  70,  78. 

Centrodesmus,  79,  315. 

Centrodeutoplasm,  163,  324. 

Centroplasm,  324. 

Centrosome,  22;  general  sketch,  50,  304; 
position,  55;  in  mitosis,  74;  a  permanent 
organ,  74;  dynamic  centre,  76;  historical 
origin,  315;  functions,  loi,  354;  in  ami- 
tosis, 115;  of  the  ovum,  125;  of  the 
spermatozoon,  137,  1 65-1 70;  in  fertiliza- 
tion, 190,  208;  degeneration  of,  186,  213; 
continuity,  74,  77,  194,  214,  321 ;  nature, 
304;    intra-nuclear,   304;    supernumerary, 

355- 
Centrosphere,  68,  85;   nature  of,  324. 

Cet'aHuin,  91. 

Ceratozamia,  reduction,  275. 

Cerebratiilus,  1 93, 194,  213,  306,  307,  32 1, 325. 

Cerianthus,  regeneration  in,  392. 

ChcEtopterus,  spindle,  81,  84;  fertilization, 
192;  sperm-centrosome,  213;  centrosomes 
de  novo,  306;   cell-division,  391. 

Chara,  spermatozoids,  143. 


Chilomonas,  32,  40,  192. 

Chironomus,  spireme-nuclei,  36. 

Chorion,  132. 

Chromatic  figure,  69;  origin,  72;  varieties, 
86;   in  fertilization,  181,  204. 

Chromatin,  -^y,  i"  meristem,  37;  in  mitosis, 
65,  86;  in  cancer-cells.  98;  of  the  egg- 
nucleus,  126;  elimination  of,  in  cleavage, 
147,426;  in  oogenesis,  233,  276;  staining- 
reactions,  334-340;  morphological  organi- 
zation, 37,  245,  294;  chemical  nature,  332, 
404;  relations  to  linin,  302;  physiological 
changes,  338;  as  the  idioplasm,  352;  in 
development,  405,  425,  431. 

Chromatin-granules,  37;  in  mitosis,  112;  in 
reduction,  248;  general  significance,  301- 
304;    relations  to  linin,  302, 

Chromatophore,  53;  in  the  ovum,  133;  in 
fertilization,  229. 

Chromiole,  302. 

Chromomere  (see  Chromatin-granule),  37, 
301. 

Chromoplast,  53. 

Chromosomes,  67,  70,  86,  112;  number  of, 
67,  206;  bivalent  and  plurivalent,  87; 
division,  112;  of  the  primordial  germ- 
cell,  148;  in  fertilization,  182,  204;  inde- 
pendence in  fertilization,  204;  reduction, 
238,  243,  248;  in  early  germ-nuclei,  273; 
conjugation  of,  257;  in  parthenogenesis, 
281;  individuality  of,  294;  composition  of, 
301;  chemistry,  334,  336;  history  in  ger- 
minal vesicle,  338;   in  dwarf  larvae,  296. 

Ciliated  cells,  44,  57. 

Ciotia,  egg-axis,  379. 

Clavelina,  cleavage,  369,  381. 

Cleavage,  in  general,  10 ;  geometrical  rela- 
tions, 362;  Sachs's  rules,  362;  Hertwig's 
rules,  364;  modifications  of,  366;  spiral, 
368;  reversal  of,  368;  unequal,  370;  under 
pressure,  375,411 ;  promorphology  of,  378; 
bilateral,  381;  rhythm,  366,  388;  mosaic 
theory,  399,  423;   half  cleavage,  410. 

Cleavage-nucleus,  204. 

Cleavage-planes,  362;   axial  relations,  378. 

Clepsine,  nephridial  cell,  45;  polar  rings, 
202 ;   cleavage,  370. 

Closteriiim,  conjugation  and  reduction,  280. 

Cockroach,  amitosis,  115;  orientation  of  egg, 

384. 
Coelenterates,  germ-cells,  146;    regeneration, 

392,  393»  430- 
Conjugation,    in    unicellular    animals,    222; 
unicellular  plants,  228,  280;   physiological 
meaning,  178,  223. 


INDEX  OF  SUBJECTS 


479 


Contractility,  theory  of  mitosis,  loo;   inade- 
quacy, 1 06. 

Copepods,  reduction,  251. 

Corixa^  ovum,  383, 

Corpuscule  central,  310,  314. 

Crepichda,    fertilization,     210 ;     dwarfs    and 
giants,  389;    cleavage,  323,  423. 

Cross-furrow,  368. 

Crustacea,  spermatozoa,  142. 

Ctenophores,  experiments  on  eggs,  41 S. 

Cucurbita,  346. 

Cuticular,  54. 

Cyanophyceae,  nucleus,  31,  39. 

Cycads,  spermatozoids,  144,  173;  fertiliza- 
tion, 218,  221. 

Cyclops, 0x^,12"^',  primordial  germ-cells,  148; 
fertilization,  188;  reduction,  25 1;  attrac- 
tion-sphere, 325;   axial  relations,  385. 

Cytoplasm,  21,  41,  293,  303;  of  the  ovum, 
130;  of  the  spermatozoon,  134;  morpho- 
logical relations  to  nucleus,  302;  to  archo- 
plasm,  316,  319;  chemical  relations  to 
nucleus,  333-341 ;  physiological  relations 
to  nucleus,  341 ;  in  inheritance,  352-354, 
359 ;  in  development,  398, 42 1 ;  origin,  431. 

Cytosome,  322. 

Dendrobczna,  metamerism,  390. 
Determinants,  245. 

Deutoplasm,   131 ;    deposit,   153;     effect   on 
cleavage,    366,    371 ;     rearrangement    by 
gravity,  422. 
Development,   1-12;   and  cell-division,  388; 
mosaic  theory,  399,421;  theory  of  Nageli, 
402;     Roux-Weismann    theory,    404;     of 
single  blastomeres,  399,  409,  418;   of  egg- 
fragments,  296,  353,  419;   De  Vries's  the- 
ory,  413;     Hert wig's    theory,    415,   432; 
Driesch's    theory,  394,  415;    partial,  409, 
419;   half  and  whole,  419;    nature  of,  413; 
external  conditions,  428;   and  metabolism, 
430;   unknown  factor,  431 ;    rhythm,  432; 
adaptive  character,  433, 
Diaptomus,  250. 

Diatoms,  mitosis,  92;   centrosome,  51. 
Dinnlula,  79,  314. 

Diemyctyliis,  yolk,  153;    yolk-nuclei,  156. 
Differentiation,  361 ;     theory    of    De   Vries, 
404;     of    Weismann,    405;     nature    and 
causes,    413;     of    the    nuclear  substance, 
425;   and  cell-division,  427. 
Dipsacus,  346. 
Dispermy,  355. 
Double  embryos,  410,  422. 
Drosera,  350. 


Dwarfs,  formation  of,  353.  4,0,  422:   size  of 
cells,  389. 

Dyads  (Zwciergruppen;,  239,  241 ;    in  par- 
thenogenesis, 284. 
Dyaster,  70. 

Dycyemids,  centrosome,  51. 
Dytiscus,  ovarian  eggs,  153.  :}4q. 

Earthworm,  ova,  152;  spermatozoon.  165; 
yolk-nucleus,  154;  polar  rings,  156.  202; 
spermatogenesis,  257;   tclublasts,  374. 

Echinoderms,  protoplasm,  28.  44,  29 1;  sper- 
matozoa, 137;  fertilization,  188.  212; 
polyspermy,  194,  198;  dwarf  larva-,  353, 
410;  half  cleavage,  410:  eggs  under  press- 
ure, 41 1 ;    modified  larva.*,  428. 

Echinus,  fertilization,  210;  centrosome,  314; 
dwarf  larv.-e.  353;    number  of  cells,  389. 

Ectosphere,  324. 

Egg-axis,  378;  promorphological  signifi- 
cance, 379;  determination,  386;  alteration 
of,  422. 

Egg- fragments,  fertilization.  194;  develop- 
ment, 352. 

Elasmobranchs,  spermatozoon,  140,  167,  169; 
germinal  vesicle,  245,  273;  reduction,  257. 
Embryo-sac,  218,  263. 
Enchylema,  23. 
End-knob,  136. 
Endoplasm,  41. 
End-piece,  140. 
End-plate,  91. 
Energid,  19,  30. 
Entosphere,  324. 
Envelopes,  of  the  egg,  132. 
Epigenesis,  8,  432. 
Equatorial  plate,  68. 
Ei/uist'ium,  mit«)sis,  85. 
Ergastoplasm,  322. 
Erysiphe,  mitosis,  '8>2. 
Eiiclucta,  tetrads,  2^0. 
EugUna,  mitosis,  91,  31;;. 
Euglypha,  mitosis,  89,  95. 
Evolution  (preformation),  S,  399,  452. 
Evolution,  theory  of,  2,  8. 
Exoplasm,  41. 

Fertilization,  general  aspect.  9:  physiologi- 
cal meaning,  iSo;  general  sketch,  iSo; 
Ascaris,  182;  mouse,  185;  sea-urchin,  188; 
Nereis,  188;  Cychps,  '188:  7'haiassema, 
Cfuctoptertts,  193.  195;  pathological,  198; 
partial,  kk).  194;  of  J/ico^/c^w./.  196,  20S; 
in  plants,  215;  egg-fragments,  194;  Ho- 
veri's  theory,  192,  211. 


48o 


INDEX   OF  SUBJECTS 


Fishes,  pigment-cells,  102 ;  periblast-nuclei, 
117;  spermatozoa,  137;  young  ova,  116; 
single  blastomeres,  410. 

Flagellates,  diffused  nuclei,  39. 

Follicle,  of  the  egg,  150. 

Forficula,  nurse-cells,  151. 

Fragmentation,  64. 

Fritillaria,  spireme,  112;  fertilization, 
219. 

Frog,  tetrads,  259;  egg-axis,  378;  first  cleav- 
age-plane, 380;  Roux's  puncture  experi- 
ment, 399;  post-generation,  409;  pressure- 
experiments,  410;  effect  of  gravity  on  the 
egg,  422;  development  of  single  blasto- 
meres, 399,  408,  422;  double  embryos, 
422. 

Fiicus,  143,  217,  221. 

Ganglion-cell,  48;   centrosome  in,  51,  314. 

Gemmae,  291. 

Gemmules,  12,  291. 

Genoblasts,  243. 

Geophilus,  deutoplasm,  154,  158;  yolk-nu- 
cleus, 156. 

Germ,  7,  396. 

Germ-cells,  general,  8,  9;  detailed  account, 
122;  of  plants,  133,  142;  origin,  144; 
growth  and  differentiation,  150;  union, 
196;  results  of  union,  200 ;  maturation, 
233;   early  history  of  nuclei,  272. 

Germinal  localization,  theory  of,  397. 

Germinal  spot,  124. 

Germinal  vesicle,  124,  125;  early  history, 
273;    movements)  349;    position,  387. 

Germ-nuclei,  of  the  ovum,  125;  of  the 
spermatozoon,  135;  of  plants,  216;  stain- 
ing-reactions,  175;  in  fertilization,  182, 
188;  equivalence,  182,  205;  paths,  202; 
movements,  204;  union,  204;  indepen- 
dence, 204,  299;  in  Infusoria,  224;  early 
history,  272. 

Giant-cells,  31;   microcentrum,  314. 

Gingko,  173. 

Globulin,  331,  333. 

Granules  (see  Microsomes),  of  Altmann,  290; 
nuclear,  37,  303;  chromophilic,  23,  48; 
in  general,  289. 

Gravity,  effect  on  the  egg,  131,  422. 

Gregarines,  mitosis,  89;    polar  body,  278. 

Ground-substance,  of  protoplasm,  23;  of 
nucleus,  36. 

Growth,  and  cell-division,  58,  388. 

Gryllotalpa,  reduction,  249. 

Guinea-pig,  spermatogenesis,  1 70;  matura- 
tion, 277. 


Heliozoa,  92,  103. 

Helix,  163,  168,  259. 

Ilenierocaliis,  306. 

Heterocope,  tetrads,  250. 

Heterokinesis,  406. 

Histon,  334,  336. 

Homoeokinesis,  406. 

Hydrophiliis,  orientation  of  &gg,  384. 

Id,  in  reduction,  245;   in  inheritance,  406. 

Idant,  245. 

Idioblast,  291. 

Idioplasm,  theory  of,  401;  as  chromatin, 
403;   action  of,  406,  414,  431,  432. 

Idiosome,  291. 

Idiozome,  163,  165,  324. 

Ilyanassa,  partial  development,  419. 

Infusoria,  nuclei,  31,  224;  mitosis,  90;  con- 
jugation, 223;   reduction,  277. 

Inheritance,  of  acquired  characters,  12, 
433  ;  Weismann's  theory,  12;  through 
the  nucleus,    351-354;    and   metabolism, 

430. 
Inotagmata,  291. 
Insect-eggs,  132,  386. 
Interzonal  fibres,  70. 
Iris,  267. 

Isopods,  metamerism,  390. 
Isotropy,  of  the  egg,  384,  417. 

Karyokinesis  (see  Mitosis),  64. 
Karyokinetic    figure    (see    Mitotic   Figure), 

69. 
Karyolymph,  36. 
Karyoplasm,  21. 
Karyosome,  34. 
Kinoplasm   (archoplasm),  54,    77,    82,   173, 

322. 

Lanthanin,  38. 

Lepidoptera,  sex,  144. 

Leucocytes,  structure,  102;  division,  117; 
centrosome,  309;    attraction-sphere,  326. 

Leucoplasts,  of  plant-ovum,  133. 

Z/7/?/;«,  mitosis,  83;  spireme,  112;  fertiliza- 
tion, 219;   reduction,  265-271. 

Umax,  germ-nuclei,  204. 

liniulus,  158. 

Linin,  32;  relations  to  cytoreticulum  and 
chromatin,  302. 

liparis,  281. 

locusta,  orientation  of  egg,  384. 

loligo,  spindle,  81 ;    cleavage,  381. 

lumbricus,    yolk-nucleus,    157;      reduction, 

257- 


INDEX   or  SUBJECTS 


481 


Macrobdella,  305. 

Macrogamete,  226. 

Macromeres,  371. 

Mammals,   spermatozoa,    139,    169;     young 

ova,  155. 
Mantle-fibres,  78,  105. 
Mar  sill  a,  175. 

Maturation   (see  Reduction),  234;   theoreti- 
cal significance,  243;    of  parthenogenetic 
eggs,  280;   nucleus  in,  353. 
Medusze,  dwarf  embryos,  410. 
ISIeristem,  nuclei  of,  340. 
Metamerism,  390. 
Metanucleus,  128. 
Metaphase,  69. 
Metaplasm,  19. 
Micellae,  291. 

Microcentrum,  31 1,  315,  324. 
Microgamete,  226. 
Micrumeres,  371. 
Micropyle,  124,  133. 

Microsomes,  23;    of  the  egg-cytoplasm,  131 ; 
nature  of,  2S9,  290,  293;   of  the  astral  sys- 
tems, 318,  326;   of  the  nucleus,  301,  303; 
relation    to    centrosome,    315;     staining- 
reactions,  337. 
Microsphere,  324. 
Microzyma,  291. 
Mid-body,  71,  78. 
Middle-piece,    135,    139;    origin,   161,   165- 

170;   in  fertilization,  187,  212, 
Mitosis,  64;    general  outline,  65;   modifica- 
tions of,  77;  heterotypical,  86;  in  unicellu- 
lar forms,  87;  pathological,  88;  multipolar, 
97;   mechanism  of,  100;  physiological  sig- 
nificance, 351;    Roux-Weismann  concep- 
tion of,  245,  406. 
Mitosome,  165. 
Mitotic    figure    (see    Mitosis,  Spindle),    69; 

origin,  72;   varieties,  78. 
Molgida,  158. 

Mouse,  fertilization,  185,  193. 
Aftisca,  ovum,  142. 
Myriapods,  spermatozoa,  142;    yolk-nucleus, 

156. 
Myzostoma,  fertilization,  196,  208. 

Naias,  266. 

Nebenkern,  pancreas-cells,  44;  of  spermatid, 

163,  165. 
Nebenkorper,  164,  165. 
Nectiiriis,  pancreas-cells,  44. 
Nematodes,  germ-nuclei,  184. 
Nereis,  asters,  49;    perivitelline  layer,   131 ; 

ovum,  129;  deutoplasm,  131 ;  fertilization. 


191 ;  attraction-sj^here  and  centr<».  .lue, 
325;  cleavage.  366,  36*);  prcssurf-t-Mn  ri- 
ments  on,  411. 

Nerve-cell,  48. 

Net-knot,  34. 

i\W//7//<'(/,  mitosis,  93;  flagclluni,  171 ;  con- 
jugation, 227;   sphere,  319. 

Nuclear  stains,  335. 

Nuclein,  H,  332;  staining-reactions,  334; 
physiological  signilicancc,  340. 

Nuclein-bascs,  331. 

Nucleinic  acid,  T,}f,  332-334;  stainmg-rcac- 
tions,     334  ;      ])hysiological     signiticance, 

340. 

Nucleo-ali)umin,  331,  334. 

Nucleo-proteid,  331,  334. 

Nucleolus,  }^T^\  in  mitosis,  67;  of  the  ovum, 
126;   physiological  meaning,  128, 

Nucleoplasm,  21. 

Nucleus,  general  structure  and  functions, 
31 ;  finer  structure,  37;  polarity,  36,  294; 
chemistry,  41;  in  mitosis,  65;  of  the 
ovum,  123;  of  the  spermatozoon,  135,  137; 
relation  to  cytoplasm,  302;  morpht)|ogital 
composition,  294;  in  organic  synthesis, 
340,  430 ;  physiolog)',  341;  position  and 
movements,  346;  in  fertilization,  181,352; 
in  maturation,  353;  in  later  tievelopmcnt, 
425;  in  metabolism  and  inheritance,  430: 
in  inheritance  and  development.  341,  35S, 
405.  425,  431 ;  control  of  the  cell,  426. 
Nurse-cells,  15 1. 

CEdigoniiim,  fertilization,    181;     membrane, 

346. 

Onoclea,  175. 

Oocyte,  236, 

Oogenesis,  234,  230. 

Oogonium,  236. 

Otisphere,  133. 

Op/iryotrochn,  amitosis,  1 15;  nursc-cclls, 
151;    fertilization,  iS«),  193;   tetrads,  25S. 

((possum,  siK-rmalozoa,  142. 

Organization,  289,  2<)l;  i>f  the  nu- 1-  1.  ^.^, 
301;   of  the  egg,  397,  433- 

Origin  of  species.  3. 

Ch/fiun,i<t,  reduction,  275. 

Ovary,  123;   of  Cauthoiampius,  273. 

(^vuin,  in  general,  8,  9:  detailed  an-unt, 
124;  nucleus,  125;  cytoplasm,  130;  en- 
velopes, 132;  «>f  plants.  133;  origin  and 
growth,  150;  fertilization.  178;  effects  of 
spermatozoon  upon.  201 ;  maturation,  236; 
parthenogenetic,  280;  promorphology, 
378;   bilaterality,  382. 


2  I 


482 


INDEX   OF  SUBJECTS 


Oxychromatin,  38,  303;     staining-reactions, 

337- 
Oxydation-ferments,  351. 

Oxytricha,  342. 

Oyster,  germ-nuclei,  staining-reactions,  175. 

Pallavicuiia,  reduction,  275. 

Pahidina,  dimorphic  spermatozoa,  141. 

Pangenesis,  12,  290,  431. 

Pangens,  291. 

Parachromatin,  41. 

Paralinin,  41, 

Parainceba,  mitosis,  94,  315. 

/'«raw^<"?«;;/,  mitosis,  91;  conjugation,  224; 
reduction,  277. 

Paranucleus,  163. 

Parthenogenesis,  theories  of,  281;  polar 
bodies  in,  280. 

Pellicle,  54. 

Fentatoma^  271. 

Felrotnyzon,  fertilization,  192,  212. 

Phallusia,  fertilization,  193,  212. 

Fhysa,  fertilization,  193,  210,  212;  reversed 
cleavage,  368. 

Physiological  units,  289. 

Fieris,  spinning-gland,  37. 

Pigment-cells,  102. 

Filnlaria,  fertilization,  216. 

Fimis,  reduction,  275. 

Flana7'ia,  regeneration,  394. 

Plant-cells,  plastids,  52;  membranes,  54; 
mitosis,  82;   cleavage-planes,  363. 

Plasma-stains,  335. 

Plasmocyte,  52. 

Plasmosome,  34. 

riasome,  291. 

Plastids,  52;  of  the  ovum,  133;  of  the  sper- 
matozoid,  143;   conjugation  of,  229. 

Plastidule,  291. 

Plastin,  41,  331. 

Plen7'ophyllidia,  78,  94. 

Podophyllum,  267. 

Polar  bodies,  181 ;  nature  and  mode  of  for- 
mation, 235-240;  division,  236;  in  par- 
thenogenesis, 281. 

Polar  rings,  156,  202, 

Polarity,  of  the  nucleus,  36;  of  the  cell,  55; 
of  the  ovum,  378;   determination  of,  382. 

Pole-plates,  91. 

Pollen-grains,  formation,  263-265. 

Pollen-tube,  218. 

Polyclades,  cleavage,  416. 

Poly  cheer  us,  276,  325. 

Polygordius,  cleavage,  368. 

Polyspermy,  198;   prevention  of,  199. 


Polystomella,  regeneration,  344. 

Polyzonium,  159. 

Porcellio,  amitosis,  116. 

Predelineation,  398. 

Preformation  (see  Evolution). 

Pressure,  experiments,  375,  410. 

Principal  cone,  loi. 

Pristiurus,  338. 

Promorphology  (see  Cleavage,  Ovum). 

Pronuclei,  202. 

Prophase,  65. 

Prostheceraus,  213,  235,  256,  259,  306. 

Prosthiostovnim,  212. 

Protamin,  334. 

Proteids,  331. 

Prothallium,  264;    chromosomes  in,  275. 

Protoplasm,  4,  5,   17,   19;   structure,  23,42, 

293;   chemistry,  331. 
Protoplast  (see  Plastid). 
Pseudo-alveolar  structure,  50. 
Pseudo-reduction,  248. 
Pteris,  253. 

Pterotrachea,  germ-nuclei,  186,  205. 
Ptychoptera,  spireme-nuclei,  35. 
Pygara,  165. 
Pyrenin,  41. 
Pyrenoid,  133. 
Pyrrhoco7-is,  165,  248. 

Quadrille  of  centres,  210. 

Rat,  spermatogenesis,  170. 

Reduction,  general  outline,  234;  parallel 
between  the  two  sexes,  241 ;  theoretical 
significance,  243;  detailed  account,  246; 
in  plants,  263;  Strasburger's  theory  of, 
275;  in  unicellular  forms,  277;  by  conju- 
gation, 257;   modes  contrasted,  247. 

Regeneration,  Weismann's  theory,  406;  in 
frog-embryo,  409;  nature  of,  425,  427; 
in  coelenterates,  430;    of  lens,  433. 

Rejuvenescence,  179,  224. 

Keiiilla,  ovum,  132, 

Rhabdoiejua,  amitosis,  1 15. 

KhyncJielinis,  fertilization,  192,  193,  212; 
cleavage,  370. 

Rotifers,  sex,  145. 

Sagitta,  number  of  chromosomes,  184;  pri- 
mordial germ-cells,  146;  germ-nuclei,  184; 
spermaster,  191. 

Salamander,  epidermis,  3;  spermatogonia, 
20;  mitosis  in,  71,  78;  pathological  mito- 
sis, 98;  leucocytes,  102;  spermatozoa, 
140;   maturation,  259, 


IXDEX   OF  SCB/ECTS 


483 


Sargns,  pigment-cells,  103. 
ScyUimii,  263. 

Segmentation  (see  Cleavage). 
Sehrginelia,  spermatozoicls,  197. 
Senescence,  179. 
Sepia,  spindle,  81. 
Sertoli-cells,  284. 

Sex,  9;   determination  of,  144;   Minot's  the- 
ory of,  243. 
Siphonophures,  amitosis,  117. 
Soma,  13. 
Somacule,  291. 
Somatic  cells,  122;  number  of  chromosomes. 


2->  T 
33- 


Spermary,  123, 

Spermatid,  161,  163;  development  into  sper- 
matozoon, 164. 

Spermatocyte,  161,  241. 

Spermatogenesis  (see  Reduction),  234;  gen- 
eral outline,  parallel  with  oogenesis,  241. 

Spermatogonium,  161,  241. 

Spermatozeugma,  142. 

Spermatozoid,  structure  and  origin,  142, 
172;   in  fertilization,  217,  221. 

Spermatozoon,  discovery,  9;  structure,  134; 
essential  parts,  135;  giant,  141 ;  double, 
142;  unusual  forms,  142;  of  plants,  142; 
formation,  1 60;  in  fertilization,  181,  192; 
entrance  into  ovum,  197. 

Sperm-centrosome,  135,  164-171;  in  fertili- 
zation, 192,  211-215,  221. 

Sperm-nucleus,  135;  origin,  164-171;  in 
fertilization,  182,  190;  rotation,  188;  path 
in  the  egg,  202;  in  inheritance,  353; 
chemistry,  334. 

Sphcir echinus,  fertilization,  193,  210;  num- 
ber of  cells,  389;  hybrids,  353;  regenera- 
tion, 393. 

Spindle  (see  Amphiaster,  Central  Spindle) ,  68 ; 
origin,  72,  79, 82;  in  IVotozoa,9o;  conjuga- 
tion of,  227;  nature  of,  316;   position,  375. 

Spireme,  65. 

Spirochona,  mitosis,  90. 

Spirogyra,  nucleolus,  67;  amitosis,  1 19; 
conjugation,  229;   reduction,  2S0. 

Spongioplasm,  25. 

Spontaneous  generation,  7. 

Stem-cells,  148. 

Stentor,  regeneration,  342. 

Siylonychia,  senescence,  224. 

Stypocanlon,  mitosis,  82. 

Siirirelia,  94. 

Syml)iosis,  53,  292. 

Syiiapta,  cleavage,  364. 

Syncytium,  59. 


Teloblasts,  371,  390. 

Telophase,  71. 

Tetrads  (Vicrcrgruppen),  238:  origin,  246; 
in  Ascaris,  241,  253;  in  arthropods,  248; 
ring-shaped,  248;  in  amphibia,  259;  ori- 
gin   by    conjugation,    257;    formulas  for, 

247- 
Tetramitus,  40,  92. 

Thalassema,  spindle,  81 ;  fcrtili/ati.m,  1^3, 
194,  213;  reduction,  259,  263;  cenlro- 
some,  321 ;   attraction-sphere,  325. 

Thalassicolla,  experiments  on,  344, 

Thysanozpon,  212,  259,  326. 

Tonoplast,  53. 

ToxopncHstes,  cleavage,  lO;  mitosis,  107; 
ovum,  126;  spermatozoon,  134;  fertiliza- 
tion, 188;  paths  of  germ-nuclei,  202; 
polar  bodies,  114;   double  cleavage,  355. 

Trachelocerca,  diffused  nuclei,  40. 

Trilliiiin,  269, 

Tritou,  140,  212,  263,  277. 
Trophoplasm,  322,  401. 

Tiibiilaria,  regeneration,  430. 

Tunicates,  egg-axis,  379;   cleavage,  381. 

Unicellular  organisms,  5;  mitosis,  88;  con- 
jugation, 222;  reduction,  277;  experi- 
ments on,  342. 

Unio,  centrosome  and  aster,  314;  cleavage. 
381. 

Urostyla,  40. 

Vacuole,  50,  53. 

J'linessa,  ovarian  egg,  153. 
Variations,  ii;   origin  of,  433. 

Wutchcria,  meml)rane,  348. 
\'italism,  394,  417. 

\'itelline  membrane,  132;  of  egg-fragments 
132;    formation  of,  198;    function,  I99. 

J'o/vox,  germ-cells,  133. 

I'ordiellit,  conjugation,  226. 

Xiphidiutn,  271. 

Vellow  cells  (of  Kadiolaria),  53. 
Yolk  (see  Deulojilasm),  152. 
Volk-nucleus,  155. 
VulU-plates,  131. 

ZitrniiJ,  173,  221. 
ZirpJhca,  259,  263. 
Zwischenkorper  (mid-body),  71. 
Zygnema,  membrane,  346. 
Zygospore,  228. 


Columbia  University  Biological  Series. 

EDITKI)   r.V 
HENRY    FAIRFIELD    OSBORN, 

Da  Contu  PfofesHor  cf  Zai'i/nr/i/  in  ('nh(i,itil,i  f'iticer«itu 

A  M  > 

EDMUND    B.    WILSON, 

ProjetiHvr  <if  Zooluyy  in  (vluinhiu  i'ldicrHiti/. 


This  series  is  founded  upon  a  course  of  popular  Universitv 
lectures  given  during  the  winter  of  ISD'^'-o,  in  connection  witli 
the  opening  of  the  new  department  of  Biology  in  Colunibia 
College.  The  lectures  are  in  a  measure  consecutive  in  charac- 
ter, illustrating  phases  in  the  discovery  and  ai>[»lication  of  tlie 
theory  of  Evolution.  Thus  the  tirst  course  outlined  the  de- 
velopment of  the  Descent  theory;  the  second,  the  a]>]»licati(>n 
of  this  theory  to  the  problem  of  the  ancestry  of  the  Vertebrates, 
largely  based  upon  embryological  data;  the  third,  the  applica- 
tion of  the  Descent  theory  to  the  interpretation  of  the  structure 
and  phylogeny  of  the  Fishes  or  lowest  Vertebrates,  cliietly  based 
upon  comparative  anatomy  ;  the  fourth,  u])on  the  problems  of 
individual  development  and  Inheritance,  chiefly  based  upon  the 
structure  and  functions  of  the  cell. 

Since  their  original  delivery  the  lectures  have  been  carofully 
rewritten  and  illustrated  so  as  to  adapt  them  to  the  use  of  (\d- 
lege  and  University  students  and  of  general  readers.  The  vol- 
umes as  at  present  arranged  for  include: 

I.  From  the  Greeks  to  Darwin.     By  Henry  Fairfield 

OSBORX. 

II.  Aiiipliioxus  and  the  Ancestry  of  tlie  Vertebrates. 

By  Arthur  Wili.f.y. 

III.   Fishes,  Livinir  and  F'ossil.     By  Bashfokp  Dkan. 

lY.   The    Cell    in    Development    and    Iiiiierilance.       Uy 
Edmi^xd  B.   Wilson. 

T.  The   Foundations   of  Zooloirv.     Bv  Wimiam    Kinn 

Brooks. 

THE   MACMILLAN   COMPANY. 

66    FIFTH    AVENUE,  NEW    YORK. 


I.    FROM  THE   GREEKS  TO   DARWIN. 

THE  DEVELOPMENT   OF    THE  EVOLUTION   IDEA. 

BY 

HENRY    FAIRFIELD    OSBORN,    Sc.D.,   Princeton. 

Da  Costa  Professor  of  Zoology  in  Columbia   University. 
8vo.    Cloth.    $2.00,  net. 


This  opening  volume, "  From  the  Greeks  to  Darwin/'  is  an 
outline  of  the  development  from  the  earliest  times  of  the  idea  of 
the  origin  of  life  by  evolution.  It  brings  together  in  a  continu- 
ous treatment  the  progress  of  this  idea  from  the  Greek  philoso- 
plier  Thales  (640  B.C.)  to  Darwin  and  Wallace.  It  is  based 
partly  upon  critical  studies  of  the  original  authorities,  partly 
upon  the  studies  of  Zeller,  Perrier,  Quatrefages,  Martin,  and 
other  writers  less  known  to  English  readers. 

This  history  differs  from  the  outlines  which  have  been  pre- 
viously published,  in  attempting  to  establish  a  complete  conti- 
nuity of  thought  in  the  growth  of  the  various  elements  in  the 
Evolution  idea,  and  especially  in  the  more  critical  and  exact 
study  of  the  pre-Darwinian  w^riters,  such  as  Buffon,  Goethe, 
Erasmus  Darwin,  Treviranus,  Lamarck,  and  St.  Hilaire,  about 
whose  actual  share  in  the  establishment  of  the  Evolution  theory 
vague  ideas  are  still  current. 

TABLE    OF    CONTENTS. 
I.  The  Anticipation^  and  Interpretation  of  Nature. 
II.  Among  the  Greeks. 

III.  The  Theologians  and  Natural  Philosophers.. 

IV.  The  Evolutionists  of  the  Eighteenth  Century. 
V.  From  Lamarck  to  St.  Hilaire. 

VI.  The  First  Half-century  and  Darwin. 
In  the  opening  chapter  the  elements  and  environment  of  the 
Evolution  idea  are  discussed,  and  in  the  second  chapter  the  re- 
markable parallelism  between  the  growth  of  this  idea  in  Greece 
and  in  modern  times  is  pointed  out.  In  the  succeeding  chap- 
ters the  various  periods  of  European  thought  on  the  subject  are 
covered,  concluding  with  the  first  half  of  the  present  century, 
especially  with  the  development  of  the  Evolution  idea  in  the 
mind  of  Darwin. 


II.    AMPHIOXUS  AND  THE  ANCESTRY 
OF  THE  VERTEBRATES. 


BY 


ARTHUR   WILLEY,    B.Sc.    LOND., 

Tutor  in  Biology,  Columbia   Fnirersit;/ :  Balfour  Student  of  th* 
Unirernity  of  Cambridge. 

8vo.    Cloth.    $2.50,  net. 


The  purpose  of  this  vohime  is  to  consider  tlie  proV)h^ni  of  tlie 
ancestry  of  the  Vertebrates  from  the  stand i)oint  of  the  anat- 
omy and  development  of  Ampliioxns  and  other  members  of  the 
group  Protochordata.  The  work  opens  with  an  Introchiction, 
in  which  is  given  a  brief  historical  sketch  of  the  sj)ecii  hit  ions 
of  the  celebrated  anatomists  and  embryologists,  from  Ktionnc 
Geoffroy  St.  Hilaire  down  to  our  own  day,  upon  this  problem. 
The  remainder  of  the  first  and  the  whole  of  tlie  second  cliai»ter 
is  devoted  to  a  detailed  account  of  the  anatomy  of  Ampiiioxus 
as  compared  with  that  of  higher  Vertebrates.  The  third  chapter 
deals  with  the  embrvonic  and  larval  development  of  Am}>hioxu8, 
while  the  fourth  deals  more  briefly  with  the  anatomy,  embryoloiry, 
and  relationships  of  the  Ascidians;  then  the  other  allied  forms, 
Balanoglossus,  Cephalodiscus,  are  described. 

The  work  concludes  with  a  series  of  di.<cussions  touch- 
ing the  problem  proposed  in  the  Introduction,  in  wliich  it  is 
attempted  to  define  certain  general  in-inciplcs  of  Kvolution  by 
which  the  descent  of  the  Vertebrates  from  Invertebrate  ancestors 
may  be  supposed  to  have  taken  i)lacc. 

The  work  contains  an  extensive  bibliography,  full  notes,  and 
135  illustrations. 

TABLE   OF    CONTENTS. 

Introduction. 

Chapter    I.  Anatomy  of  Ami'Iiioxus. 
II.  Ditto. 

III.  Development  of  Ami'Iiioxu^;. 

IV.  The  Ascidians. 

V.  The  Protochordata   in'   Tin:ii:   Ixklation  t<> 
the  Prorlem  or  Vkktkhkati:  Descent. 


III.    FISHES,    LIVING   AND   FOSSIL. 

^.Y  IXTRODUCrORY  STUDY. 

BY 

BASHFORD   DEAN,   Ph.D.,  Columbia, 

Instructor  in  Biology,   Columbia    University. 
8vo.    Cloth.    $2.50,  net. 


This  work  has  been  prepared  to  meet  the  needs  of  the  oren- 
eral  student  for  a  concise  knowledge  of  the  Fishes.  -It  contains 
a  review  of  the  four  larger  groups  of  the  strictly  fishlike  fornis, 
Sharks,  Chimaeroids,  Teleostomes,  and  the  Dipnoans,  and  adds 
to  this  a  chapter  on  the  Lampreys.  It  presents  in  figures  the 
prominent  members,  living  and  fossil,  of  each  group;  illustrates 
characteristic  structures;  adds  notes  upon  the  important  phases 
of  development,  and  formulates  the  views  of  investigators  as  to 
relationships  and  descent. 

The  recent  contributions  to  the  knowledge  of  extinct  Fishes 
are  taken  into  special  account  in  the  treatment  of  the  entire 
subject,  and  restorations  have  been  attempted,  as  of  Dinichthys, 
Ctenodus,  and  Cladoselache. 

The  writer  has  also  indicated  diagrammatically,  as  far  a? 
generally  accepted,  the  genetid'  relationships  of  fossil  and  living 
forms. 

The  aim  of  the  book  has  been  mainlv  to  furnish  the  student 
with  a  well-marked  ground-plan  of  Ichthyology,  to  enable  him  to 
better  understand  special  works,  such  as  those  of  Smith  Wood- 
ward and  Giinther.  The  work  is  fullv  illustrated,  mainlv  from 
the  writer's  original  pen-drawings. 

TABLE    OF    CONTENTS. 

CHAPTER 

I.  Fishes.  Their  Esseniial  Characters.  Siiarks,  Chimaeroids,  Teleo- 
stomes, aud  Lung-tishes.  Their  Appearance  iu  Time  and  their 
Distribution. 

II.  The  Lampreys.  Their  Position  with  Reference  to  Fishes.  Bdel- 
lostoma,  jVIyxine,  Petromyzon,   Palaeospondylus. 

IIL   The  Shakk  Group.     Anatomical  Cluiracters.     Its  E.xtiuct  3Iembers, 
AcaLthodiau,  Cladoselachid,  Xeuacauthid,  Cestraciouts. 

IV.  Chimaer<jids.     Structures  of  Callorhynchus  and  Chimaeia.     Squalo- 
raja  aud  Myriacantlius.     Life-habits  and  Probable  Relationships. 

V.  Teleostomes.  The  Forms  of  Recent  "  Ganoids."  Habits  and  Dis- 
tribution. The  Relations  of  Prominent  Extinct  Foi-ms.  Crosso- 
pterygians.     Typical  "  Bony  Fishes. " 

VI.  The  Evolution  of  the  Groups  of  Fishes.     Aquatic  Metnraerism. 

Numerical  Lines.     Evolution  of  Gill-cleft  Characters,  Paired  and 

Unpaired  Fins,  Aquatic  Sense-organs. 
VII.  The  Development  of  Fishes.     Prominent  Features  iu  Embr3'onic 

and  Larval  Development  of  jMwmbers  of  each  Group.     Summaries. 


V.    The  Foundations  of  Zoology 

By  WILLIAM    KEITH    BROOKS, 

Professor  of  Zoology,  Johns  Hopkins  Uttivcrsitv- 

8vo.     Cloth,     Price  $2.50  net. 


EXTRACT   FROM   PREFACE. 

"I  shall  try  to  show  that  life  is  response  to  the  order  of  nature;  in  fact,  this 
thesis  is  the  text  of  most  of  the  lectures:  but  if  it  he  admitted,  it  follows  thai 
biology  is  the  study  of  response,  and  that  the  study  of  that  order  of  nature  to 
which  response  is  made  is  as  well  within  its  province  as  the  study  of  the  livmg  or- 
ganism which  responds,  for  all  the  knowledge  we  can  get  of  both  these  aspects  of 
nature  is  needed  as  a  preparation  for  the  study  of  that  relation  between  them  which 
constitutes  life.  Our  interest  in  all  branches  gf  science  is  vital  interest.  It  is  only 
as  living  things  that  we  care  to  know.  Life  is  that  which,  when  joined  to  mind,  is 
knowledge,  —  knowledge  in  use;  and  we  may  be  sure  that  all  living  thmgs  with 
minds  like  ours  are  conscious  of  some  part  of  the  order  of  nature,  for  the  response 
in  which  life  consists  is  response  to  this  order.  The  statement  that  physical 
phenomena  are  natural  seems  to  mean  little,  but  the  phenomena  of  life  arc  so 
wonderful  that  many  hesitate,  even  at  the  present  day,  to  believe  that  nature  can  be 
such  a  wonderful  thing  as  it  must  be  if  the  actions  of  all  living  things  are  natural; 
and,  as  I  shall  try  to  find  out  in  this  course  of  lectures  what  we  mean  by  the  asser- 
tion that  living  nature  is  natural,  I  shall  now  attempt,  by  a  few  illustrations,  to  give 
a  broad  outline  of  some  of  the  most  notable  features  of  the  nature  of  living  things." 


TABLE   OF  CONTENTS  : 


I. 
II. 

III. 
IV. 

V. 

VI. 
VI. 


VII. 


Introductory. 

Huxley  and  the  Problem  of  the 

Naturalist. 
Nature  and  Nurture. 
Lamarck. 
Migration    in    its     Bearing    on 

Lamarckism. 

(I)  Zoology  and  the  Philosophy 
of  Evolution. 

(II)  An  Inherent  Error  in  the 
Views  of  Galton  and  Weis- 
mann  on  Inheritance. 

Galton  and  the  Statistical  Study 
of  Inheritance. 


VIII.     Darwin     and     the      Origin      01 
Species. 
IX.     Natural   Selection   and    the  An- 
tiquity of  Life. 
X.     Natural    Selection    an.l    N.itur.il 

Theology. 
XI.     Paley  and    the  Argument    (rom 

Contrivance. 
XII.     The  Mechanism  of  Nature. 
XIII.     Louis  Agassiz  and  George  Berke- 
ley. 


THE    MACMILl.AX    COMPANY 

66  FIFTH   AVENUE,    NEW   YORK 


tO--*:t  !i^''rw-.^i--»-'3¥Tf.'- 


1^ 


„.„    BOUND    TO     PLEASE 


,/^%^     JAN.  66 


MANCHESTER. 


North  Carolina  State  University  Libraries 


QH581  .W755 


C.2 


CELL  IN  DEVELOPMENT  AND  INHERITANCE 


S02776680  L 


